THE HABITS OF INSECTS IN RELATION TO THE FERTILISATION OF FLOWERS

The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom by Charles Darwin, is part of the HackerNoon Books Series. You can jump to any chapter in this book here. THE HABITS OF INSECTS IN RELATION TO THE FERTILISATION OF FLOWERS

THE HABITS OF INSECTS IN RELATION TO THE FERTILISATION OF FLOWERS

Bees and various other insects must be directed by instinct to search flowers for nectar and pollen, as they act in this manner without instruction as soon as they emerge from the pupa state. Their instincts, however, are not of a specialised nature, for they visit many exotic flowers as readily as the endemic kinds, and they often search for nectar in flowers which do not secrete any; and they may be seen attempting to suck it out of nectaries of such length that it cannot be reached by them. (11/1. See, on this subject Hermann Muller ‘Befruchtung’ etc. page 427; and Sir J. Lubbock’s ‘British Wild Flowers’ etc. page 20. Muller ‘Bienen Zeitung’ June 1876 page 119, assigns good reasons for his belief that bees and many other Hymenoptera have inherited from some early nectar-sucking progenitor greater skill in robbing flowers than that which is displayed by insects belonging to the other Orders.) All kinds of bees and certain other insects usually visit the flowers of the same species as long as they can, before going to another species. This fact was observed by Aristotle with respect to the hive-bee more than 2000 years ago, and was noticed by Dobbs in a paper published in 1736 in the Philosophical Transactions. It may be observed by any one, both with hive and humble-bees, in every flower-garden; not that the habit is invariably followed. Mr. Bennett watched for several hours many plants of Lamium album, L. purpureum, and another Labiate plant, Nepeta glechoma, all growing mingled together on a bank near some hives; and he found that each bee confined its visits to the same species. (11/2. ‘Nature’ 1874 June 4 page 92.) The pollen of these three plants differs in colour, so that he was able to test his observations by examining that which adhered to the bodies of the captured bees, and he found one kind on each bee.

Humble and hive-bees are good botanists, for they know that varieties may differ widely in the colour of their flowers and yet belong to the same species. I have repeatedly seen humble-bees flying straight from a plant of the ordinary red Dictamnus fraxinella to a white variety; from one to another very differently coloured variety of Delphinium consolida and of Primula veris; from a dark purple to a bright yellow variety of Viola tricolor; and with two species of Papaver, from one variety to another which differed much in colour; but in this latter case some of the bees flew indifferently to either species, although passing by other genera, and thus acted as if the two species were merely varieties. Hermann Muller also has seen hive-bees flying from flower to flower of Ranunculus bulbosus and arvensis, and of Trifolium fragiferum and repens; and even from blue hyacinths to blue violets. (11/3. ‘Bienen Zeitung’ July 1876 page 183.)

Some species of Diptera or flies keep to the flowers of the same species with almost as much regularity as do bees; and when captured they are found covered with pollen. I have seen Rhingia rostrata acting in this manner with the flowers of Lychnis dioica, Ajuga reptans, and Vici sepium. Volucella plumosa and Empis cheiroptera flew straight from flower to flower of Myosotis sylvatica. Dolichopus nigripennis behaved in the same manner with Potentilla tormentilla; and other Diptera with Stellaria holostea, Helianthemum vulgare, Bellis perennis, Veronica hederaefolia and chamoedrys; but some flies visited indifferently the flowers of these two latter species. I have seen more than once a minute Thrips, with pollen adhering to its body, fly from one flower to another of the same kind; and one was observed by me crawling about within a convolvulus with four grains of pollen adhering to its head, which were deposited on the stigma.

Fabricius and Sprengel state that when flies have once entered the flowers of Aristolochia they never escape,—a statement which I could not believe, as in this case the insects would not aid in the cross-fertilisation of the plant; and this statement has now been shown by Hildebrand to be erroneous. As the spathes of Arum maculatum are furnished with filaments apparently adapted to prevent the exit of insects, they resemble in this respect the flowers of Aristolochia; and on examining several spathes, from thirty to sixty minute Diptera belonging to three species were found in some of them; and many of these insects were lying dead at the bottom, as if they had been permanently entrapped. In order to discover whether the living ones could escape and carry pollen to another plant, I tied in the spring of 1842 a fine muslin bag tightly round a spathe; and on returning in an hour’s time several little flies were crawling about on the inner surface of the bag. I then gathered a spathe and breathed hard into it; several flies soon crawled out, and all without exception were dusted with arum pollen. These flies quickly flew away, and I distinctly saw three of them fly to another plant about a yard off; they alighted on the inner or concave surface of the spathe, and suddenly flew down into the flower. I then opened this flower, and although not a single anther had burst, several grains of pollen were lying at the bottom, which must have been brought from another plant by one of these flies or by some other insect. In another flower little flies were crawling about, and I saw them leave pollen on the stigmas.

I do not know whether Lepidoptera generally keep to the flowers of the same species; but I once observed many minute moths (I believe Lampronia (Tinea) calthella) apparently eating the pollen of Mercurialis annua, and they had the whole front of their bodies covered with pollen. I then went to a female plant some yards off, and saw in the course of fifteen minutes three of these moths alight on the stigmas. Lepidoptera are probably often induced to frequent the flowers of the same species, whenever these are provided with a long and narrow nectary, as in this case other insects cannot suck the nectar, which will thus be preserved for those having an elongated proboscis. No doubt the Yucca moth visits only the flowers whence its name is derived, for a most wonderful instinct guides this moth to place pollen on the stigma, so that the ovules may be developed on which the larvae feed. (11/4. Described by Mr. Riley in the ‘American Naturalist’ volume 7 October 1873.)With respect to Coleoptera, I have seen Meligethes covered with pollen flying from flower to flower of the same species; and this must often occur, as, according to M. Brisout, “many of the species affect only one kind of plant.” (11/5. As quoted in ‘American Nat.’ May 1873 page 270.)

It must not be supposed from these several statements that insects strictly confine their visits to the same species. They often visit other species when only a few plants of the same kind grow near together. In a flower-garden containing some plants of Œnothera, the pollen of which can easily be recognised, I found not only single grains but masses of it within many flowers of Mimulus, Digitalis, Antirrhinum, and Linaria. Other kinds of pollen were likewise detected in these same flowers. A large number of the stigmas of a plant of Thyme, in which the anthers were completely aborted, were examined; and these stigmas, though scarcely larger than a split needle, were covered not only with pollen of Thyme brought from other plants by the bees, but with several other kinds of pollen.

That insects should visit the flowers of the same species as long as they can, is of great importance to the plant, as it favours the cross-fertilisation of distinct individuals of the same species; but no one will suppose that insects act in this manner for the good of the plant. The cause probably lies in insects being thus enabled to work quicker; they have just learnt how to stand in the best position on the flower, and how far and in what direction to insert their proboscides. (11/6. Since these remarks were written, I find that Hermann Muller has come to almost exactly the same conclusion with respect to the cause of insects frequenting as long as they can the flowers of the same species: ‘Bienen Zeitung’ July 1876 page 182.) They act on the same principle as does an artificer who has to make half-a-dozen engines, and who saves time by making consecutively each wheel and part for all of them. Insects, or at least bees, seem much influenced by habit in all their manifold operations; and we shall presently see that this holds good in their felonious practice of biting holes through the corolla.

It is a curious question how bees recognise the flowers of the same species. That the coloured corolla is the chief guide cannot be doubted. On a fine day, when hive-bees were incessantly visiting the little blue flowers of Lobelia erinus, I cut off all the petals of some, and only the lower striped petals of others, and these flowers were not once again sucked by the bees, although some actually crawled over them. The removal of the two little upper petals alone made no difference in their visits. Mr. J. Anderson likewise states that when he removed the corollas of the Calceolaria, bees never visited the flowers. (11/7. ‘Gardeners’ Chronicle’ 1853 page 534. Kurr cut off the nectaries from a large number of flowers of several species, and found that the greater number yielded seeds; but insects probably would not perceive the loss of the nectary until they had inserted their proboscides into the holes thus formed, and in doing so would fertilise the flowers. He also removed the whole corolla from a considerable number of flowers, and these likewise yielded seeds. Flowers which are self-fertile would naturally produce seeds under these circumstances; but I am greatly surprised that Delphinium consolida, as well as another species of Delphinium, and Viola tricolor, should have produced a fair supply of seeds when thus treated; but it does not appear that he compared the number of the seeds thus produced with those yielded by unmutilated flowers left to the free access of insects: ‘Bedeutung der Nektarien’ 1833 pages 123-135.) On the other hand, in some large masses of Geranium phaeum which had escaped out of a garden, I observed the unusual fact of the flowers continuing to secrete an abundance of nectar after all the petals had fallen off; and the flowers in this state were still visited by humble-bees. But the bees might have learnt that these flowers with all their petals lost were still worth visiting, by finding nectar in those with only one or two lost. The colour alone of the corolla serves as an approximate guide: thus I watched for some time humble-bees which were visiting exclusively plants of the white-flowered Spiranthes autumnalis, growing on short turf at a considerable distance apart; and these bees often flew within a few inches of several other plants with white flowers, and then without further examination passed onwards in search of the Spiranthes. Again, many hive-bees which confined their visits to the common ling (Calluna vulgaris), repeatedly flew towards Erica tetralix, evidently attracted by the nearly similar tint of their flowers, and then instantly passed on in search of the Calluna.

That the colour of the flower is not the sole guide, is clearly shown by the six cases above given of bees which repeatedly passed in a direct line from one variety to another of the same species, although they bore very differently coloured flowers. I observed also bees flying in a straight line from one clump of a yellow-flowered Œnothera to every other clump of the same plant in the garden, without turning an inch from their course to plants of Eschscholtzia and others with yellow flowers which lay only a foot or two on either side. In these cases the bees knew the position of each plant in the garden perfectly well, as we may infer by the directness of their flight; so that they were guided by experience and memory. But how did they discover at first that the above varieties with differently coloured flowers belonged to the same species? Improbable as it may appear, they seem, at least sometimes, to recognise plants even from a distance by their general aspect, in the same manner as we should do. On three occasions I observed humble-bees flying in a perfectly straight line from a tall larkspur (Delphinium) which was in full flower to another plant of the same species at the distance of fifteen yards which had not as yet a single flower open, and on which the buds showed only a faint tinge of blue. Here neither odour nor the memory of former visits could have come into play, and the tinge of blue was so faint that it could hardly have served as a guide. (11/8. A fact mentioned by Hermann Muller ‘Die Befruchtung’ etc. page 347, shows that bees possess acute powers of vision and discrimination; for those engaged in collecting pollen from Primula elatior invariably passed by the flowers of the long-styled form, in which the anthers are seated low down in the tubular corolla. Yet the difference in aspect between the long-styled and short-styled forms is extremely slight.)

The conspicuousness of the corolla does not suffice to induce repeated visits from insects, unless nectar is at the same time secreted, together perhaps with some odour emitted. I watched for a fortnight many times daily a wall covered with Linaria cymbalaria in full flower, and never saw a bee even looking at one. There was then a very hot day, and suddenly many bees were industriously at work on the flowers. It appears that a certain degree of heat is necessary for the secretion of nectar; for I observed with Lobelia erinus that if the sun ceased to shine for only half an hour, the visits of the bees slackened and soon ceased. An analogous fact with respect to the sweet excretion from the stipules of Vicia sativa has been already given. As in the case of the Linaria, so with Pedicularis sylvatica, Polygala vulgaris, Viola tricolor, and some species of Trifolium, I have watched the flowers day after day without seeing a bee at work, and then suddenly all the flowers were visited by many bees. Now how did so many bees discover at once that the flowers were secreting nectar? I presume that it must have been by their odour; and that as soon as a few bees began to suck the flowers, others of the same and of different kinds observed the fact and profited by it. We shall presently see, when we treat of the perforation of the corolla, that bees are fully capable of profiting by the labour of other species. Memory also comes into play, for, as already remarked, bees know the position of each clump of flowers in a garden. I have repeatedly seen them passing round a corner, but otherwise in as straight a line as possible, from one plant of Fraxinella and of Linaria to another and distant one of the same species; although, owing to the intervention of other plants, the two were not in sight of each other.

It would appear that either the taste or the odour of the nectar of certain flowers is unattractive to hive or to humble-bees, or to both; for there seems no other reason why certain open flowers which secrete nectar are not visited by them. The small quantity of nectar secreted by some of these flowers can hardly be the cause of their neglect, as hive-bees search eagerly for the minute drops on the glands on the leaves of the Prunus laurocerasus. Even the bees from different hives sometimes visit different kinds of flowers, as is said to be the case by Mr. Grant with respect to the Polyanthus and Viola tricolor. (11/9. ‘Gardeners’ Chronicle’ 1844 page 374.) I have known humble-bees to visit the flowers of Lobelia fulgens in one garden and not in another at the distance of only a few miles. The cupful of nectar in the labellum of Epipactis latifolia is never touched by hive- or humble-bees, although I have seen them flying close by; and yet the nectar has a pleasant taste to us, and is habitually consumed by the common wasp. As far as I have seen, wasps seek for nectar in this country only from the flowers of this Epipactis, Scrophularia aquatica, Symphoricarpus racemosa (11/10. The same fact apparently holds good in Italy, for Delpino says that the flowers of these three plants are alone visited by wasps: ‘Nettarii Estranuziali, Bulletino Entomologico’ anno 6.), and Tritoma; the two former plants being endemic, and the two latter exotic. As wasps are so fond of sugar and of any sweet fluid, and as they do not disdain the minute drops on the glands of Prunus laurocerasus, it is a strange fact that they do not suck the nectar of many open flowers, which they could do without the aid of a proboscis. Hive-bees visit the flowers of the Symphoricarpus and Tritoma, and this makes it all the stranger that they do not visit the flowers of the Epipactis, or, as far as I have seen, those of the Scrophularia aquatica; although they do visit the flowers of Scrophularia nodosa, at least in North America. (11/11. ‘Silliman’s American Journal of Science’ August 1871.)

The extraordinary industry of bees and the number of flowers which they visit within a short time, so that each flower is visited repeatedly, must greatly increase the chance of each receiving pollen from a distinct plant. When the nectar is in any way hidden, bees cannot tell without inserting their proboscides whether it has lately been exhausted by other bees, and this, as remarked in a former chapter, forces them to visit many more flowers than they otherwise would. But they endeavour to lose as little time as they can; thus in flowers having several nectaries, if they find one dry they do not try the others, but as I have often observed, pass on to another flower. They work so industriously and effectually, that even in the case of social plants, of which hundreds of thousands grow together, as with the several kinds of heath, every single flower is visited, of which evidence will presently be given. They lose no time and fly very quickly from plant to plant, but I do not know the rate at which hive-bees fly. Humble-bees fly at the rate of ten miles an hour, as I was able to ascertain in the case of the males from their curious habit of calling at certain fixed points, which made it easy to measure the time taken in passing from one place to another.

With respect to the number of flowers which bees visit in a given time, I observed that in exactly one minute a humble-bee visited twenty-four of the closed flowers of the Linaria cymbalaria; another bee visited in the same time twenty-two flowers of the Symphoricarpus racemosa; and another seventeen flowers on two plants of a Delphinium. In the course of fifteen minutes a single flower on the summit of a plant of Œnothera was visited eight times by several humble-bees, and I followed the last of these bees, whilst it visited in the course of a few additional minutes every plant of the same species in a large flower-garden. In nineteen minutes every flower on a small plant of Nemophila insignis was visited twice. In one minute six flowers of a Campanula were entered by a pollen-collecting hive-bee; and bees when thus employed work slower than when sucking nectar. Lastly, seven flower-stalks on a plant of Dictamnus fraxinella were observed on the 15th of June 1841 during ten minutes; they were visited by thirteen humble-bees each of which entered many flowers. On the 22nd the same flower-stalks were visited within the same time by eleven humble-bees. This plant bore altogether 280 flowers, and from the above data, taking into consideration how late in the evening humble-bees work, each flower must have been visited at least thirty times daily, and the same flower keeps open during several days. The frequency of the visits of bees is also sometimes shown by the manner in which the petals are scratched by their hooked tarsi; I have seen large beds of Mimulus, Stachys, and Lathyrus with the beauty of their flowers thus sadly defaced.

PERFORATION OF THE COROLLA BY BEES.

I have already alluded to bees biting holes in flowers for the sake of obtaining the nectar. They often act in this manner, both with endemic and exotic species, in many parts of Europe, in the United States, and in the Himalaya; and therefore probably in all parts of the world. The plants, the fertilisation of which actually depends on insects entering the flowers, will fail to produce seed when their nectar is stolen from the outside; and even with those species which are capable of fertilising themselves without any aid, there can be no cross-fertilisation, and this, as we know, is a serious evil in most cases. The extent to which humble-bees carry on the practice of biting holes is surprising: a remarkable case was observed by me near Bournemouth, where there were formerly extensive heaths. I took a long walk, and every now and then gathered a twig of Erica tetralix, and when I had got a handful all the flowers were examined through a lens. This process was repeated many times; but though many hundreds were examined, I did not succeed in finding a single flower which had not been perforated. Humble-bees were at the time sucking the flowers through these perforations. On the following day a large number of flowers were examined on another heath with the same result, but here hive-bees were sucking through the holes. This case is all the more remarkable, as the innumerable holes had been made within a fortnight, for before that time I saw the bees everywhere sucking in the proper manner at the mouths of the corolla. In an extensive flower-garden some large beds of Salvia grahami, Stachys coccinea, and Pentstemon argutus (?) had every flower perforated, and many scores were examined. I have seen whole fields of red clover (Trifolium pratense) in the same state. Dr. Ogle found that 90 per cent of the flowers of Salvia glutinosa had been bitten. In the United States Mr. Bailey says it is difficult to find a blossom of the native Gerardia pedicularia without a hole in it; and Mr. Gentry, in speaking of the introduced Wistaria sinensis, says “that nearly every flower had been perforated.” (11/12. Dr. Ogle ‘Pop. Science Review’ July 1869 page 267. Bailey ‘American Naturalist’ November 1873 page 690. Gentry ibid May 1875 page 264.)

As far as I have seen, it is always humble-bees which first bite the holes, and they are well fitted for the work by possessing powerful mandibles; but hive-bees afterwards profit by the holes thus made. Dr. Hermann Muller, however, writes to me that hive-bees sometimes bite holes through the flowers of Erica tetralix. No insects except bees, with the single exception of wasps in the case of Tritoma, have sense enough, as far as I have observed, to profit by the holes already made. Even humble-bees do not always discover that it would be advantageous to them to perforate certain flowers. There is an abundant supply of nectar in the nectary of Tropaeolum tricolor, yet I have found this plant untouched in more than one garden, while the flowers of other plants had been extensively perforated; but a few years ago Sir J. Lubbock’s gardener assured me that he had seen humble-bees boring through the nectary of this Tropaeolum. Muller has observed humble-bees trying to suck at the mouths of the flowers of Primula elatior and of an Aquilegia, and, failing in their attempts, they made holes through the corolla; but they often bite holes, although they could with very little more trouble obtain the nectar in a legitimate manner by the mouth of the corolla.

Dr. W. Ogle has communicated to me a curious case. He gathered in Switzerland 100 flower-stems of the common blue variety of the monkshood (Aconitum napellus), and not a single flower was perforated; he then gathered 100 stems of a white variety growing close by, and every one of the open flowers had been perforated. (11/13. Dr. Ogle ‘Popular Science Review’ July 1869 page 267. Bailey ‘American Naturalist’ November 1873 page 690. Gentry ibid May 1875 page 264.) This surprising difference in the state of the flowers may be attributed with much probability to the blue variety being distasteful to bees, from the presence of the acrid matter which is so general in the Ranunculaceae, and to its absence in the white variety in correlation with the loss of the blue tint. According to Sprengel, this plant is strongly proterandrous (11/14. ‘Das Entdeckte’ etc. page 278.); it would therefore be more or less sterile unless bees carried pollen from the younger to the older flowers. Consequently the white variety, the flowers of which were always bitten instead of being properly entered by the bees, would fail to yield the full number of seeds and would be a comparatively rare plant, as Dr. Ogle informs me was the case.

Bees show much skill in their manner of working, for they always make their holes from the outside close to the spot where the nectar lies hidden within the corolla. All the flowers in a large bed of Stachys coccinea had either one or two slits made on the upper side of the corolla near the base. The flowers of a Mirabilis and of Salvia coccinea were perforated in the same manner; whilst those of Salvia grahami, in which the calyx is much elongated, had both the calyx and the corolla invariably perforated. The flowers of Pentstemon argutus are broader than those of the plants just named, and two holes alongside each other had here always been made just above the calyx. In these several cases the perforations were on the upper side, but in Antirrhinum majus one or two holes had been made on the lower side, close to the little protuberance which represents the nectary, and therefore directly in front of and close to the spot where the nectar is secreted.

But the most remarkable case of skill and judgment known to me, is that of the perforation of the flowers of Lathyrus sylvestris, as described by my son Francis. (11/15. ‘Nature’ January 8, 1874 page 189.) The nectar in this plant is enclosed within a tube, formed by the united stamens, which surround the pistil so closely that a bee is forced to insert its proboscis outside the tube; but two natural rounded passages or orifices are left in the tube near the base, in order that the nectar may be reached by the bees. Now my son found in sixteen out of twenty-four flowers on this plant, and in eleven out of sixteen of those on the cultivated everlasting pea, which is either a variety of the same species or a closely allied one, that the left passage was larger than the right one. And here comes the remarkable point,—the humble-bees bite holes through the standard-petal, and they always operated on the left side over the passage, which is generally the larger of the two. My son remarks: “It is difficult to say how the bees could have acquired this habit. Whether they discovered the inequality in the size of the nectar-holes in sucking the flowers in the proper way, and then utilised this knowledge in determining where to gnaw the hole; or whether they found out the best situation by biting through the standard at various points, and afterwards remembered its situation in visiting other flowers. But in either case they show a remarkable power of making use of what they have learnt by experience.” It seems probable that bees owe their skill in biting holes through flowers of all kinds to their having long practised the instinct of moulding cells and pots of wax, or of enlarging their old cocoons with tubes of wax; for they are thus compelled to work on the inside and outside of the same object.

In the early part of the summer of 1857 I was led to observe during some weeks several rows of the scarlet kidney-bean (Phaseolus multiflorus), whilst attending to the fertilisation of this plant, and daily saw humble- and hive-bees sucking at the mouths of the flowers. But one day I found several humble-bees employed in cutting holes in flower after flower; and on the next day every single hive-bee, without exception, instead of alighting on the left wing-petal and sucking the flower in the proper manner, flew straight without the least hesitation to the calyx, and sucked through the holes which had been made only the day before by the humble-bees; and they continued this habit for many following days. (11/16. ‘Gardeners’ Chronicle’ 1857 page 725.) Mr. Belt has communicated to me (July 28th, 1874) a similar case, with the sole difference that less than half of the flowers had been perforated by the humble-bees; nevertheless, all the hive-bees gave up sucking at the mouths of the flowers and visited exclusively the bitten ones. Now how did the hive-bees find out so quickly that holes had been made? Instinct seems to be out of the question, as the plant is an exotic. The holes cannot be seen by bees whilst standing on the wing-petals, where they had always previously alighted. From the ease with which bees were deceived when the petals of Lobelia erinus were cut off, it was clear that in this case they were not guided to the nectar by its smell; and it may be doubted whether they were attracted to the holes in the flowers of the Phaseolus by the odour emitted from them. Did they perceive the holes by the sense of touch in their proboscides, whilst sucking the flowers in the proper manner, and then reason that it would save them time to alight on the outside of the flowers and use the holes? This seems almost too abstruse an act of reason for bees; and it is more probable that they saw the humble-bees at work, and understanding what they were about, imitated them and took advantage of the shorter path to the nectar. Even with animals high in the scale, such as monkeys, we should be surprised at hearing that all the individuals of one species within the space of twenty-four hours understood an act performed by a distinct species, and profited by it.

I have repeatedly observed with various kinds of flowers that all the hive and humble-bees which were sucking through the perforations, flew to them, whether on the upper or under side of the corolla, without the least hesitation; and this shows how quickly all the individuals within the district had acquired the same knowledge. Yet habit comes into play to a certain extent, as in so many of the other operations of bees. Dr. Ogle, Messrs. Farrer and Belt have observed in the case of Phaseolus multiflorus that certain individuals went exclusively to the perforations, while others of the same species visited only the mouths of the flowers. (11/17. Dr. Ogle ‘Pop. Science Review’ April 1870 page 167. Mr. Farrer ‘Annals and Magazine of Natural History’ 4th series volume 2 1868 page 258. Mr. Belt in a letter to me.) I noticed in 1861 exactly the same fact with Trifolium pratense. So persistent is the force of habit, that when a bee which is visiting perforated flowers comes to one which has not been bitten, it does not go to the mouth, but instantly flies away in search of another bitten flower. Nevertheless, I once saw a humble-bee visiting the hybrid Rhododendron azaloides, and it entered the mouths of some flowers and cut holes into the others. Dr. Hermann Muller informs me that in the same district he has seen some individuals of Bombus mastrucatus boring through the calyx and corolla of Rhinanthus alecterolophus, and others through the corolla alone. Different species of bees may, however, sometimes be observed acting differently at the same time on the same plant. I have seen hive-bees sucking at the mouths of the flowers of the common bean; humble-bees of one kind sucking through holes bitten in the calyx, and humble-bees of another kind sucking the little drops of fluid excreted by the stipules. Mr. Beal of Michigan informs me that the flowers of the Missouri currant (Ribes aureum) abound with nectar, so that children often suck them; and he saw hive-bees sucking through holes made by a bird, the oriole, and at the same time humble-bees sucking in the proper manner at the mouths of the flowers. (11/18. The flowers of the Ribes are however sometimes perforated by humble-bees, and Mr. Bundy says that they were able to bite through and rob seven flowers of their honey in a minute: ‘American Naturalist’ 1876 page 238.) This statement about the oriole calls to mind what I have before said of certain species of humming-birds boring holes through the flowers of the Brugmansia, whilst other species entered by the mouth.

The motive which impels bees to gnaw holes through the corolla seems to be the saving of time, for they lose much time in climbing into and out of large flowers, and in forcing their heads into closed ones. They were able to visit nearly twice as many flowers, as far as I could judge, of a Stachys and Pentstemon by alighting on the upper surface of the corolla and sucking through the cut holes, than by entering in the proper way. Nevertheless each bee before it has had much practice, must lose some time in making each new perforation, especially when the perforation has to be made through both calyx and corolla. This action therefore implies foresight, of which faculty we have abundant evidence in their building operations; and may we not further believe that some trace of their social instinct, that is, of working for the good of other members of the community, may here likewise play a part?

Many years ago I was struck with the fact that humble-bees as a general rule perforate flowers only when these grow in large numbers near together. In a garden where there were some very large beds of Stachys coccinea and of Pentstemon argutus, every single flower was perforated, but I found two plants of the former species growing quite separate with their petals much scratched, showing that they had been frequently visited by bees, and yet not a single flower was perforated. I found also a separate plant of the Pentstemon, and saw bees entering the mouth of the corolla, and not a single flower had been perforated. In the following year (1842) I visited the same garden several times: on the 19th of July humble-bees were sucking the flowers of Stachys coccinea and Salvia grahami in the proper manner, and none of the corollas were perforated. On the 7th of August all the flowers were perforated, even those on some few plants of the Salvia which grew at a little distance from the great bed. On the 21st of August only a few flowers on the summits of the spikes of both species remained fresh, and not one of these was now bored. Again, in my own garden every plant in several rows of the common bean had many flowers perforated; but I found three plants in separate parts of the garden which had sprung up accidentally, and these had not a single flower perforated. General Strachey formerly saw many perforated flowers in a garden in the Himalaya, and he wrote to the owner to inquire whether this relation between the plants growing crowded and their perforation by the bees there held good, and was answered in the affirmative. Hence it follows that the red clover (Trifolium pratense) and the common bean when cultivated in great masses in fields,—that Erica tetralix growing in large numbers on heaths,—rows of the scarlet kidney-bean in the kitchen-garden,—and masses of any species in the flower-garden,—are all eminently liable to be perforated.

The explanation of this fact is not difficult. Flowers growing in large numbers afford a rich booty to the bees, and are conspicuous from a distance. They are consequently visited by crowds of these insects, and I once counted between twenty and thirty bees flying about a bed of Pentstemon. They are thus stimulated to work quickly by rivalry, and, what is much more important, they find a large proportion of the flowers, as suggested by my son, with their nectaries sucked dry. (11/19. ‘Nature’ January 8, 1874 page 189.) They thus waste much time in searching many empty flowers, and are led to bite the holes, so as to find out as quickly as possible whether there is any nectar present, and if so, to obtain it.

Flowers which are partially or wholly sterile unless visited by insects in the proper manner, such as those of most species of Salvia, of Trifolium pratense, Phaseolus multiflorus, etc., will fail more or less completely to produce seeds if the bees confine their visits to the perforations. The perforated flowers of those species, which are capable of fertilising themselves, will yield only self-fertilised seeds, and the seedlings will in consequence be less vigorous. Therefore all plants must suffer in some degree when bees obtain their nectar in a felonious manner by biting holes through the corolla; and many species, it might be thought, would thus be exterminated. But here, as is so general throughout nature, there is a tendency towards a restored equilibrium. If a plant suffers from being perforated, fewer individuals will be reared, and if its nectar is highly important to the bees, these in their turn will suffer and decrease in number; but, what is much more effective, as soon as the plant becomes somewhat rare so as not to grow in crowded masses, the bees will no longer be stimulated to gnaw holes in the flowers, but will enter them in a legitimate manner. More seed will then be produced, and the seedlings being the product of cross-fertilisation will be vigorous, so that the species will tend to increase in number, to be again checked, as soon as the plant again grows in crowded masses.

CHAPTER XII. GENERAL RESULTS.

The first and most important of the conclusions which may be drawn from the observations given in this volume, is that cross-fertilisation is generally beneficial, and self-fertilisation injurious. This is shown by the difference in height, weight, constitutional vigour, and fertility of the offspring from crossed and self-fertilised flowers, and in the number of seeds produced by the parent-plants. With respect to the second of these two propositions, namely, that self-fertilisation is generally injurious, we have abundant evidence. The structure of the flowers in such plants as Lobelia ramosa, Digitalis purpurea, etc., renders the aid of insects almost indispensable for their fertilisation; and bearing in mind the prepotency of pollen from a distinct individual over that from the same individual, such plants will almost certainly have been crossed during many or all previous generations. So it must be, owing merely to the prepotency of foreign pollen, with cabbages and various other plants, the varieties of which almost invariably intercross when grown together. The same inference may be drawn still more surely with respect to those plants, such as Reseda and Eschscholtzia, which are sterile with their own pollen, but fertile with that from any other individual. These several plants must therefore have been crossed during a long series of previous generations, and the artificial crosses in my experiments cannot have increased the vigour of the offspring beyond that of their progenitors. Therefore the difference between the self-fertilised and crossed plants raised by me cannot be attributed to the superiority of the crossed, but to the inferiority of the self-fertilised seedlings, due to the injurious effects of self-fertilisation.

With respect to the first proposition, namely, that cross-fertilisation is generally beneficial, we likewise have excellent evidence. Plants of Ipomoea were intercrossed for nine successive generations; they were then again intercrossed, and at the same time crossed with a plant of a fresh stock, that is, one brought from another garden; and the offspring of this latter cross were to the intercrossed plants in height as 100 to 78, and in fertility as 100 to 51. An analogous experiment with Eschscholtzia gave a similar result, as far as fertility was concerned. In neither of these cases were any of the plants the product of self-fertilisation. Plants of Dianthus were self-fertilised for three generations, and this no doubt was injurious; but when these plants were fertilised by a fresh stock and by intercrossed plants of the same stock, there was a great difference in fertility between the two sets of seedlings, and some difference in their height. Petunia offers a nearly parallel case. With various other plants, the wonderful effects of a cross with a fresh stock may be seen in Table 7/C. Several accounts have also been published of the extraordinary growth of seedlings from a cross between two varieties of the same species, some of which are known never to fertilise themselves; so that here neither self-fertilisation nor relationship even in a remote degree can have come into play. (12/1. See ‘Variation under Domestication’ chapter 19 2nd edition volume 2 page 159.) We may therefore conclude that the above two propositions are true,—that cross-fertilisation is generally beneficial and self-fertilisation injurious to the offspring.

That certain plants, for instance, Viola tricolor, Digitalis purpurea, Sarothamnus scoparius, Cyclamen persicum, etc., which have been naturally cross-fertilised for many or all previous generations, should suffer to an extreme degree from a single act of self-fertilisation is a most surprising fact. Nothing of the kkind has been observed in our domestic animals; but then we must remember that the closest possible interbreeding with such animals, that is, between brothers and sisters, cannot be considered as nearly so close a union as that between the pollen and ovules of the same flower. Whether the evil from self-fertilisation goes on increasing during successive generations is not as yet known; but we may infer from my experiments that the increase if any is far from rapid. After plants have been propagated by self-fertilisation for several generations, a single cross with a fresh stock restores their pristine vigour; and we have a strictly analogous result with our domestic animals. (12/2. Ibid chapter 19 2nd edition volume 2 page 159.) The good effects of cross-fertilisation are transmitted by plants to the next generation; and judging from the varieties of the common pea, to many succeeding generations. But this may merely be that crossed plants of the first generation are extremely vigorous, and transmit their vigour, like any other character, to their successors.

Notwithstanding the evil which many plants suffer from self-fertilisation, they can be thus propagated under favourable conditions for many generations, as shown by some of my experiments, and more especially by the survival during at least half a century of the same varieties of the common pea and sweet-pea. The same conclusion probably holds good with several other exotic plants, which are never or most rarely cross-fertilised in this country. But all these plants, as far as they have been tried, profit greatly by a cross with a fresh stock. Some few plants, for instance, Ophrys apifera, have almost certainly been propagated in a state of nature for thousands of generations without having been once intercrossed; and whether they would profit by a cross with a fresh stock is not known. But such cases ought not to make us doubt that as a general rule crossing is beneficial, any more than the existence of plants which, in a state of nature, are propagated exclusively by rhizomes, stolons, etc. (their flowers never producing seeds), (12/3. I have given several cases in my ‘Variation under Domestication’ chapter 18 2nd edition volume 2 page 152.) (their flowers never producing seeds), should make us doubt that seminal generation must have some great advantage, as it is the common plan followed by nature. Whether any species has been reproduced asexually from a very remote period cannot, of course, be ascertained. Our sole means for forming any judgment on this head is the duration of the varieties of our fruit trees which have been long propagated by grafts or buds. Andrew Knight formerly maintained that under these circumstances they always become weakly, but this conclusion has been warmly disputed by others. A recent and competent judge, Professor Asa Gray, leans to the side of Andrew Knight, which seems to me, from such evidence as I have been able to collect, the more probable view, notwithstanding many opposed facts. (12/4. ‘Darwiniana: Essays and Reviews pertaining to Darwinism’ 1876 page 338.)

The means for favouring cross-fertilisation and preventing self-fertilisation, or conversely for favouring self-fertilisation and preventing to a certain extent cross-fertilisation, are wonderfully diversified; and it is remarkable that these differ widely in closely allied plants,—in the species of the same genus, and sometimes in the individuals of the same species. (12/5. Hildebrand has insisted strongly to this effect in his valuable observations on the fertilisation of the Gramineae: ‘Monatsbericht K. Akad. Berlin’ October 1872 page 763.) It is not rare to find hermaphrodite plants and others with separated sexes within the same genus; and it is common to find some of the species dichogamous and others maturing their sexual elements simultaneously. The dichogamous genus Saxifraga contains proterandrous and proterogynous species. (12/6. Dr. Engler ‘Botanische Zeitung’ 1868 page 833.) Several genera include both heterostyled (dimorphic or trimorphic forms) and homostyled species. Ophrys offers a remarkable instance of one species having its structure manifestly adapted for self-fertilisation, and other species as manifestly adapted for cross-fertilisation. Some con-generic species are quite sterile and others quite fertile with their own pollen. From these several causes we often find within the same genus species which do not produce seeds, while others produce an abundance, when insects are excluded. Some species bear cleistogene flowers which cannot be crossed, as well as perfect flowers, whilst others in the same genus never produce cleistogene flowers. Some species exist under two forms, the one bearing conspicuous flowers adapted for cross-fertilisation, the other bearing inconspicuous flowers adapted for self-fertilisation, whilst other species in the same genus present only a single form. Even with the individuals of the same species, the degree of self-sterility varies greatly, as in Reseda. With polygamous plants, the distribution of the sexes differs in the individuals of the same species. The relative period at which the sexual elements in the same flower are mature, differs in the varieties of Pelargonium; and Carriere gives several cases, showing that the period varies according to the temperature to which the plants are exposed. (12/7. ‘Des Varieties’ 1865 page 30.)

This extraordinary diversity in the means for favouring or preventing cross- and self-fertilisation in closely allied forms, probably depends on the results of both processes being highly beneficial to the species, but directly opposed in many ways to one another and dependent on variable conditions. Self-fertilisation assures the production of a large supply of seeds; and the necessity or advantage of this will be determined by the average length of life of the plant, which largely depends on the amount of destruction suffered by the seeds and seedlings. This destruction follows from the most various and variable causes, such as the presence of animals of several kinds, and the growth of surrounding plants. The possibility of cross-fertilisation depends mainly on the presence and number of certain insects, often of insects belonging to special groups, and on the degree to which they are attracted to the flowers of any particular species in preference to other flowers,—all circumstances likely to change. Moreover, the advantages which follow from cross-fertilisation differ much in different plants, so that it is probable that allied plants would often profit in different degrees by cross-fertilisation. Under these extremely complex and fluctuating conditions, with two somewhat opposed ends to be gained, namely, the safe propagation of the species and the production of cross-fertilised, vigorous offspring, it is not surprising that allied forms should exhibit an extreme diversity in the means which favour either end. If, as there is reason to suspect, self-fertilisation is in some respects beneficial, although more than counterbalanced by the advantages derived from a cross with a fresh stock, the problem becomes still more complicated.

As I only twice experimented on more than a single species in a genus, I cannot say whether the crossed offspring of the several species within the same genus differ in their degree of superiority over their self-fertilised brethren; but I should expect that this would often prove to be the case from what was observed with the two species of Lobelia and with the individuals of the same species of Nicotiana. The species belonging to distinct genera in the same family certainly differ in this respect. The effects of cross- and self-fertilisation may be confined either to the growth or to the fertility of the offspring, but generally extends to both qualities. There does not seem to exist any close correspondence between the degree to which their offspring profit by this process; but we may easily err on this head, as there are two means for ensuring cross-fertilisation which are not externally perceptible, namely, self-sterility and the prepotent fertilising influence of pollen from another individual. Lastly, it has been shown in a former chapter that the effect produced by cross and self-fertilisation on the fertility of the parent-plants does not always correspond with that produced on the height, vigour, and fertility of their offspring. The same remark applies to crossed and self-fertilised seedlings when these are used as the parent-plants. This want of correspondence probably depends, at least in part, on the number of seeds produced being chiefly determined by the number of the pollen-tubes which reach the ovules, and this will be governed by the reaction between the pollen and the stigmatic secretion or tissues; whereas the growth and constitutional vigour of the offspring will be chiefly determined, not only by the number of pollen-tubes reaching the ovules, but by the nature of the reaction between the contents of the pollen-grains and ovules.

There are two other important conclusions which may be deduced from my observations: firstly, that the advantages of cross-fertilisation do not follow from some mysterious virtue in the mere union of two distinct individuals, but from such individuals having been subjected during previous generations to different conditions, or to their having varied in a manner commonly called spontaneous, so that in either case their sexual elements have been in some degree differentiated. And secondly, that the injury from self-fertilisation follows from the want of such differentiation in the sexual elements. These two propositions are fully established by my experiments. Thus, when plants of the Ipomoea and of the Mimulus, which had been self-fertilised for the seven previous generations and had been kept all the time under the same conditions, were intercrossed one with another, the offspring did not profit in the least by the cross. Mimulus offers another instructive case, showing that the benefit of a cross depends on the previous treatment of the progenitors: plants which had been self-fertilised for the eight previous generations were crossed with plants which had been intercrossed for the same number of generations, all having been kept under the same conditions as far as possible; seedlings from this cross were grown in competition with others derived from the same self-fertilised mother-plant crossed by a fresh stock; and the latter seedlings were to the former in height as 100 to 52, and in fertility as 100 to 4. An exactly parallel experiment was tried on Dianthus, with this difference, that the plants had been self-fertilised only for the three previous generations, and the result was similar though not so strongly marked. The foregoing two cases of the offspring of Ipomoea and Eschscholtzia, derived from a cross with a fresh stock, being as much superior to the intercrossed plants of the old stock, as these latter were to the self-fertilised offspring, strongly supports the same conclusion. A cross with a fresh stock or with another variety seems to be always highly beneficial, whether or not the mother-plants have been intercrossed or self-fertilised for several previous generations. The fact that a cross between two flowers on the same plant does no good or very little good, is likewise a strong corroboration of our conclusion; for the sexual elements in the flowers on the same plant can rarely have been differentiated, though this is possible, as flower-buds are in one sense distinct individuals, sometimes varying and differing from one another in structure or constitution. Thus the proposition that the benefit from cross-fertilisation depends on the plants which are crossed having been subjected during previous generations to somewhat different conditions, or to their having varied from some unknown cause as if they had been thus subjected, is securely fortified on all sides.

Before proceeding any further, the view which has been maintained by several physiologists must be noticed, namely, that all the evils from breeding animals too closely, and no doubt, as they would say, from the self-fertilisation of plants, is the result of the increase of some morbid tendency or weakness of constitution common to the closely related parents, or to the two sexes of hermaphrodite plants. Undoubtedly injury has often thus resulted; but it is a vain attempt to extend this view to the numerous cases given in my Tables. It should be remembered that the same mother-plant was both self-fertilised and crossed, so that if she had been unhealthy she would have transmitted half her morbid tendencies to her crossed offspring. But plants appearing perfectly healthy, some of them growing wild, or the immediate offspring of wild plants, or vigorous common garden-plants, were selected for experiment. Considering the number of species which were tried, it is nothing less than absurd to suppose that in all these cases the mother-plants, though not appearing in any way diseased, were weak or unhealthy in so peculiar a manner that their self-fertilised seedlings, many hundreds in number, were rendered inferior in height, weight, constitutional vigour and fertility to their crossed offspring. Moreover, this belief cannot be extended to the strongly marked advantages which invariably follow, as far as my experience serves, from intercrossing the individuals of the same variety or of distinct varieties, if these have been subjected during some generations to different conditions.

It is obvious that the exposure of two sets of plants during several generations to different conditions can lead to no beneficial results, as far as crossing is concerned, unless their sexual elements are thus affected. That every organism is acted on to a certain extent by a change in its environment, will not, I presume, be disputed. It is hardly necessary to advance evidence on this head; we can perceive the difference between individual plants of the same species which have grown in somewhat more shady or sunny, dry or damp places. Plants which have been propagated for some generations under different climates or at different seasons of the year transmit different constitutions to their seedlings. Under such circumstances, the chemical constitution of their fluids and the nature of their tissues are often modified. (12/8. Numerous cases together with references are given in my ‘Variation under Domestication’ chapter 23 2nd edition volume 2 page 264. With respect to animals, Mr. Brackenridge ‘A Contribution to the Theory of Diathesis’ Edinburgh 1869, has well shown that the different organs of animals are excited into different degrees of activity by differences of temperature and food, and become to a certain extent adapted to them.) Many other such facts could be adduced. In short, every alteration in the function of a part is probably connected with some corresponding, though often quite imperceptible change in structure or composition.

Whatever affects an organism in any way, likewise tends to act on its sexual elements. We see this in the inheritance of newly acquired modifications, such as those from the increased use or disuse of a part, and even from mutilations if followed by disease. (12/9. ‘Variation under Domestication’ chapter 12 2nd edition volume 1 page 466.) We have abundant evidence how susceptible the reproductive system is to changed conditions, in the many instances of animals rendered sterile by confinement; so that they will not unite, or if they unite do not produce offspring, though the confinement may be far from close; and of plants rendered sterile by cultivation. But hardly any cases afford more striking evidence how powerfully a change in the conditions of life acts on the sexual elements, than those already given, of plants which are completely self-sterile in one country, and when brought to another, yield, even in the first generation, a fair supply of self-fertilised seeds.

But it may be said, granting that changed conditions act on the sexual elements, how can two or more plants growing close together, either in their native country or in a garden, be differently acted on, inasmuch as they appear to be exposed to exactly the same conditions? Although this question has been already considered, it deserves further consideration under several points of view. In my experiments with Digitalis purpurea, some flowers on a wild plant were self-fertilised, and others were crossed with pollen from another plant growing within two or three feet’s distance. The crossed and self-fertilised plants raised from the seeds thus obtained, produced flower-stems in number as 100 to 47, and in average height as 100 to 70. Therefore the cross between these two plants was highly beneficial; but how could their sexual elements have been differentiated by exposure to different conditions? If the progenitors of the two plants had lived on the same spot during the last score of generations, and had never been crossed with any plant beyond the distance of a few feet, in all probability their offspring would have been reduced to the same state as some of the plants in my experiments,—such as the intercrossed plants of the ninth generation of Ipomoea,—or the self-fertilised plants of the eighth generation of Mimulus,—or the offspring from flowers on the same plant,—and in this case a cross between the two plants of Digitalis would have done no good. But seeds are often widely dispersed by natural means, and one of the above two plants or one of their ancestors may have come from a distance, from a more shady or sunny, dry or moist place, or from a different kind of soil containing other organic or inorganic matter. We know from the admirable researches of Messrs. Lawes and Gilbert that different plants require and consume very different amounts of inorganic matter. (12/10. ‘Journal of the Royal Agricultural Society of England’ volume 24 part 1.) But the amount in the soil would probably not make so great a difference to the several individuals of any particular species as might at first be expected; for the surrounding species with different requirements would tend, from existing in greater or lesser numbers, to keep each species in a sort of equilibrium, with respect to what it could obtain from the soil. So it would be even with respect to moisture during dry seasons; and how powerful is the influence of a little more or less moisture in the soil on the presence and distribution of plants, is often well shown in old pasture fields which still retain traces of former ridges and furrows. Nevertheless, as the proportional numbers of the surrounding plants in two neighbouring places is rarely exactly the same, the individuals of the same species will be subjected to somewhat different conditions with respect to what they can absorb from the soil. It is surprising how the free growth of one set of plants affects others growing mingled with them; I allowed the plants on rather more than a square yard of turf which had been closely mown for several years, to grow up; and nine species out of twenty were thus exterminated; but whether this was altogether due to the kinds which grew up robbing the others of nutriment, I do not know.

Seeds often lie dormant for several years in the ground, and germinate when brought near the surface by any means, as by burrowing animals. They would probably be affected by the mere circumstance of having long lain dormant; for gardeners believe that the production of double flowers and of fruit is thus influenced. Seeds, moreover, which were matured during different seasons, will have been subjected during the whole course of their development to different degrees of heat and moisture.

It was shown in the last chapter that pollen is often carried by insects to a considerable distance from plant to plant. Therefore one of the parents or ancestors of our two plants of Digitalis may have been crossed by a distant plant growing under somewhat different conditions. Plants thus crossed often produce an unusually large number of seeds; a striking instance of this fact is afforded by the Bignonia, previously mentioned, which was fertilised by Fritz Muller with pollen from some adjoining plants and set hardly any seed, but when fertilised with pollen from a distant plant, was highly fertile. Seedlings from a cross of this kind grow with great vigour, and transmit their vigour to their descendants. These, therefore, in the struggle for life, will generally beat and exterminate the seedlings from plants which have long grown near together under the same conditions, and will thus tend to spread.

When two varieties which present well-marked differences are crossed, their descendants in the later generations differ greatly from one another in external characters; and this is due to the augmentation or obliteration of some of these characters, and to the reappearance of former ones through reversion; and so it will be, as we may feel almost sure, with any slight differences in the constitution of their sexual elements. Anyhow, my experiments indicate that crossing plants which have been long subjected to almost though not quite the same conditions, is the most powerful of all the means for retaining some degree of differentiation in the sexual elements, as shown by the superiority in the later generations of the intercrossed over the self-fertilised seedlings. Nevertheless, the continued intercrossing of plants thus treated does tend to obliterate such differentiation, as may be inferred from the lessened benefit derived from intercrossing such plants, in comparison with that from a cross with a fresh stock. It seems probable, as I may add, that seeds have acquired their endless curious adaptations for wide dissemination, not only that the seedlings would thus be enabled to find new and fitting homes, but that the individuals which have been long subjected to the same conditions should occasionally intercross with a fresh stock. (12/11. See Professor Hildebrand’s excellent treatise ‘Verbreitungsmittel der Pflanzen’ 1873.)

From the foregoing several considerations we may, I think, conclude that in the above case of the Digitalis, and even in that of plants which have grown for thousands of generations in the same district, as must often have occurred with species having a much restricted range, we are apt to over-estimate the degree to which the individuals have been subjected to absolutely the same conditions. There is at least no difficulty in believing that such plants have been subjected to sufficiently distinct conditions to differentiate their sexual elements; for we know that a plant propagated for some generations in another garden in the same district serves as a fresh stock and has high fertilising powers. The curious cases of plants which can fertilise and be fertilised by any other individual of the same species, but are altogether sterile with their own pollen, become intelligible, if the view here propounded is correct, namely, that the individuals of the same species growing in a state of nature near together, have not really been subjected during several previous generations to quite the same conditions.

Some naturalists assume that there is an innate tendency in all beings to vary and to advance in organisation, independently of external agencies; and they would, I presume, thus explain the slight differences which distinguish all the individuals of the same species both in external characters and in constitution, as well as the greater differences in both respects between nearly allied varieties. No two individuals can be found quite alike; thus if we sow a number of seeds from the same capsule under as nearly as possible the same conditions, they germinate at different rates and grow more or less vigorously. They resist cold and other unfavourable conditions differently. They would in all probability, as we know to be the case with animals of the same species, be somewhat differently acted on by the same poison, or by the same disease. They have different powers of transmitting their characters to their offspring; and many analogous facts could be given. (12/12. Vilmorin as quoted by Verlot ‘Des Varieties’ pages 32, 38, 39.) Now, if it were true that plants growing near together in a state of nature had been subjected during many previous generations to absolutely the same conditions, such differences as those just specified would be quite inexplicable; but they are to a certain extent intelligible in accordance with the views just advanced.

As most of the plants on which I experimented were grown in my garden or in pots under glass, a few words must be added on the conditions to which they were exposed, as well as on the effects of cultivation. When a species is first brought under culture, it may or may not be subjected to a change of climate, but it is always grown in ground broken up, and more or less manured; it is also saved from competition with other plants. The paramount importance of this latter circumstance is proved by the multitude of species which flourish and multiply in a garden, but cannot exist unless they are protected from other plants. When thus saved from competition they are able to get whatever they require from the soil, probably often in excess; and they are thus subjected to a great change of conditions. It is probably in chief part owing to this cause that all plants with rare exceptions vary after being cultivated for some generations. The individuals which have already begun to vary will intercross one with another by the aid of insects; and this accounts for the extreme diversity of character which many of our long cultivated plants exhibit. But it should be observed that the result will be largely determined by the degree of their variability and by the frequency of the intercrosses; for if a plant varies very little, like most species in a state of nature, frequent intercrosses tend to give uniformity of character to it.

I have attempted to show that with plants growing naturally in the same district, except in the unusual case of each individual being surrounded by exactly the same proportional numbers of other species having certain powers of absorption, each will be subjected to slightly different conditions. This does not apply to the individuals of the same species when cultivated in cleared ground in the same garden. But if their flowers are visited by insects, they will intercross; and this will give to their sexual elements during a considerable number of generations a sufficient amount of differentiation for a cross to be beneficial. Moreover, seeds are frequently exchanged or procured from other gardens having a different kind of soil; and the individuals of the same cultivated species will thus be subjected to a change of conditions. If the flowers are not visited by our native insects, or very rarely so, as in the case of the common and sweet pea, and apparently in that of the tobacco when kept in a hothouse, any differentiation in the sexual elements caused by intercrosses will tend to disappear. This appears to have occurred with the plants just mentioned, for they were not benefited by being crossed one with another, though they were greatly benefited by a cross with a fresh stock.

I have been led to the views just advanced with respect to the causes of the differentiation of the sexual elements and of the variability of our garden plants, by the results of my various experiments, and more especially by the four cases in which extremely inconstant species, after having been self-fertilised and grown under closely similar conditions for several generations, produced flowers of a uniform and constant tint. These conditions were nearly the same as those to which plants, growing in a garden clear of weeds, are subjected, if they are propagated by self-fertilised seeds on the same spot. The plants in pots were, however, exposed to less severe fluctuations of climate than those out of doors; but their conditions, though closely uniform for all the individuals of the same generation, differed somewhat in the successive generations. Now, under these circumstances, the sexual elements of the plants which were intercrossed in each generation retained sufficient differentiation during several years for their offspring to be superior to the self-fertilised, but this superiority gradually and manifestly decreased, as was shown by the difference in the result between a cross with one of the intercrossed plants and with a fresh stock. These intercrossed plants tended also in a few cases to become somewhat more uniform in some of their external characters than they were at first. With respect to the plants which were self-fertilised in each generation, their sexual elements apparently lost, after some years, all differentiation, for a cross between them did no more good than a cross between the flowers on the same plant. But it is a still more remarkable fact, that although the seedlings of Mimulus, Ipomoea, Dianthus, and Petunia which were first raised, varied excessively in the colour of their flowers, their offspring, after being self-fertilised and grown under uniform conditions for some generations, bore flowers almost as uniform in tint as those on a natural species. In one case also the plants themselves became remarkably uniform in height.

The conclusion that the advantages of a cross depend altogether on the differentiation of the sexual elements, harmonises perfectly with the fact that an occasional and slight change in the conditions of life is beneficial to all plants and animals. (12/13. I have given sufficient evidence on this head in my ‘Variation under Domestication’ chapter 18 volume 2 2nd edition page 127.) But the offspring from a cross between organisms which have been exposed to different conditions, profit in an incomparably higher degree than do young or old beings from a mere change in the conditions. In this latter case we never see anything like the effect which generally follows from a cross with another individual, especially from a cross with a fresh stock. This might, perhaps, have been expected, for the blending together of the sexual elements of two differentiated beings will affect the whole constitution at a very early period of life, whilst the organisation is highly flexible. We have, moreover, reason to believe that changed conditions generally act differently on the several parts or organs of the same individual (12/14. See, for instance, Brackenridge ‘Theory of Diathesis’ Edinburgh 1869.); and if we may further believe that these now slightly differentiated parts react on one another, the harmony between the beneficial effects on the individual due to changed conditions, and those due to the interaction of differentiated sexual elements, becomes still closer.

That wonderfully accurate observer, Sprengel, who first showed how important a part insects play in the fertilisation of flowers, called his book ‘The Secret of Nature Displayed;’ yet he only occasionally saw that the object for which so many curious and beautiful adaptations have been acquired, was the cross-fertilisation of distinct plants; and he knew nothing of the benefits which the offspring thus receive in growth, vigour, and fertility. But the veil of secrecy is as yet far from lifted; nor will it be, until we can say why it is beneficial that the sexual elements should be differentiated to a certain extent, and why, if the differentiation be carried still further, injury follows. It is an extraordinary fact that with many species, flowers fertilised with their own pollen are either absolutely or in some degree sterile; if fertilised with pollen from another flower on the same plant, they are sometimes, though rarely, a little more fertile; if fertilised with pollen from another individual or variety of the same species, they are fully fertile; but if with pollen from a distinct species, they are sterile in all possible degrees, until utter sterility is reached. We thus have a long series with absolute sterility at the two ends;—at one end due to the sexual elements not having been sufficiently differentiated, and at the other end to their having been differentiated in too great a degree, or in some peculiar manner.

The fertilisation of one of the higher plants depends, in the first place, on the mutual action of the pollen-grains and the stigmatic secretion or tissues, and afterwards on the mutual action of the contents of the pollen-grains and ovules. Both actions, judging from the increased fertility of the parent-plants and from the increased powers of growth in the offspring, are favoured by some degree of differentiation in the elements which interact and unite so as to form a new being. Here we have some analogy with chemical affinity or attraction, which comes into play only between atoms or molecules of a different nature. As Professor Miller remarks: “Generally speaking, the greater the difference in the properties of two bodies, the more intense is their tendency to mutual chemical action...But between bodies of a similar character the tendency to unite is feeble.” (12/15. ‘Elements of Chemistry’ 4th edition 1867 part 1 page 11. Dr. Frankland informs me that similar views with respect to chemical affinity are generally accepted by chemists.) This latter proposition accords well with the feeble effects of a plant’s own pollen on the fertility of the mother-plant and on the growth of the offspring; and the former proposition accords well with the powerful influence in both ways of pollen from an individual which has been differentiated by exposure to changed conditions, or by so-called spontaneous variation. But the analogy fails when we turn to the negative or weak effects of pollen from one species on a distinct species; for although some substances which are extremely dissimilar, for instance, carbon and chlorine, have a very feeble affinity for each other, yet it cannot be said that the weakness of the affinity depends in such cases on the extent to which the substances differ. It is not known why a certain amount of differentiation is necessary or favourable for the chemical affinity or union of two substances, any more than for the fertilisation or union of two organisms.

Mr. Herbert Spencer has discussed this whole subject at great length, and after stating that all the forces throughout nature tend towards an equilibrium, remarks, “that the need of this union of sperm-cell and germ-ccell is the need for overthrowing this equilibrium and re-establishing active molecular change in the detached germ—a result which is probably effected by mixing the slightly-different physiological units of slightly-different individuals.” (12/16. ‘Principles of Biology’ volume 1 page 274 1864. In my ‘Origin of Species’ published in 1859, I spoke of the good effects from slight changes in the condition of life and from cross-fertilisation, and of the evil effects from great changes in the conditions and from crossing widely distinct forms (i.e., species), as a series of facts “connected together by some common but unknown bond, which is essentially related to the principle of life.”) But we must not allow this highly generalised view, or the analogy of chemical affinity, to conceal from us our ignorance. We do not know what is the nature or degree of the differentiation in the sexual elements which is favourable for union, and what is injurious for union, as in the case of distinct species. We cannot say why the individuals of certain species profit greatly, and others very little by being crossed. There are some few species which have been self-fertilised for a vast number of generations, and yet are vigorous enough to compete successfully with a host of surrounding plants. We can form no conception why the advantage from a cross is sometimes directed exclusively to the vegetative system, and sometimes to the reproductive system, but commonly to both. It is equally inconceivable why some individuals of the same species should be sterile, whilst others are fully fertile with their own pollen; why a change of climate should either lessen or increase the sterility of self-sterile species; and why the individuals of some species should be even more fertile with pollen from a distinct species than with their own pollen. And so it is with many other facts, which are so obscure that we stand in awe before the mystery of life.

Under a practical point of view, agriculturists and horticulturists may learn something from the conclusions at which we have arrived. Firstly, we see that the injury from the close breeding of animals and from the self-fertilisation of plants, does not necessarily depend on any tendency to disease or weakness of constitution common to the related parents, and only indirectly on their relationship, in so far as they are apt to resemble each other in all respects, including their sexual nature. And, secondly, that the advantages of cross-fertilisation depend on the sexual elements of the parents having become in some degree differentiated by the exposure of their progenitors to different conditions, or from their having intercrossed with individuals thus exposed, or, lastly, from what we call in our ignorance spontaneous variation. He therefore who wishes to pair closely related animals ought to keep them under conditions as different as possible. Some few breeders, guided by their keen powers of observation, have acted on this principle, and have kept stocks of the same animals at two or more distant and differently situated farms. They have then coupled the individuals from these farms with excellent results. (12/17. ‘Variation of Animals and Plants under Domestication’ chapter 17 2nd edition volume 2 pages 98, 105.) This same plan is also unconsciously followed whenever the males, reared in one place, are let out for propagation to breeders in other places. As some kinds of plants suffer much more from self-fertilisation than do others, so it probably is with animals from too close interbreeding. The effects of close interbreeding on animals, judging again from plants, would be deterioration in general vigour, including fertility, with no necessary loss of excellence of form; and this seems to be the usual result.

It is a common practice with horticulturists to obtain seeds from another place having a very different soil, so as to avoid raising plants for a long succession of generations under the same conditions; but with all the species which freely intercross by aid of insects or the wind, it would be an incomparably better plan to obtain seeds of the required variety, which had been raised for some generations under as different conditions as possible, and sow them in alternate rows with seeds matured in the old garden. The two stocks would then intercross, with a thorough blending of their whole organisations, and with no loss of purity to the variety; and this would yield far more favourable results than a mere exchange of seeds. We have seen in my experiments how wonderfully the offspring profited in height, weight, hardiness, and fertility, by crosses of this kind. For instance, plants of Ipomoea thus crossed were to the intercrossed plants of the same stock, with which they grew in competition, as 100 to 78 in height, and as 100 to 51 in fertility; and plants of Eschscholtzia similarly compared were as 100 to 45 in fertility. In comparison with self-fertilised plants the results are still more striking; thus cabbages derived from a cross with a fresh stock were to the self-fertilised as 100 to 22 in weight.

Florists may learn from the four cases which have been fully described, that they have the power of fixing each fleeting variety of colour, if they will fertilise the flowers of the desired kind with their own pollen for half-a-dozen generations, and grow the seedlings under the same conditions. But a cross with any other individual of the same variety must be carefully prevented, as each has its own peculiar constitution. After a dozen generations of self-fertilisation, it is probable that the new variety would remain constant even if grown under somewhat different conditions; and there would no longer be any necessity to guard against intercrosses between the individuals of the same variety.

With respect to mankind, my son George has endeavoured to discover by a statistical investigation whether the marriages of first cousins are at all injurious, although this is a degree of relationship which would not be objected to in our domestic animals; and he has come to the conclusion from his own researches and those of Dr. Mitchell that the evidence as to any evil thus caused is conflicting, but on the whole points to its being very small. From the facts given in this volume we may infer that with mankind the marriages of nearly related persons, some of whose parents and ancestors had lived under very different conditions, would be much less injurious than that of persons who had always lived in the same place and followed the same habits of life. Nor can I see reason to doubt that the widely different habits of life of men and women in civilised nations, especially amongst the upper classes, would tend to counterbalance any evil from marriages between healthy and somewhat closely related persons.

Under a theoretical point of view it is some gain to science to know that numberless structures in hermaphrodite plants, and probably in hermaphrodite animals, are special adaptations for securing an occasional cross between two individuals; and that the advantages from such a cross depend altogether on the beings which are united, or their progenitors, having had their sexual elements somewhat differentiated, so that the embryo is benefited in the same manner as is a mature plant or animal by a slight change in its conditions of life, although in a much higher degree.

Another and more important result may be deduced from my observations. Eggs and seeds are highly serviceable as a means of dissemination, but we now know that fertile eggs can be produced without the aid of the male. There are also many other methods by which organisms can be propagated asexually. Why then have the two sexes been developed, and why do males exist which cannot themselves produce offspring? The answer lies, as I can hardly doubt, in the great good which is derived from the fusion of two somewhat differentiated individuals; and with the exception of the lowest organisms this is possible only by means of the sexual elements, these consisting of cells separated from the body, containing the germs of every part, and capable of being fused completely together.

It has been shown in the present volume that the offspring from the union of two distinct individuals, especially if their progenitors have been subjected to very different conditions, have an immense advantage in height, weight, constitutional vigour and fertility over the self-fertilised offspring from one of the same parents. And this fact is amply sufficient to account for the development of the sexual elements, that is, for the genesis of the two sexes.

It is a different question why the two sexes are sometimes combined in the same individual and are sometimes separated. As with many of the lowest plants and animals the conjugation of two individuals which are either quite similar or in some degree different, is a common phenomenon, it seems probable, as remarked in the last chapter, that the sexes were primordially separate. The individual which receives the contents of the other, may be called the female; and the other, which is often smaller and more locomotive, may be called the male; though these sexual names ought hardly to be applied as long as the whole contents of the two forms are blended into one. The object gained by the two sexes becoming united in the same hermaphrodite form probably is to allow of occasional or frequent self-fertilisation, so as to ensure the propagation of the species, more especially in the case of organisms affixed for life to the same spot. There does not seem to be any great difficulty in understanding how an organism, formed by the conjugation of two individuals which represented the two incipient sexes, might have given rise by budding first to a monoecious and then to an hermaphrodite form; and in the case of animals even without budding to an hermaphrodite form, for the bilateral structure of animals perhaps indicates that they were aboriginally formed by the fusion of two individuals.

It is a more difficult problem why some plants and apparently all the higher animals, after becoming hermaphrodites, have since had their sexes re-separated. This separation has been attributed by some naturalists to the advantages which follow from a division of physiological labour. The principle is intelligible when the same organ has to perform at the same time diverse functions; but it is not obvious why the male and female glands when placed in different parts of the same compound or simple individual, should not perform their functions equally well as when placed in two distinct individuals. In some instances the sexes may have been re-separated for the sake of preventing too frequent self-fertilisation; but this explanation does not seem probable, as the same end might have been gained by other and simpler means, for instance dichogamy. It may be that the production of the male and female reproductive elements and the maturation of the ovules was too great a strain and expenditure of vital force for a single individual to withstand, if endowed with a highly complex organisation; and that at the same time there was no need for all the individuals to produce young, and consequently that no injury, on the contrary, good resulted from half of them, or the males, failing to produce offspring.

There is another subject on which some light is thrown by the facts given in this volume, namely, hybridisation. It is notorious that when distinct species of plants are crossed, they produce with the rarest exceptions fewer seeds than the normal number. This unproductiveness varies in different species up to sterility so complete that not even an empty capsule is formed; and all experimentalists have found that it is much influenced by the conditions to which the crossed species are subjected. The pollen of each species is strongly prepotent over that of any other species, so that if a plant’s own pollen is placed on the stigma some time after foreign pollen has been applied to it, any effect from the latter is quite obliterated. It is also notorious that not only the parent species, but the hybrids raised from them are more or less sterile; and that their pollen is often in a more or less aborted condition. The degree of sterility of various hybrids does not always strictly correspond with the degree of difficulty in uniting the parent forms. When hybrids are capable of breeding inter se, their descendants are more or less sterile, and they often become still more sterile in the later generations; but then close interbreeding has hitherto been practised in all such cases. The more sterile hybrids are sometimes much dwarfed in stature, and have a feeble constitution. Other facts could be given, but these will suffice for us. Naturalists formerly attributed all these results to the difference between species being fundamentally distinct from that between the varieties of the same species; and this is still the verdict of some naturalists.

The results of my experiments in self-fertilising and cross-fertilising the individuals or the varieties of the same species, are strikingly analogous with those just given, though in a reversed manner. With the majority of species flowers fertilised with their own pollen yield fewer, sometimes much fewer seeds, than those fertilised with pollen from another individual or variety. Some self-fertilised flowers are absolutely sterile; but the degree of their sterility is largely determined by the conditions to which the parent plants have been exposed, as was well exemplified in the case of Eschscholtzia and Abutilon. The effects of pollen from the same plant are obliterated by the prepotent influence of pollen from another individual or variety, although the latter may have been placed on the stigma some hours afterwards. The offspring from self-fertilised flowers are themselves more or less sterile, sometimes highly sterile, and their pollen is sometimes in an imperfect condition; but I have not met with any case of complete sterility in self-fertilised seedlings, as is so common with hybrids. The degree of their sterility does not correspond with that of the parent-plants when first self-fertilised. The offspring of self-fertilised plants suffer in stature, weight, and constitutional vigour more frequently and in a greater degree than do the hybrid offspring of the greater number of crossed species. Decreased height is transmitted to the next generation, but I did not ascertain whether this applies to decreased fertility.

I have elsewhere shown that by uniting in various ways dimorphic or trimorphic heterostyled plants, which belong to the same undoubted species, we get another series of results exactly parallel with those from crossing distinct species. (12/18. ‘Journal of the Linnean Society Botany’ volume 10 1867 page 393.) Plants illegitimately fertilised with pollen from a distinct plant belonging to the same form, yield fewer, often much fewer seeds, than they do when legitimately fertilised with pollen from a plant belonging to a distinct form. They sometimes yield no seed, not even an empty capsule, like a species fertilised with pollen from a distinct genus. The degree of sterility is much affected by the conditions to which the plants have been subjected. (12/19. ‘Journal of the Linnean Society Botany’ volume 8 1864 page 180.) The pollen from a distinct form is strongly prepotent over that from the same form, although the former may have been placed on the stigma many hours afterwards. The offspring from a union between plants of the same form are more or less sterile, like hybrids, and have their pollen in a more or less aborted condition; and some of the seedlings are as barren and as dwarfed as the most barren hybrid. They also resemble hybrids in several other respects, which need not here be specified in detail,—such as their sterility not corresponding in degree with that of the parent plants,—the unequal sterility of the latter, when reciprocally united,—and the varying sterility of the seedlings raised from the same seed-capsule.

We thus have two grand classes of cases giving results which correspond in the most striking manner with those which follow from the crossing of so-called true and distinct species. With respect to the difference between seedlings raised from cross and self-fertilised flowers, there is good evidence that this depends altogether on whether the sexual elements of the parents have been sufficiently differentiated, by exposure to different conditions or by spontaneous variation. It is probable that nearly the same conclusion may be extended to heterostyled plants; but this is not the proper place for discussing the origin of the long-styled, short-styled and mid-styled forms, which all belong to the same species as certainly as do the two sexes of the same species. We have therefore no right to maintain that the sterility of species when first crossed and of their hybrid offspring, is determined by some cause fundamentally different from that which determines the sterility of the individuals both of ordinary and of heterostyled plants when united in various ways. Nevertheless, I am aware that it will take many years to remove this prejudice.

There is hardly anything more wonderful in nature than the sensitiveness of the sexual elements to external influences, and the delicacy of their affinities. We see this in slight changes in the conditions of life being favourable to the fertility and vigour of the parents, while certain other and not great changes cause them to be quite sterile without any apparent injury to their health. We see how sensitive the sexual elements of those plants must be, which are completely sterile with their own pollen, but are fertile with that of any other individual of the same species. Such plants become either more or less self-sterile if subjected to changed conditions, although the change may be far from great. The ovules of a heterostyled trimorphic plant are affected very differently by pollen from the three sets of stamens belonging to the same species. With ordinary plants the pollen of another variety or merely of another individual of the same variety is often strongly prepotent over its own pollen, when both are placed at the same time on the same stigma. In those great families of plants containing many thousand allied species, the stigma of each distinguishes with unerring certainty its own pollen from that of every other species.

There can be no doubt that the sterility of distinct species when first crossed, and of their hybrid offspring, depends exclusively on the nature or affinities of their sexual elements. We see this in the want of any close correspondence between the degree of sterility and the amount of external difference in the species which are crossed; and still more clearly in the wide difference in the results of crossing reciprocally the same two species;—that is, when species A is crossed with pollen from B, and then B is crossed with pollen from A. Bearing in mind what has just been said on the extreme sensitiveness and delicate affinities of the reproductive system, why should we feel any surprise at the sexual elements of those forms, which we call species, having been differentiated in such a manner that they are incapable or only feebly capable of acting on one another? We know that species have generally lived under the same conditions, and have retained their own proper characters, for a much longer period than varieties. Long-continued domestication eliminates, as I have shown in my ‘Variation under Domestication,’ the mutual sterility which distinct species lately taken from a state of nature almost always exhibit when intercrossed; and we can thus understand the fact that the most different domestic races of animals are not mutually sterile. But whether this holds good with cultivated varieties of plants is not known, though some facts indicate that it does. The elimination of sterility through long-continued domestication may probably be attributed to the varying conditions to which our domestic animals have been subjected; and no doubt it is owing to this same cause that they withstand great and sudden changes in their conditions of life with far less loss of fertility than do natural species. From these several considerations it appears probable that the difference in the affinities of the sexual elements of distinct species, on which their mutual incapacity for breeding together depends, is caused by their having been habituated for a very long period each to its own conditions, and to the sexual elements having thus acquired firmly fixed affinities. However this may be, with the two great classes of cases before us, namely, those relating to the self-fertilisation and cross-fertilisation of the individuals of the same species, and those relating to the illegitimate and legitimate unions of heterostyled plants, it is quite unjustifiable to assume that the sterility of species when first crossed and of their hybrid offspring, indicates that they differ in some fundamental manner from the varieties or individuals of the same species.

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