The Effects of Cross & Self-Fertilisation in the Vegetable Kingdom Part 62

As most of the plants on which I experimented were grown in my garden or in pots under gla.s.s, 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 compet.i.tion with other plants. The paramount importance of this latter circ.u.mstance is proved by the mult.i.tude of species which flourish and multiply in a garden, but cannot exist unless they are protected from other plants. When thus saved from compet.i.tion 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 s.e.xual 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 s.e.xual 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 s.e.xual 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 circ.u.mstances, the s.e.xual 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 s.e.xual 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 s.e.xual 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 s.e.xual elements of two differentiated beings will affect the whole const.i.tution 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 s.e.xual 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 s.e.xual 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 s.e.xual 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 a.n.a.logy 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 a.n.a.logy 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-establis.h.i.+ng 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 a.n.a.logy of chemical affinity, to conceal from us our ignorance. We do not know what is the nature or degree of the differentiation in the s.e.xual 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 const.i.tution common to the related parents, and only indirectly on their relations.h.i.+p, in so far as they are apt to resemble each other in all respects, including their s.e.xual nature. And, secondly, that the advantages of cross-fertilisation depend on the s.e.xual 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 compet.i.tion, 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 const.i.tution. 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 relations.h.i.+p 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. Mitch.e.l.l 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 cla.s.ses, 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 s.e.xual 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 as.e.xually. Why then have the two s.e.xes 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 s.e.xual 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, const.i.tutional 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 s.e.xual elements, that is, for the genesis of the two s.e.xes.

It is a different question why the two s.e.xes 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 s.e.xes 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 s.e.xual 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 s.e.xes 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 s.e.xes, 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 s.e.xes 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 s.e.xes 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. .h.i.therto been practised in all such cases. The more sterile hybrids are sometimes much dwarfed in stature, and have a feeble const.i.tution. 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 a.n.a.logous 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 const.i.tutional 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 cla.s.ses 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 s.e.xual 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 s.e.xes 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 s.e.xual 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 s.e.xual 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 s.e.xual 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 s.e.xual 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 s.e.xual 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 s.e.xual elements having thus acquired firmly fixed affinities. However this may be, with the two great cla.s.ses 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 a.s.sume 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.