1902 Encyclopedia > Hybridism

Hybridism




HYBRIDISM. The Latin word hybrida, or hibrida, a hybrid or mongrel, is commonly derived from the Greek v^pis, an insult or outrage, with special reference to lust; hence an outrage on nature, a mongrel.

As a general rule animals or plants belonging to distinct species are not able, when crossed with each other, to produce offspring. There are, however, innumerable excep-tions to this rule; and hybridism is the word employed to denote these exceptions. It is an abstract term which signifies the more or less fertile crossing of distinct species. In scientific usage, the term " hybrid" is exclusively reserved to denote the result of a fertile cross between two distinct species, while the term " mongrel " is the one which is as exclusively reserved to denote the result of a fertile cross between two varieties of the same species.

Until recently the interest attaching to hybridism was almost entirely of a practical nature, and arose from the fact, which is of considerable importance in horticulture, that hybrids are often found to present characters somewhat different from those of either parent species. But of late years the subject has acquired a high degree of scientific interest in relation to the theory of descent. On this account it has been so carefully and thoroughly treated by Mr Darwin that a brief exposition of its main facts and principles must necessarily be little more than acondensation of his already closely packed material.

Looking first to the general facts and principles of hybridism, apart from their bearing upon the theory of descent, the following may be regarded as the most important :—

1. The laws governing the production of hybrids are identical, or nearly identical, in the animal and vegetable kingdoms.

2. The sterility which so generally attends the crossing of two specific forms is to be distinguished as of two kinds, which, although often confounded by naturalists, are in reality quite distinct. For the sterility may obtain between the two parent species when first crossed, or it may first assert itself in their hybrid progeny. In the latter case the hybrids, although possibly produced without any appearance of infertility on the part of their parent species, nevertheless prove more or less infertile among themselves, and also with members of either parent species,

3. The degree of both kinds of infertility varies in the case of different species, and in that of their hybrid progeny, from absolute sterility up to complete fertility. Thus, to take the case of plants, " when pollen from a plant of one family is placed on the stigma of a plant of a distinct family, it exert3 no more influence than so much inorganic dust. From this absolute zero of fertility, the pollen of different species, applied to the stigma of some one species of the same genus, yields a perfect gradation in the number of seeds produced, up to nearly complete, or even quite complete, fertility; so, in hybrids themselves, there are some which never have produced, and probably never would produce, even with the pollen of the purparents, a single fertile seed; but in some of these cases a first trace of fertility may be detected, by the pollen of one of the pure parent species causing the flower of the hybrid to wither earlier than it otherwise would have done; and the early withering of the flower is well known to be a sign of incipient fertilization. From this extreme degree of sterility we have self- fertilized hybrids producing a greater and greater number of seeds up to perfect fertility."

4. Although there is, as a rule, a certain parallelism, there is no fixed relation between the degree of sterility manifested by the parent species when crossed and that which is manifested by their hybrid progeny. There are many cases in which two pure species can be crossed with unusual facility, while the resulting hybrids are remark ably sterile; and, contrariwise, there are species which can only be crossed with extreme difficulty, though the hybrids, when produced, are very fertile. Even within the limits of the same genus, these two opposite cases may occur.

5. When two species are reciprocally crossed, i.e., male A with female B, and male B with female A, the degree of sterility often differs greatly in the two cases. The sterility of the resulting hybrids may differ likewise.

6. The degree of sterility of first crosses and of hybrids runs, to a certain extent, parallel with the systematic affinity of the forms which are united. " For species belonging to distinct genera can rarely, and those belong, ing to distinct families can never, be crossed. The parallelism, however, is far from complete ; for a multitude of closely allied species will not unite, or unite with extreme difficulty, whilst other species, widely different from each other, can be crossed with perfect facility. Nor does the difficulty depend on ordinary constitutional differ-ences ; for annual and perennial plants, deciduous and evergreen trees, plants flowering at different seasons, in-habiting different stations, and naturally living under the most opposite climates, can often be crossed with ease. The difficulty or facility apparently depends exclusively on the sexual constitution of the species which are crossed, or on their sexual elective affinity."

Such being the principal facts of hybridism, we may next consider the relation which they bear to the theory of descent. It is obvious that the most important point of contact between the former and the latter consists in this —that, although hybridism is occasionally possible as an exception to the general infertility of species inter se, it is only, as it were, a partial exception; for, even when produced, the hybrid progeny almost invariably manifest some greater or less degree of sterility, and this not only when crossed among themselves, but even when crossed with either of their parent species. The main facts of hybridism thus at first sight seem to support the time-honoured doctrine that there are placed between all species the barriers of mutual sterility, for the purpose of preventing any admixture of specific qualities by heredity, and so for the purpose of maintaining the immutability of specific types. And the apparent support which this doctrine thus receives from the main facts of hybridism is still further strengthened when these facts are contrasted with those which are supplied by the breeding of our domestic " varieties." For, in the latter case, and as an almost invariable rule, neither the organisms when crossed nor their resulting progeny show any indications of sterility, although the two parent varieties may differ from one another even more widely than do many natural species which are wholly infertile when crossed. This very general distinction between natural species and domestic varieties has appeared to many competent persons in the present generation so profound and significant that they deem it to be in itself sufficient to discredit, if not to negative, the whole theory of the transmutation of species.

Now, when this distinction is thus posited as an objection to the theory of descent, we must first of all remember that this theory does not require the possibility of the commingling of specific types; it requires, indeed, that specific types should not be immutably fixed, but it does not require that the causes of their mutation should depend upon their ! mutual crossing. The whole difficulty, therefore, which the theory of descent has here to meet is to explain why it is that natural species are fenced about, as it were, with the mysterious barriers of sterility, while no such seeming care appears to have been taken in the case of our domestic breeds—.even though in the latter case artificial selection by the breeders may have pro'duced more visible difference between the two parent races than that which natural selection is supposed to have produced between two natural species.

In answer to this difficulty, the most important consideration to begin with is one that is very generally lost sight of. The consideration is that mutual sterility between organic forms has been constituted by naturalists the chief criterion of specific distinction, and therefore it is merely to argue in a circle to maintain that specific distinction is of some transcendental nature because it is so invariably associated with this mutual sterility. If it were not for the fact of their mutual sterility, this and that species would probably not have been classified as such; and therefore it is now scarcely to be considered a matter of any great significance that all species present more or less of the peculiarity in virtue of which they are recognized as species. Or, otherwise stated, on the supposition that species have had a derivative origin, whenever the modification of a specific type has proceeded sufficiently far to induce sterility with allied types, the modified type is, for this reason, classified as a distinct species; otherwise, upon the sup-position, the species could scarcely have become separated out as a distinct type. The argument which points to such sterility as evidence against this supposition is, therefore, BO far inconclusive.

The case, however, ought not in fairness to be stated quite so strongly as this; for mutual sterility, although the chief, is not the only criterion of specidc distinction. In forming their classifications, naturalists endeavour as much as possible to have regard to organisms in the totality of their structures and functions. It may therefore still be maintained that, although the above consideration as to mutual sterility being selected as the chief criterion of specificdistinction greatly mitigates the force of the argument that natural species differ from artificial breeds in being more or less sterile with one another, still this consideration does not altogether destroy that argument. For, on the one hand, it is not mutual sterility alone which is taken as a test of specific distinction ; and, on the other hand, it generally so happens that the other qualities distinctive of any given species do not differ more widely from those which are distinctive of allied species than is the case with many of our domestic breeds. It therefore still remains a significant circumstance that, along with the differences distinctive of natural species, there almost invariably goes the protective attribute of mutual sterility; while the possibly greater differences distinctive of our domestic breeds are unaccompanied by any such protective attribute. But, again, this more refined objection can be met and satisfactorily excluded by the general consideration that " as species rarely or never become modified in one character, without being at the same time modified in many, and as systematic affinity includes all visible resemblances and dissimilarities, any difference in sexual constitution between two species would naturally stand in more or less close relation with their systematic position."

But we are not confined to this general consideration alone. There are several other general considerations which tend still further to mitigate the difficulty, and there are several particular facts which together prove that the alleged distinction between natural species and domestic varieties is one, not of kind, but of degree. We shall, therefore, next proceed to state these general considerations and particular facts.

Upon the theory of descent, mutual sterility between specific types is nothing more than the expression of some certain amount of modification having taken place in the reproductive system of a changing form, which up to that time, and but for the fact of this modification, would have been classified by naturalists as a mere variety. Now the causes wdiich act upon the reproductive system, both of animals and plants, and whether in the direction of sterility or prolificness, are at present hopelessly obscure. We cannot, therefore, expect to distinguish the causes which in the case of any given species have determined sterility. Nor is it necessary, for the meeting of the present difficulty, that this should be done; it is enough for this purpose to show that the causes which thus act upon the reproductive system are much too indistinct to admit of any argument being raised upon them. And this it is most easy to show;: for it is not too much to say that the reproductive sj'stem is, generally speaking, of all parts of an organism the most delicately susceptible to slight changes in the conditions of life. Mr Darwin has adduced a vast array of facts on. this head in his Variation of Animals and Plants undef Domestication. As a result of this delicacy, there arises an apparent capaciousness in the ways and degrees in which the reproductive system is affected by slight changes in the con-ditions of life, by too close interbreeding, by grafting, and by many other causes. Thus, for example, the influences of domestication produce more or less sterility in numberless species of wild animals and plants, while in other species— and this, as we shall presently see, is a matter of great im-portance in the present connexion—such influences are favourable to fertility. Now if we suppose, as in consistency we must suppose, that throughout the course of evolution the reproductive system has always been characterized by a sensitiveness to slight changes similar to that which we now observe, and if we remember that, in any case where these slight changes were sufficient to cause mutual sterility between the modified descendants of a common progenitor, a distinction of species must necessarily have arisen,—we shall cease to regard the present sterility of species inter se-as anything more than what might be expected a priori, supposing the theory of descent with gradual modification to be true.





As evidence of the apparent capriciousness with which sterility may be manifested, owing to the slight and imperceptible causes on which it depends, special allusion must here be made to a highly remarkable and significant fact that has been brought to light by the direct experiments of Mr Darwin. The following is his account of these experiments:—

" Several plants belonging to distinct orders present two forms, which exist in about equal numbers, and which differ in no respect except in their reproductive organs,—one form having a long pistil with short stamens, the other a short pistil with long stamens; both with differently-sized pollen grains. With trimorphic plants there are three forms likewise differing in the length of their pistils and stamens, in the size and colour of the pollen grains, and in some other respects; and, as in each of the three forms there are two sets of stamens, there are altogether six sets of stamens and three kinds of pistils. These organs are so proportioned in length to each other that, in any two of the forms, half the stamens in each stand on a level with the stigma of the third form. Now I have shown, and the result has been confirmed by other observers, that in order to obtain full fertility with other plants, it is necessary that the stigma of the one form should be fertilized by pollen taken from the stamens of corresponding height in the other form. So that with dimorphic species two unions, which may be called legitimate, are fully fertile, and two which may be called illegitimate are more or less infertile. With trimorphic species six unions are legitimate or fully fertile, and twelve are illegitimate or more or less infertile. The infertility which may be observed in various dimorphic and trimorphic plants when they are illegitimately fertilized, that is, by pollen taken from stamens not corresponding in height with the pistil, differs much in degree up to absolute and utter sterility,-— just in the same manner as occurs in crossing distinct species."


the theory of descent has here to meet is to explain why it is that natural species are fenced about, as it were, with the mysterious barriers of sterility, while no such seeming care appears to have been taken in the case of our domestic breeds—.even though in the latter case artificial selection by the breeders may have pro'duced more visible difference between the two parent races than that which natural selection is supposed to have produced between two natural species.

In answer to this difficulty, the most important consideration to begin with is one that is very generally lost sight of. The consideration is that mutual sterility between organic forms has been constituted by naturalists the chief criterion of specific distinction, and therefore it is merely to argue in a circle to maintain that specific distinction is of some transcendental nature because it is so invariably associated with this mutual sterility. If it were not for the fact of their mutual sterility, this and that species would probably not have been classified as such; and therefore it is now scarcely to be considered a matter of any great significance that all species present more or less of the peculiarity in virtue of which they are recognized as species. Or, otherwise stated, on the supposition that species have had a derivative origin, whenever the modification of a specific type has proceeded sufficiently far to induce sterility with allied types, the modified type is, for this reason, classified as a distinct species; otherwise, upon the sup-position, the species could scarcely have become separated out as a distinct type. The argument which points to such sterility as evidence against this supposition is, therefore, BO far inconclusive.

The case, however, ought not in fairness to be stated quite so strongly as this; for mutual sterility, although the chief, is not the only criterion of specidc distinction. In forming their classifications, naturalists endeavour as much as possible to have regard to organisms in the totality of their structures and functions. It may therefore still be maintained that, although the above consideration as to mutual sterility being selected as the chief criterion of specificdistinction greatly mitigates the force of the argument that natural species differ from artificial breeds in being more or less sterile with one another, still this consideration does not altogether destroy that argument. For, on the one hand, it is not mutual sterility alone which is taken as a test of specific distinction ; and, on the other hand, it generally so happens that the other qualities distinctive of any given species do not differ more widely from those which are distinctive of allied species than is the case with many of our domestic breeds. It therefore still remains a significant circumstance that, along with the differences distinctive of natural species, there almost invariably goes the protective attribute of mutual sterility; while the possibly greater differences distinctive of our domestic breeds are unaccompanied by any such protective attribute. But, again, this more refined objection can be met and satisfactorily excluded by the general consideration that " as species rarely or never become modified in one character, without being at the same time modified in many, and as systematic affinity includes all visible resemblances and dissimilarities, any difference in sexual constitution between two species would naturally stand in more or less close relation with their systematic position."

But we are not confined to this general consideration alone. There are several other general considerations which tend still further to mitigate the difficulty, and there are several particular facts which together prove that the alleged distinction between natural species and domestic varieties is one, not of kind, but of degree. We shall, therefore, next proceed to state these general considerations and particular facts.

Upon the theory of descent, mutual sterility between specific types is nothing more than the expression of some certain amount of modification having taken place in the reproductive system of a changing form, which up to that time, and but for the fact of this modification, would have been classified by naturalists as a mere variety. Now the causes wdiich act upon the reproductive system, both of animals and plants, and whether in the direction of sterility or prolificness, are at present hopelessly obscure. We cannot, therefore, expect to distinguish the causes which in the case of any given species have determined sterility. Nor is it necessary, for the meeting of the present difficulty, that this should be done; it is enough for this purpose to show that the causes which thus act upon the reproductive system are much too indistinct to admit of any argument being raised upon them. And this it is most easy to show;: for it is not too much to say that the reproductive sj'stem is, generally speaking, of all parts of an organism the most delicately susceptible to slight changes in the conditions of life. Mr Darwin has adduced a vast array of facts on. this head in his Variation of Animals and Plants undef Domestication. As a result of this delicacy, there arises an apparent capaciousness in the ways and degrees in which the reproductive system is affected by slight changes in the con-ditions of life, by too close interbreeding, by grafting, and by many other causes. Thus, for example, the influences of domestication produce more or less sterility in numberless species of wild animals and plants, while in other species— and this, as we shall presently see, is a matter of great im-portance in the present connexion—such influences are favourable to fertility. Now if we suppose, as in consistency we must suppose, that throughout the course of evolution the reproductive system has always been characterized by a sensitiveness to slight changes similar to that which we now observe, and if we remember that, in any case where these slight changes were sufficient to cause mutual sterility between the modified descendants of a common progenitor, a distinction of species must necessarily have arisen,—we shall cease to regard the present sterility of species inter se-as anything more than what might be expected a priori, supposing the theory of descent with gradual modification to be true.

As evidence of the apparent capriciousness with which sterility may be manifested, owing to the slight and imperceptible causes on which it depends, special allusion must here be made to a highly remarkable and significant fact that has been brought to light by the direct experiments of Mr Darwin. The following is his account of these experiments:—

" Several plants belonging to distinct orders present two forms, which exist in about equal numbers, and which differ in no respect except in their reproductive organs,—one form having a long pistil with short stamens, the other a short pistil with long stamens; both with differently-sized pollen grains. With trimorphic plants there are three forms likewise differing in the length of their pistils and stamens, in the size and colour of the pollen grains, and in some other respects; and, as in each of the three forms there are two sets of stamens, there are altogether six sets of stamens and three kinds of pistils. These organs are so proportioned in length to each other that, in any two of the forms, half the stamens in each stand on a level with the stigma of the third form. Now I have shown, and the result has been confirmed by other observers, that in order to obtain full fertility with other plants, it is necessary that the stigma of the one form should be fertilized by pollen taken from the stamens of corresponding height in the other form. So that with dimorphic species two unions, which may be called legitimate, are fully fertile, and two which may be called illegitimate are more or less infertile. With trimorphic species six unions are legitimate or fully fertile, and twelve are illegitimate or more or less infertile. The infertility which may be observed in various dimorphic and trimorphic plants when they are illegitimately fertilized, that is, by pollen taken from stamens not corresponding in height with the pistil, differs much in degree up to absolute and utter sterility,-— just in the same manner as occurs in crossing distinct species."

In this case we appear to have actual evidence of different stages of increasing sterility in transitu, and this even within the limits of the same natural species. And if even such evidence as this can be resisted, there still remains one very important fact, which directly affects the whole alleged distinction between the sterility of natural species and the fertility of domestic breeds. This fact is that plants belonging to several species of the genus Passi-flora have been amply proved, not only to be completely fertile with plants belonging to other species, but even to be as completely infertile with plants belonging to their own. Thus fruit could not be obtained from P. alata and P. racemosa except by reciprocally fertilizing them with each other's pollen; and similar facts have been observed by several experimentalists with regard to four or five other species of this genus. The fullest details on the subject are those given by Mr Scott in the Journal of the Linnean Society, vol. viii. p. 168. Plants belonging to three species of the genus, viz., P. racemosa, cosrulea, and alata, flowered for many years in Edinburgh, but, though repeatedly fertilized by Mr Scott and others with their own pollen, never produced seed. But when mutually crossed in various ways they all produced seed. After quoting this case Mr Darwin adds :—

" Returning to P. alata, I have received (1866) some interesting odetails from Sir Robinson Munro. Three plants, including one in England, have already been mentioned which were inveterately self-sterile, and Mr Munro informs me of several others which, after repeated trials during many years, have been found in the same predicament. At some other places, however, this species fruits readily when fertilized with its own pollen. At Taymouth Castle there is a plant which was formerly grafted by Mr Donaldson on a distinct species, name unknown, and ever since the operation it has produced fruit in abundance by its own pollen, so that this small and unnatural change in the state of this plant has restored its self-fertility. Some of the seedlings from the Taymouth Castle plant were found to be not only sterile with their own pollen, but with each other's pollen and with the pollen of distinct species. Pollen from the Taymouth plant failed to fertilize certain plants of the same species, but was successful on one plant in the Edinburgh Botanic Gardens. Seedlings were raised from this latter union, and some of their flowers were fertilized by Mr Munro with their own pollen; but they were found to be as self-impotent as the mother plant had always proved, except when fertilized by the grafted Taymouth plant, and except, as we shall see, when ferti-lized by her own seedlings. Yet Mr Munro fertilized eighteen flowers on the self-impotent mother plant with pollen from these her own self-impotent seedlings, and obtained, remarkable as the fact is, eighteen fine capsules full of excellent seed. I have met with no case in regard to plants which shows so well as this of P. alata on what small and mysterious causes complete fertility or complete sterility depends."





These cases in the genus Passiflora, although so highly remarkable, are not wholly unique. There is not, indeed, any other case of a natural species, the members of which are only fertile with members of another species; but there are several cases of natural species, the members of which are self-impotent, though freely fertile either with other plants of the same species, or with plants of different though allied species. This may perhaps be regarded as a transitional stage between the ordinary condition of plants and the extraordinary condition that obtains among species of the genus Passiflora. It occurs in individual plants of certain species of Lobelia and Verbascum, and among several genera of orchids. The cases of the latter are particularly remarkable, inasmuch as Fritz Muller found from numerous experiments, not only that individual plants belonging to the several species were not fertilized by their own pollen, while freely so by pollen taken from distinct species, and even from distinct genera, but that the plant's own pollen was positively deleterious to its stigma, and acted as a poison to the destruction of the flower.

So much, then, for the facts which go to prove on what slight constitutional differences sterility may depend, and the consequent probability there is that it should generally be found to accompany a change of organization which is sufficiently great to be regarded by naturalists as a specific distinction. But the pleading must not end here. For there still remains to be adduced the fact, already mentioned as one of the general facts of hybridism, that " the degree of both kinds of fertility varies in the case of different species, and in that of their hybrid progeny, from absolute sterility up to complete fertility." As a matter of fact, the distinction between natural species and domestic varieties, upon which the whole discussion has hitherto proceeded, is in itself untenable ; the infertility of natural species when crossed, although without question the general rule, is nevertheless not the invariable rule. We need not point to the highly anomalous case of Passiflora recently mentioned in another connexion, and probably to be explained as the result of cultivation; for we appear to have sufficient evidence without it. It is true that a great deal of negative evidence has been published upon this point by very competent experimentalists; but it seems impossible to resist the positive evidence of the Hon. and Bev. W. Herbert, whose distinguished success in hybridizing Mr Darwin attributes to " great horticultural skill, and to his having hot-houses at his command." Of his many results, which from being of a positive kind can scarcely be suspected of inaccuracy, it will be enough to quote the following :—" Every ovule in a pod of Crinum capense fertilized by C. revolutum produced a plant, which I never saw to occur in a case of its natural fecundation." Thus, as Mr Darwin in alluding to this case remarks, " we have perfect, or even more than commonly perfect, fertility in a first cross between two distinct species."

So far, then, as one side of the question before us is concerned, or that relating to the mutual infertility of natural species, enough has been said to show that it presents no real difficulty to the theory of descent. Indeed, in view of all that has now been said, the difficulty, as Mr Darwin has observed, is not so much to account for the sterility of natural species, as it is to account for the con-tinued, or even increased, fertility of our domestic varieties. Turning, therefore, to this other side of the question, we have to remember that the very same sensitiveness of the reproductive system, which in some cases leads to infertility under a change in the conditions of life, in other cases leads to increased fertility under an apparently similar change in the conditions of life. Thus it is that domestication produces such apparently capricious results with regard to fertility—inducing all degrees of infertility in some wild species, while not at all impairing, or even increasing, fertility in others. Consequently, when the question is as to why our domestic varieties do not become sterile inter se when so many natural species have become so, the answer is that the mere fact of their domestication proves that their wild or parent stocks must have been some of those species whose reproductive systems were not highly sensitive to changes in their condition of life, and therefore species which " might be expected to produce varieties little liable to have their reproductive systems injuriously affected by the act of crossing with other varieties which had originated in a like manner." Thus, on the inherently necessary view that our domestic varieties have all proceeded from species which were not easily affected in the direction of sterility, we are not surprised that under variation their reproductive systems should continue to manifest a high degree of tolerance. To this must be added that domestication, if it does not produce sterility, seems well calculated to increase fertility. For if the causes inducing sterility (whatever they may be) are absent, ample and regular nutrition, combined with innumerable lesser benefits attending domestication, may well be supposed to favour fertility. And, as a matter of fact, according to Pallas, there is a great deal of evidence to indicate that prolonged domesti- I cation has a tendency to eliminate sterility; so that wild species which when first domesticated intercross with difficulty, become in time able to intercross with facility. Such, for instance, appears to have been the case with the dog; for on the one hand all the domestic varieties of this animal are now freely fertile among themselves, and, on the other hand, there is independent evidence that these varieties have sprung from more than one natural species. Again, mention must not be omitted of the important fact that, although in the case of none of our varieties of domesticated animals is there any evidence of mutual sterility, yet among our varieties of domesticated plants a few cases have been observed of complete mutual sterility, which is in every way analogous to that which occurs between natural species. Thus, Gartner observed this to be the case with certain varieties of maize and Verbascum, Kolreuter with one kind of tobacco, and other experiment-alists with sundry varieties of gourd and melon. And here, let it be observed, we have the exact counterpart that evolutionists would desire to the experiments of Herbert above mentioned; for while he was able to break down the general distinction between natural species and domestic varieties on the side of proving perfect fertility between certain natural species, these experimentalists have broken down the distinction on the side of proving perfect sterility between certain of our domestic varieties. Therefore, we may conclude that this side of the question, or that as to the fertility of our domestic varieties, presents as harmless an aspect towards the theory of descent as we have already seen to be presented by the other side of the question, or that as to the sterility of natural species.

Finally, there are two complementary considerations to be adduced, which may now be stated together. One is that the general principles of hybridism, as briefly stated at the beginning of this article, are really far from indicative of having been instituted with any design of simply prevent-ing species from intercrossing. For, upon the view that they were so instituted, scarcely any one of them admits of a rational explanation. Thus, upon this view, no reason can be assigned why the degree of sterility should be so extremely variable in different species, when it must be supposed equally important that all species should be kept distinct; nor can it be said why the degree of sterility should vary even among individuals of the same species. Neither can it be said why some species should cross with facility, and yet produce sterile hybrids, while other species cross with difficulty, and yet produce fertile hybrids. Why should species living in countries remote from one another, and therefore not able in a state of nature to come together, nevertheless prove as sterile inter se as species inhabiting the same country'! Why, again, should there often be so great a difference in the result of a reciprocal cross between two species? Or why, indeed, should the production of hybrids have been permitted at all 1 As Mr Darwin ob-serves, " to grant to species the special power of producing hybrids, and then to stop their further propagation by different degrees of sterility, not strictly related to the facility of the first union between their parents, seems a strange arrangement."

The other and complementary consideration which has to be mentioned is that, on the counter supposition of all these general principles of hybridism being " simply inci-dental," or dependent on unknown differences in the repro-ductive systems of species,—on this supposition we meet with sundry differences between wild species and domestic varieties which are fully analogous to their difference of fertility, and which yet cannot reasonably be supposed to serve any transcendental purpose. Thus, again to quote Mr Darwin:—

"Some allied species of trees cannot be grafted on each other-all varieties can be so grafted. Some allied animals are affected in a very different manner by the same poison, but with varieties no such case until recently was known, but now it has been proved that immunity from certain poisons stands in some cases in correla-tion with the colour of the hair. The period of gestation generally differs much with distinct species, but with varieties until lately no such difference had been observed. The time required for the ger-mination of seeds differs in an analogous manner, and I am not aware that any difference in this respect has as yet been detected with varieties. Here we have various physiological differences, and no doubt others could be added, between one species and another of the same genus, which do not occur, or occur with extreme rarity, in the case of varieties; and these differences are apparently wholly or in chief part incidental on other constitutional differences, just in the same manner as the sterility of crossed species is incidental on differences confined to the sexual system. Why, then, should these latter differences, however serviceable they may indirectly be in keeping the inhabitants of the same country distinct, be thought of such paramount importance in comparison with other incidental and functional differences ? No sufficient answer to this question can be given."

Upon the whole, therefore, it may be concluded that the difficulty which the facts of hybridism seem at first sight to raise against the theory of descent may be explained in harmony with the main requirements of that theory.

Animal Hybrids.—A few words may here be added with special reference to animal hybrids. As a general statement it may be said that hybrids, not only between specific but generic forms, are more easily produced in the case of animals than in that of plants. The hybrids, how-ever, when produced are, as a general rule, more sterile. Indeed, it is doubtful whether there is any single instance of a perfectly fertile hybrid having emanated from a cross between two animal species. Mr Darwin, however, says— " I have reason to believe that the hybrids from Gervulus vaginalis and Reevesii, and from Phasianus colchicus with P. torquatns, are perfectly fertile." Also M. Quatrefages states that the hybrid progeny of two moths (Bombyx cyniliia and B. arrindia) showed themselves to be fertile inter se for eight generations. The hare and rabbit are said occasionally to breed together, and their offspring to be highly fertile when crossed with either parent species. Lastly, Mr Darwin observes :—

'1 The hybrids from the common and Chinese geese (A. cygnoides), species which are so different that they have sometimes been ranked in distinct genera, have often bred in this country with either pure parent, and in several instances inter se. This was effected by Mr Eyton, who raised two hybrids from the same parents, but from differ-ent hatches ; and from these two birds he raised no less than eight hybrids (grandchildren of the pure geese) from one nest. In India, however, these cross-bred geese must be far more fertile ; for I am assured by two eminently capable judges, namely, Mr Blyth and Captain Hutton, that whole flocks of these crossed geese are kept in various parts of the country ; and, as they are kept for profit where neither pure parent-species exists, they must certainly be highly of perfectly fertile."1

It is somewhat remarkable that hitherto direct experi-ments on the hybridization of animals have been so few in number as compared with those on the hybridization of plants. This is the more to be regretted, because, as already observed, animals appear to display a somewhat greater aptitude for hybridizing than plants, and consequently furnish better material for ascertaining the furthest limits of systematic affinity within which a cross may prove fertile. But here direct experiments are wanting, and all we can say with certainty is that in animals, as in plants, no authentic instance is on record of progeny resulting from a union of two individuals separated from one another by more than a generic distinction.

1 Since the above was published, Mr Darwin has himself procured two of these hybrid geese, and from them (brother and sister) raised five " extremely fine birds from two hatches." These five hybrids " re-sembled in every detail their hybrid parents" (see Nature, Jan. 1, 1880).

Graft-Hybridism.—The only other subject of importance that falls under the present heading, is that which has been appropriately called " graft-hybridism." It is well known that, when two varieties or allied species are grafted together, each retains its dis-tinctive characters. But to this general, if not universal, rule there are on record several alleged exceptions, in which either the scion is said to have partaken of the qualities of the stock, the stock of the scion, or each to have affected the other. Supposing any of these influences to have been exerted, the resulting product would deserve to be called a graft-hybrid. It is clearly a matter of great interest to ascertain whether such formation of hybrids by grafting is really possible; for, if even one instance of such formation could be unequivocally proved, it would show that sexual and asexual reproduction are essentially identical.

The cases of alleged graft-hybridism are exceedingly few, con-sidering the enormous number of grafts that are made every year by horticulturists, and have been so made for centuries. Of these cases the most celebrated are those of Adam's laburnum (Cytisus Adami) and the bizzarria orange. Adam's laburnum is now flour-ishing in numerous places throughout Europe, all the trees having been raised as cuttings from the original graft, which was made by inserting a bud of the purple laburnum into a stock of the yellow. M. Adam, who made the graft, has left on record that from it there sprang the existing hybrid. There can be no question as to the truly hybrid character of the latter—all the peculiarities of both parent species being often blended in the same raceme, flower, or even petal; but until the experiment shall have been successfully repeated, there must always remain a strong suspicion that, not-withstanding the assertion and doubtless the belief of M. Adam, the hybrid arose as a cross in the ordinary way of seminal repro-duction. Similarly, the bizzarria orange, which is unquestionably a hybrid between the bitter orange and the citron, —since it presents the remarkable spectacle of these two different fruits blended into one,—is stated by the gardener who first succeeded in producing it to have arisen as a graft-hybrid; but here again a similar doubt, similarly due to the need of corroboration, attaches to the statement. And the same remark applies to the still more wonderful case of the so-called trifacial orange, which blends three distinct kinds of fruit in one, and which is said to have been produced by artifi-cially splitting and uniting the seeds taken from the three distinct species, the fruits of which now occur blended in the triple hybrid.

The other instances of alleged graft-hybridism are too numerous to be here noticed in detail; they refer to jessamine, ash, hazel, vine, hyacinth, potato, beet, and rose. Of these the cases of the vine, beet, and rose are the strongest as evidence of graft-hybridization, from the fact that some of them were produced as the result of careful experiments made by very competent experimentalists. On the whole, the results of some of these experiments, although so few in number, must be regarded as making out a strong case in favour of the possibility of graft-hybridism. For it must always be remembered that in experiments of this kind, negative evidence, however great in amount, may be logically dissipated by a single positive result.

History and Literature.—From time immemorial the leading facts of hybridism have been known in the case of the horse and the ass. The knowledge of corresponding facts as occurring in the vegetable kingdom necessarily dates from a time subsequent to that at which the sexual functions of plants became known, i.e., towards the end of the 17th century. The earliest recorded obser-vation of a hybrid plant is one by Gmelin ; the next is that of Thomas Fairchild, who in the second decade of the 18th century produced the cross which is still grown in gardens under the name of " Fairchild's Sweet "William." Later on in that century Linnaeus made a number of experiments on the cross fertilization of plants, and produced various hybrids ; but it was reserved for the labori-ous investigations of Kblreuter, towards the end of that century (1751-1799), to found and largely to build the existing structure of our scientific knowledge upon this subject. To him also belongs the credit of first discovering the part played by insects in the fertilization of flowers. He published most of his results at the St Petersburg Academy of Sciences. Next in order of time deserve mention the works of Graf Lavola (Discorso delta Irrilabilita d'al-cuni Fiori nnovamente scoperta, 1764), and of Conrad Sprengel (Das entdeckte Geheimniss der Natur im Bau und in der Befruchtung der Blumen, 1793). The latter work is full of interesting observations on the connexion between the structure of flowers and the visita-tion of insects. Next we come to the celebrated horticulturist, Thomas Andrew Knight, who from 1787 for a number of years de-voted an immense amount of labour, with a large measure of results, to the improvement of fruit trees and vegetables by crossing. He published in the Philosophical Transactions and in the Trans. Hort. Soc. During the first quarter of the present century the only names that in the present connexion call for mention are those of J. E. Smith (Flora Britannica, 1800), Villars (Bo. Coll. Bot., 1809), Hoppe (Neues Bot. Taschenb., 1810), Guillemin and Dumas ("Obs. sur 1'Hybridite d. Plant.," in Mem. Paris Soc. Hist. Nat., 1833), Henschel, and Godron. During this period, besides Knight, already mentioned, there were two other English experimentalists at work, whose names deserve to be placed in the first rank among those which are associated with this subject. These are Sweet, who published an important work on Geraniacece, and Herbert, whose work on Amaryllidece, together with sundry publications in the Journal of the Horticultural Society, very materially advanced both the facts and the theory of hybridism. We say " theory," because it was in these publications that Herbert carried on his celebrated controversy with Knight regarding the alleged sterility of hybrids. In 1828 there was published a prize essay by Wiegmann on a thesis which was set several years before by the Berlin Academy of Sciences, and which embodied the question whether hybrid plants are necessarily sterile. We next come, in the second quarter of the present century, to the laborious researches of Gartner, the number of whose experiments in hybridizing has certainly not been surpassed, and probably has not been equalled, by that of any other experimentalist. His principal work is Vcrsuche und Beobachtungen iiber die Bastarderzeugung im Pflanzenreiche. In connexion with this period we may also mention the names of Braun, Wallroth, Zucearini, Meyer, Ziz, Koch, Schiede, Lasch, Reichenbach, A. P. de Candolle, Wimmer, Hornschuh, and Nageli. In 1854 a research of value was published by Klotzsch (Vcrhandl. Kgl. Prcuss. Akad. Berlin), and others later by Begel, Godron, and Jordon. In 1860 a prize was offered by the French Academy of Sciences for the best essay on hybridism, with special reference to three points,—the fertility or sterility of hybrids, the cause of their sterility, and the constancy of type manifested by fertile hybrids. In 1865 this prize was awarded jointly to Naudin and Godron, the latter name being identified with researches upon the character of hybrids which deserve to be considered among the most im-portant of the present century. The next work of note appeared in 1865, by Max Wrichura, on Die Bastardbefruchtung in Pflan-zenrcich, &'c. He combined, in one complex hybrid, six different species of Salix ; confirmed, in opposition to Godron, the doctrine of Kblreuter, Herbert, Gartner, and Naudin, that a hybrid is best fertilized by its own pollen ; and found, in opposition to Naudin, that the progeny of hybrid willows retains its hybrid character. In 1865-6 Nageli published his important observations on naturally produced hybrids (Sitzungsber. Akad. Miinchen, Math. Phys.). The highly important experiments of Darwin on dimorphic and trimor-phic plants have been already alluded to. Those who within still more recent years have contributed to the literature of hybridism are Caspary, Mendel, Seden, Dominy, Kellermann, Fr. Schultz, Timbal-Lagrave, Grenier, A. Kerner, Wirtgen, Miehalet, Bitschl, Beckhaus, P. Ascherson, R. von tlechtritz, J. Schmalhausen, C. Haussknecht, V. von Borbas, Kuntze, Henniger, and W. O. Focke. The last-named author has just published an elaborate and valuable work on hybridism in plants (Die Pflanzen-Mischlinge, Berlin, 1881), giving a tabular series of all the known vegetable hybrids, and treating the entire subject in a very comprehensive manner.

On the subject of animal hybrids there is virtually no literature, save scattered records of fertile crosses among sundry species confined in various menageries ; and these are without interest as bearing on any of the principles of hybridism. (G. J. K.)




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