Hybridity

“ Others, on the contrary, of our most acute observers, having acquired convincing evidence of natural hybridity in a few cases, have generalized their conclusions; they have supposed natural hybrids to be of constant and frequent occurrence; and they have ascribed to this cause alone the majority of variations from the supposed typical forms of species, or even attributed to original hybridisations the multitude of nearly allied, but constant species, in several of the largest genera. ”

“ Hybridity is a problem which lies at the root of a philosophical conception of the much-used and much-vexed term "species." We all agree that the various breeds of Fowls and Pigeons represent but one species, because we know their life-histories. ”

“ However, let us examine into hybridity so far as it may serve to produce new, or modify existing races, as Blumenbach and Flourens have admitted.252 Let us only remark that these two authors, like most monogenists, in placing hybridity, as the modifying cause, in the same rank as climate or medium, commit a great error. Hybridity, even in giving it the creative power which some have desired, goes entirely into the second rank, for it supposes a pre-existing plurality. ”

“ It is assumed that unlimited prolificness is a certain test of community of origin.
We, on the contrary, contend that there is no abrupt line of demarcation; that no complete laws of hybridity have yet been established; that there is a regular gradation in the prolificness of the species, and that, according to the best lights we now possess, there is a continued series from perfect sterility to perfect prolificacy. The degrees may be expressed in the following language:—
1. That in which hybrids never reproduce; in other words, where the mixed progeny begins and ends with the first cross. ”

“ Again, in pre-Darwinian times it was so universally the practice to argue in a circle, and declare that the fertility of the offspring of a cross proved the identity of species of the parents, that experiments in hybridity were usually made between very remote species and even between species of different genera, to avoid the possibility of the reply: "They are both really the same species;" and the sterility of the hybrid offspring of such remote crosses of course served to strengthen the popular belief. ”

“ But these two fundamental propositions, admitted as axioms, do they express the truth? Can this triumphant syllogism, of which they are the premises, stand? Is it true that only animals of the same species can produce a prolific progeny? Is65 it true that all human crossings are eugenesic? To upset the syllogism of the monogenists, and to deprive their system of any scientific base, it might be sufficient that the first of the above questions should be answered in the negative. ”

“ There is, in fact, one set of these peculiarities which the theory of selective modification, as it stands at present, is not wholly competent to explain, and that is the group of phenomena which I mentioned to you under the name of Hybridism and which I explained to consist in the sterility of the offspring of certain species when crossed one with another. It matters not one whit whether this sterility is universal, or whether it exists only in a single case. ”

“ Recapitulation of the arguments from hybrids.—I shall now conclude this subject by summing up, in a few words, the results to which I have been led by the consideration of the phenomena of hybrids. It appears that the aversion of individuals of distinct species to the sexual union is common to animals and plants; and that it is only when the species approach near to each other in their organization and habits, that any offspring are produced from their connexion. ”

“ But if you take members of two distinct species, however similar they may be to each other and make them breed together, you will find a check, with some modifications and exceptions, however, which I shall speak of presently. If you cross two such species with each other, then,—although you may get offspring in the case of the first cross, yet, if you attempt to breed from the products of that crossing, which are what are called HYBRIDS—that is, if you couple a male and a female hybrid—then the result is that in ninety-nine cases out of a hundred you will get no offspring at all ”

“ There are real obstacles (of which I will say more below) in natural conditions to hybrid-breeding between any two species which occur naturally in the same territory. Thus the idea of a species is expanded so as to be not merely "a group of individuals of constant likeness in form and characteristics," but we add to that definition a living or constitutional quality expressed by the words, "which produce fertile offspring by pairing with one another, but do not pair with the members of other species." ”

“ 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 belonging 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. ”

“ Hybridization differs from ordinary crossing only in degree. A species is only a race which has assumed greater fixity. The purposes and results of crossing within the species and of hybridization of different species are essentially alike. The formation of the new individual by the sexual relation of two parents is in itself a species of crossing, giving each new individual in its degree new traits or new combinations. ”

“ Perhaps on close inspection you may find in some cases a slight difference, some character being not as fully developed in the hybrid as in the species. But when such is not the case, or where the opportunity for such a closer examination is wanting, a hybrid may easily be taken for a specimen of the pure race. Now take the seeds of your plant and sow them. If you had not supposed it to be hybrid you will be astonished at finding among its progeny some of a wholly different type. You will be led to conclude that you are observing a sudden change in structure such as is usually called a sport. ”

“ That the fertility of hybrids is not related to the degree in which they resemble in external appearance either parent. And lastly, that the facility of making a first cross between any two species is not always governed by their systematic affinity or degree of resemblance to each other. ”

“ Theory of Hybridism.—Charles Darwin was interested in hybridism as an experimental side of biology, but still more from the bearing of the facts on the theory of the origin of species. It is obvious that although hybridism is occasionally possible as an exception to the general infertility of species inter se, the exception is still more minimized when it is remembered that the hybrid progeny usually display some degree of sterility. ”

“ It is a truly wonderful fact—the wonder of which we are apt to overlook from familiarity—that all animals and all plants throughout all time and space should be related to each other in group subordinate to group, in the manner which we everywhere behold—namely, varieties of the same species most closely related together, species of the same genus less closely and unequally related together, forming sections and sub-genera, species of distinct genera much less closely related, and genera related in different degrees, forming sub-families, families, orders, sub-classes, and classes. ”

“ The conception of variety or species has a different value in every small or large department of systematic Zoology and Botany.
But the conception of species has just as little any fixed physiological value. In respect to this we must especially insist that the question of hybrid offspring, the last corner of refuge of all the defenders of the constancy of species, has at present lost all significance as bearing on the conception of species. ”

“ The hybrid of two such elementary species is in some way incomplete and unnatural. In the ordinary course of things all individuals derive [254] their qualities from both parents; for each single mark they possess at least two units. Practically but not absolutely equal, these two opponents always work together and give to the offspring a likeness to both parents. No unpaired qualities occur in normal offspring; these constitute the essential features of the hybrids of species and are at the same time the cause of their wide deviations from the ordinary rules. ”

“ This, as well as the complicated relations of the mixture of races, and the various forms of hybrids, is the great difficulty lying in the way of tracing the human pedigree in its individual branches, species, races, varieties, etc. ”

“ Why should there often be so great a difference in the result of a reciprocal cross between the same two species? Why, it may even be asked, has the production of hybrids been permitted? 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 to be a strange arrangement. ”

“ I have made so many experiments and collected so many facts, showing on the one hand that an occasional cross with a distinct individual or variety increases the vigour and fertility of the offspring, and on the other hand that very close interbreeding lessens their vigour and fertility, that I cannot doubt the correctness of this conclusion. Hybrids are seldom raised by experimentalists in great numbers ”

“ In nearly the same manner as it is admitted by naturalists that hybridism, from inducing sterility, is of service in keeping the forms of life distinct and fitted for their proper places; so, when species are rendered highly variable by changed conditions of life, the free inter-crossing of the varying individuals will tend to keep each form fitted for its proper place in nature ”

“ For, you see, if you have not done that you have not strictly fulfilled all the conditions of the problem; you have not shown that you can produce, by the cause assumed, all the phenomena which you have in nature. Here are the phenomena of Hybridism staring you in the face, and you cannot say, 'I can, by selective modification, produce these same results.' Now, it is admitted on all hands that, at present, so far as experiments have gone, it has not been found possible to produce this complete physiological divergence by selective breeding. ”

“ Now, as crosses in interbreeding can take place between the different varieties or families of these several species, and can not take place between the species themselves—between those which I thus class as species—the limits of such crosses become important facts in considering the theory of evolution, because they narrow the inquiry to the possibility of effecting a propagation of one species out of another species. Take any animal which has become a completed and final product—a peculiar and distinct creature—whether made so by aboriginal creation or produced by what you call evolution. ”

“ For our present purposes we are to deal with hybrids only in so far as they afford the means of a still better distinction between elementary species and varieties. I will try to show that these two contrasting groups behave in quite a different manner, when subjected to crossing experiments, and that the hope is justified that some day crosses may become the means of deciding in any given instance, what is to be called a species, and what a variety, on physiologic grounds. ”

“ Independently of the question of fertility and sterility, in all other respects there seems to be a general and close similarity in the offspring of crossed species, and of crossed varieties. If we look at species as having been specially created, and at varieties as having been produced by secondary laws, this similarity would be an astonishing fact. But it harmonises perfectly with the view that there is no essential distinction between species and varieties. ”

“ A closer scrutiny of the facts, however, makes the term hybridism less isolated and more vague. The words species and genus, and still more subspecies and variety, do not correspond with clearly marked and sharply defined zoological categories, and no exact line can be drawn between the various kinds of crossings from those between individuals apparently identical to those belonging to genera universally recognized as distinct. ”

“ As if to emphasise the kinship of all of life’s forms and to render incontrovertible the fact of universal evolution, Nature compels every individual to commence existence at the same place, and to recapitulate in his individual evolution the phylogenetic journeyings of his species. ”

“ It is said that such a test is to be found in the phenomena of hybridization—in the results of crossing races as compared with the results of crossing species.
So far as the evidence goes at present, individuals, of what are certainly known to be mere races produced by selection, however distinct they may appear to be, not only breed freely together, but the offspring of such crossed races are also perfectly fertile with one another. ”

“ That true hybrids raised from entirely distinct species, though they lose in fertility, often gain in size and constitutional vigour, is certain. It would be superfluous to quote any facts; for all experimenters, Kölreuter, Gärtner, Herbert, Sageret, Lecoq, and Naudin, have been struck with the wonderful vigour, height, size, tenacity of life, precocity, and hardiness of their hybrid productions. ”