the theory of descent has here to meet is to expl un why it is that natural species are fenced about, as it were, with the mysterious barriers of sterility, while no such seeming I 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 produced 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 con sideration 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, GO far inconclusive. The case, however, ought not in fairness to be stated quits so strongly as this ; for mutual sterility, although the chief, is not the only criterion of specific 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 specific distinction 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, tins 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." 423 Upon the theory of descent, mutual sterility between specific types is nothing mure 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 which 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 system h, 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 under Domestication. As a result of this delicacy, there arises an apparent capriciousness 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 w r e 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 dill er 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 nnd 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 form should be fertilized by pollen taken from the stigma of the one . . But we are not confined to this general consideration stamens of corresponding height in the other form. So that with urn QPVPIMI nfhnr rr,p1 nn a ^nrafin dimorphic species two unions, which may be called legitimate, nre 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 arc illegitimately fertilized, that is, by pollen taken from stamens not corresponding in height with the istil, differs much in degree up to absolute and utter sterility, 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.
just in the same manner as occurs in crossing distinct species."
