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Show 122 .D A.R WIN IAN A. poles with as good tails for swimming as any of their kindred, although as tadpoles they never enter the water; that the Guinea-pig is furnished with incisor teeth which it never uses, as it sheds them before birth· that embryos of mamJ;Ilals and birds have branchial slits and arteries running in loops, in imitation or reminiscence of the arrangement which is permanent in fishes; and that thousands of animals and plants have rudimentary organs which, at least in numerous cases are wholly useless to their possessors, etc., etc. Upon' a derivative theory this morphological conformity is explained by community of descent; and it has not been explained in any other way. Naturalists are constantly speaking of "related species," of the "affinity" of a genus or other group, and of "family resemblance "-vaguely conscious that · these terms of kinship are something more than mere metaphors, but unaware of the grounds of their aptness. Mr. Darwin assures them that they have been talking derivative doctrine all their lives-as M. J omdain talked prose-without knowing it. If it is difficult and in many cases practically impossible to fix the limits of species, it is still more so to fix those of genera; and those of tribes and families are still less susceptible of exact natural circumscription. Intermediate forms occur, connecting one group with another in a manner sadly perplexing to systematists, except to those who have ceased to expect ~bsolute limitations in Nature. All this blending could hardly fail to suggest a former material connection among allied forms, such as that which the hypothesis of derivation demands. NATURAL SELECTION, ETC. 123 Here it weuld not be amiss to consider the general principle of gradation throughout organic Nature-a principle which answers in a general way to t~e Law of Continuity in the inorganic world, or rather JS so analogous to it that both may fairly be expressed by the Leibnitzian axiom, Natura non agit saltatim. As an axiom or philo$ophical principle, used to test modal laws or hypotheses, this in strictness belongs only to physics. In the investigation of Nature at large, at least in the organic world, nobody would -qndertake to apply this principle as a test of the validity of any theory or supposed law. But naturalists of enlarged views will not fail to infer the principle from the phenomena they investigate-to perceive that the rule holds, under due qualifications and altered forms, throughout the realm of Nature; although we do not suppose that Nature in the organic world makes no distinct steps, but only short and serial steps-not infinitely fine gradations, but no long leaps, or few· of them. To glance at a few illustrations out of many that present themselves. It would be thought that the distinction between tJ:e two organic kingdoms was broad and absolute. Plants and animals belong to two very different categories, fulfill opposite offices, and, as to the mass of them, are so un1ike that the difficulty of the ordinary observer would be to find points of comparison. Without entering into details, which would fill an article, we may safely say that the difficulty with the naturalist is all the other way-that all these broad differences vanish one by one as we approach the lower confines of the two kingdoms, and that no abso- |