Miscellaneous. 383 



foot of a dog and of a horse, the wing of a bat, and the fin of a por- 

 poise are fundamentally identical ; that the long neck of the giraffe 

 has the same and no more bones than the short one of the elephant; 

 that the eggs of Surinam frogs hatch into tadpoles 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 em- 

 bryos of Mammals 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, &c. Upon a derivative theory this mor- 

 phological 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 without knowing it. 



If it is difficult, and in some 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 cir- 

 cumscription. Intermediate forms occur, connecting one group with 

 another in a manner sadly perplexing to systematists, except to those 

 who have ceased to expect absolute limitations in nature. All this 

 blending could hardly fail to suggest a former material connexion 

 among allied forms, such as that which a hypothesis of derivation 

 demands. 



Here it would not be amiss to consider the general principle of 

 gradation throughout organic nature, — a principle which answers in 

 a general way to the law of continuity in the inorganic world, or rather 

 is so analogous to it that both may be fairly expressed by the Leib- 

 nitzian axion, Natura non agit saltatim. As an axiom or philosophical 

 principle, used to test model 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 undertake 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 phsenomena 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 them- 

 selves. It would be thought that the distinction between the two 

 organic kingdoms was broad and absolute. Plants and animals be- 

 long to two very different categories, fulfil opposite offices, and, as to 



