122 DARWINIANA. 



poles with as good tails for swimming as any of theii 

 kindred, although as tadpoles they never enter the wa- 

 ter ; that the Guinea-pig is furnished with incisor teeth 

 which it never uses, as it sheds them before birth; 

 that embryos of mammals and birds have branchial 

 slits and arteries running in loops, in imitation or remi- 

 niscence 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 apt- 

 ness. Mr. Darwin assures them that they have been 

 talking derivative doctrine all their lives — as M. Jour- 

 dain talked prose — without knowing it. 



If it is difficult and in many cases practically im- 

 possible 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 circumscrip- 

 tion. Intermediate forms occur, connecting one group 

 with another in a manner sadly perplexing to sys- 

 tematists, except to those who have ceased to expect 

 absolute limitations in Nature. All this blending 

 could hardly fail to suggest a former material connec- 

 tion among allied forms, such as that which the 

 hypothesis of derivation demands. 



