244 EVOLUTION 



barnacle and active shrimp, of sluggish 

 beetle and nimble gnat; or again within the 

 same orders, as moth and butterfly — always 

 the same dichotomy of passive and active. 

 Here essentially lies the secret of the diver- 

 gence between ascidian and vertebrate, to 

 take a great difference, or that between toad 

 and frog for a small one; or again this is the 

 main contrast between reptile and bird, 

 Diplodocus and swallow. This lies within 

 the perpetual redifferentiation of these, 

 witness, among reptiles, the sluggish and 

 often colossal tortoises, yet the active and 

 supple snakes; and these (of course) again 

 differentiating anew, here the huge and 

 passive python, and there the small and 

 nimble fer de lance. Once more, for birds, 

 see the contrast of the massive pedestrian 

 dodo with his pigeon cousins; or of giant 

 chickens like the ostrich and emu tribes with 

 exquisite but tiny adults, say the humming- 

 birds. The kinship of elephant and coney, 

 the contrast of stony glyptodon and gigantic 

 sloth with nimble lemur and agile monkey, 

 and again of bear and dog, of dog and cat, 

 of sheep and goat, are thus re-interpreted 

 together, no longer as for the early Darwin- 

 ians as so many machine-like combinations 

 of innumerable indefinite variations exter- 

 nally selected from among yet more innumer- 



