$2 CHAPTERS ON EVOLUTION. 



same plan ? Or, again, why should the appendages of the bodies of 

 these animals, which resemble each other far more closely in the 

 early stages than in the adults, present a striking correspondence of 

 type that is only marked by the modifications they undergo through 

 adaptation to varied ends ? Why, again, should the mouth-parts of 

 a butterfly, adapted, as every one knows, for suction, be formed of 

 essentially similar and corresponding parts to those which are found 

 in the biting mouth of a beetle ? And why should the arm of man, 

 the wing of the bird or bat, the fore-limb of the horse, the paddle of 

 the whale or dolphin, and the fore-limb of the frog, as will be more 

 fully shown in a future chapter, be constructed on one and the 

 same type ? 



The answer to these pertinent inquiries can only be found in 

 some conception which demands and postulates some intelligible 

 relationship between the varied and yet fundamentally similar parts. 

 Mr. Spencer, speaking of similar facts in the structure of Articulate 

 animals, asks, " What, now, can be the meaning of this community 

 of structure among these hundreds of thousands of species filling the 

 air, burrowing in the earth, swimming in the water, creeping about 

 among the seaweed, and having such enormous differences of size, 

 outline, and substance, as that no community would be suspected 

 between them ? Why under the down-covered body of the moth, 

 and under the hard wing-cases of the beetle, should there be 

 discovered the same number of divisions as in the calcareous frame- 

 work of the lobster? It cannot be by chance" continues Mr. 

 Spencer, " that there exist just twenty segments in all these hundreds 

 of thousands of species. There is no reason to think that it was 

 necessary, in the sense that no other number would have made a 

 possible organism. And to say that it is the result of design to say 

 that the Creator folio wed this pattern throughout, merely for the purpose 

 of maintaining the pattern is to assign a motive which, if avowed 

 by a human being, we should call whimsical No rational interpre- 

 tation of this and hosts of like morphological truths, can be given 

 except by the hypothesis of evolution ; and from the hypothesis of 

 evolution they are corollaries. If organic forms have arisen from 

 common stocks by perpetual divergences and redivergences if 

 they have continued to inherit, more or less clearly, the characters 

 of ancestral races ; then there will naturally result these communities 

 of fundamental structure among extensive assemblages of creatures 

 that have severally become modified in countless ways and degrees, 

 in adaptation to their respective modes of life." Choosing thus the 

 doctrine of evolution, we can clearly enough account for the general 

 likeness exhibited by the various members of an animal type. The 

 animals of each type resemble each other because they are descended 

 from a common stock. " Descent with modification " is the key 



