20 THE WHENCE AND THE WHITHER OF MAN 



tion, " Why this progressive system of forms, and what 

 does it all mean ? ' 



Let us experiment a little in forming our own classifi- 

 cation of a few vertebrates. We see a bat flying 

 through the air. We mistake it for a bird. But a 

 glance at it shows that it is a mammal. It is covered 

 with hair. It has fore and hind legs. Its wings are 

 membranes stretched between the fingers and along 

 the sides of the body. It has teeth. It suckles its 

 young. In all these respects it differs from birds. It 

 differs from mammals only in its wings. But we re- 

 member that flying squirrels have a membrane stretch- 

 ing along the sides of the body and serving as a para- 

 chute, though not as wings. We naturally consider the 

 wings as a sort of after-thought superinduced on the 

 mammalian structure. We do not hesitate to call it a 

 mammal. 



The whale makes us more trouble ; it certainly looks 

 remarkably like a fish. But the fin of its tail is hori- 

 zontal, not vertical. Its front flippers differ altogether 

 from the corresponding fins of fish ; their bones are the 

 same as those occurring in the forelegs of mammals, 

 only shorter and more crowded together. Later we 

 find that it has lungs, and a heart with four chambers 

 instead of only two, as in fish. The vertebrae of its 

 backbone are not biconcave, but flat in front and behind. 

 And, finally, we discover that it suckles its young. It, 

 too, is in all its deep-seated characteristics a mammal. 

 It is fish-like only in characteristics which it might eas- 

 ily have acquired in adaptation to its aquatic life. And 

 there are other aquatic mammals, like the seals, in 

 which these characteristics are much less marked. 

 Their adaptation has evidently not gone so far. 



