620 



COLLEGE ZOOLOGY 



Equus 

 (Recent) 



Pliohippus 

 (Pliocene) 



Mesohippus 

 (Oligocene) 



Eohippus 

 (Eocene) 



Figure 438. Evolution of the horse. Principal features of change in the horses through time 

 are: (1) increased size; (2) increased length and mobility of the neck; (3) greater head size; 

 (4) elongation of limbs; (5) reduction of toes; and (6) changes in teeth. 



animals has been carefully worked out by 

 paleontologists, but none quite so much in 

 detail as that of the horse. Nevertheless, 

 they show how much can be learned of the 

 ancestors of vertebrates from a study of 

 fossil forms. 



THEORIES TO ACCOUNT 

 FOR ORGANIC EVOLUTION 



Early belief in 

 organic evolution 



The idea of organic evolution did not 

 originate with Charles Darwin, as many 



people seem to believe, but from the time 

 of the Greek philosopher Aristotle (384- 

 322 B.C.), it has had a steady growth. Dar- 

 win, however, brought together an enormous 

 mass of evidence in its favor and proposed 

 a very plausible theory (natural selection) 

 to account for it. He was so widely criticized 

 by those who did not accept the theoiy, 

 and so strongly defended by those who did, 

 that the whole civilized world became con- 

 scious of the struggle and, for the most part, 

 gave Darwin credit for the idea. 



The immediate predecessors of Charles 

 Darwin were Buffon, Erasmus Darwin, and 

 Lamarck, and many others followed, espe- 



