64 DOCTRINE OF EVOLUTION 



we have reached forms so highly developed that they 

 would not exhibit the same kind of developmental 

 history, but would have their own mode of growing up. 

 This is not so, for like the adult fish, the larval tadpole, 

 and the embryo chick, an embryo of a cat or a man 

 is at one time constructed with a series of gill-clefts 

 and with blood-vessels and skeletal supports of fishlike 

 nature that are everywhere associated with gills. 

 The embryos of wildcats and dogs, rabbits and rats, 

 pigs, deer, and sheep, and of all other mammalia, possess 

 similar structures. Thus they all pass through a stage 

 which is found also in the development of reptiles, 

 birds, and amphibia, — a stage which corresponds to 

 the fish throughout its life. Unless these facts mean 

 that the great classes of vertebrates have originated 

 together from the same or closely similar ancestors, 

 they are unintelligible ; for we cannot see why a cat or 

 a chick should have to be essentially fishlike at any 

 time unless this is so. Comparative anatomy states 

 as we have learned that the amphibia as a class have 

 evolved from and have out-developed the fishes, that 

 reptiles have progressed still higher, and that birds 

 and mammals have originated from reptilian ancestors 

 along roads that have diverged beyond the immediate 

 parent class. Because the members of each class have 

 to pass along the same path trodden by their many 

 varied ancestors, although at express speed, as it were, 

 the similarity of the earliest stages in their develop- 

 ment is explained, for during these periods they are 

 traversing a path over which their ancestors passed 

 together. 

 The places where the developing embryos depart 



