EMBRYOLOGY AND EVOLUTION 



that of the frogs, and finally a metanephros (or hind-kidney) 

 like that of reptiles, birds, and mammals, which alone sur- 

 vives in the adult. His brain, eye, ear, in fact, all his organs, 

 pass through stages in development that are characteristic 

 of lower vertebrates. Even in those adult features that are 

 distinctively human, such as the peculiar form of the hand 

 and the foot, the number of bones in the ankle and wrist, 

 the number of pairs of ribs, the absence of a tail and the rela- 

 tive hairlessness of the skin — in all these respects the human 

 foetus resembles anthropoid apes more than adult man. 



Why are not these and a hundred other structures made 

 directly? Why this roundabout process of making a man? 

 There is no answer but evolution. 



Embryology bears indubitable testimony to the truth of 

 evolution; ancestral history is repeated in individual history, 

 but the record is like an ancient palimpsest that has been 

 erased and written over again and again. Traces of the old 

 record are still there; some are obscure, some are almost 

 entirely obliterated, but wherever they are decipherable they, 

 tell the old, old story of the common origin of animal and 

 man — their similarities and their kinship. 



The Causes of Development and Evolution 



Ontogeny recapitulates certain stages and features of 

 phylogeny, but the whole course of evolution through past 

 geologic ages can be followed only by a study of fossils. The 

 record is necessarily incomplete, for we rarely find all the 

 stages of evolution represented in fossils. Nevertheless 

 palaeontology is a certain guide to the general succession of 

 living things during past ages. But evolution is a present 

 process, going on to-day, and to see it at work it is not neces- 

 sary to explore "the dark backward and abysm of time." To 

 be sure, it goes slowly; species are not made in a day any 



[75] 



