STRUCTURE AND DEVELOPMENT 63 



that the order in which the stages follow is the one 

 which comparative anatomy independently proves to 

 be the order of the evolution of fishes, amphibia, rep- 

 tiles, and birds. Why, now, should it be necessary for 

 a developing bird to follow this order? The answer 

 has been found in the immense array of embryological 

 facts that investigators have verified and classified, 

 that all tell the same story. It is, that birds have 

 arisen by evolution from ancestors which were really 

 as simple as the members of these lower classes. It 

 seems then that the only way a bird of to-day can 

 become itself is to traverse the path along which its 

 progenitors had progressed in evolution. Stating its 

 conclusions precisely, science formulates the principle 

 in the following words : individual development is a 

 brief resume of the history of the species in past timeSj 

 or, more technically, ontogeny recapitulates phytogeny. 

 To be sure, the full history is not reviewed in detail, 

 for the chick embryo does not actually swim in water 

 and breathe by means of gills. Only a condensed 

 account of evolution of its kind is presented by an 

 embryo during its development ; as Huxley and 

 Haeckel have put it, whole lines and paragraphs and 

 even pages are left out; many false passages of a 

 later date are inserted as the result of peculiar larval 

 and embryonic needs and adjustments. But in its 

 major statements and as a general outline, the account 

 is a trustworthy natural document submitted as evi- 

 dence that higher species of to-day have evolved from 

 ancestors which must have been like some of the present 

 lower animals. 



Coming now to the mammalia, it might seem that 



