350 THE EVOLUTION OF MAK. 



vitiated, and abbreviated, in the course of time, that w© 

 can gain but little certain information as to the tribal- 

 history from the kenogenetic phenomena of their germ- 

 history. Here, therefore, Comparative Anatomy must come 

 to our help, and it often affords much more important and 

 trustworthy disclosures as to Phylogeny than Ontogeny 

 is able to impart. It is, therefore, most important, if the 

 fundamental law of Biogeny is to be correctly and critically 

 applied, to keep its two sides continually in view. The 

 first haif of this fundamental law of evolution enables us to 

 use Phylogeny, as it shows us how to gain an approximate 

 knowledge of the history of the tribe from that of the 

 germ : the germ-form reproduces, by Heredity, the corre- 

 sponding tribal form (Palingenesis). The other half of 

 the law, however, limits this guiding principle, and calls 

 attention to the foresight with which it must be employed ; 

 it shows us that the original reproduction of the Phylogeny 

 in the Ontogeny has been in many ways altered, vitiated, 

 and abbreviated, in the course of millions of years. The 

 germ-form has deviated, by Adaptation, from the corre- 

 sponding tribal form (Kenogenesis) ; the greater this devia- 

 tion, the more are we compelled to employ Comparative 

 Anatomy in the study of Phylogeny. 



Perhaps in no other system of organs of the human body 

 is this so greatly the case as in the vascular system (vas- 

 cular, or circulatory apparatus), the development of which 

 we wiU now examine. If we attempted to infer the 

 original structural features of our older animal ancestors 

 solely from the phenomena which the individual develop- 

 ment of these organ-systems, in the embryo of Man and of 

 other high Vertebrates, exhibit, we should obtain wholly 



