356 THE EVOLUTION OF MAN. 



This constant and regular circulation of the blood alone 

 makes the complex change of substance with the higher 

 animals possible. 



Important as is the vascular system in the more highly 

 developed and differentiated animal body, it is not, however, 

 an apparatus as indispensable to animal life as is generally 

 supposed. In the older theory of medicine the blood was 

 regarded as the real source of life, and " humoral pathology" 

 referred most diseases to " corrupt blood-mixture." Simi- 

 larly, the blood plays the most important part in the pre- 

 vailing, obscure conception of Heredity. Just as half-blood, 

 pure blood, etc., etc., are yet common phrases, so it is widely 

 believed that the transmission, by Heredity, of definite 

 morphological and physiological characters from the parent 

 to the child "lies in the blood." That this customary 

 notion is entirely false, is easily seen from the fact that, 

 neither in the act of procreation is the blood of the parents 

 directly transmitted to the procreated germ, nor does the 

 embryo acquire blood at an early period. As we have 

 already seen, not only the sejjaration of the four secondary 

 germ-layers, but also the beginning of the most impor- 

 tant organs, takes place, in the embryos of all Vertebrates, 

 before the rudiment of the vascular systems, of the heart 

 and blood, is formed. In accordance with this ontogenetic 

 fact, we must, from a phylogenetic point of view, regard the 

 vascular system as the most recent, the intestinal system, 

 on the contrary, as the oldest formation of the animal body. 

 The origin of the vascular system is, at least, much later 

 than that of the intestinal system. If the fundamental law 

 of Biogeny is rightly appreciated, it is possible, from the 

 ontogenetic sequence, in which the various organs of the 



