320 THE EVOLUTION OF JVIAN 



anatomical features. If, liowever, the subsequent stages of 

 development, which throw light on their predecessors, 

 are accurately noted, and especially, if the transverse 

 sections in the preceding figures and in Plate IV., repre- 

 senting the complete vertebrate body and its germ, are 

 carefully compared, the reader will probably obtain a clear 

 conception of the main features of mammalian Ontogeny. 

 A close and thoughtful comparison of the transverse sections 

 is of the greatest -importance in this respect. 



It is true, however, that a deeper, phylogenetic know- 

 ledge of these complex processes can only be gained with 

 the aid of Comparative Anatomy and Ontogeny. These 

 teach us that the ontogenetic process wdiich we have 

 described as resulting in the formation of the Vertebrate 

 must be explained as kenogenetic, and that, in consequence 

 of continual embryonic adaptation, these processes have 

 departed very widely from the original palingenetic form. 

 The Amphioxus alone of all living Vertebrates has, in con- 

 sequence of tenacious heredity, approximately retained the 

 palingenetic form.^^ (Cf Chapters XIII. and XIV.) 



As yet we have paid no attention to the various sections 

 which are distinguishable in the length of the body : the 

 head, neck, breast, abdomen, tail, etc. The transverse 

 sections do not help us in this respect, and we must, there- 

 fore, closely observe the articulation in the longitudinal axi.s 

 of the mammalian body. 



