I08 THE EVOLUTION OF MAN. 



all essential points in the highest degree similar, even 

 though the outward forms are extremely unlike. Man, 

 accordingly, in all essential features of internal organization 

 so closely resembles other Mammals, that no comparative 

 anatomist has ever doubted that he belongs to that class. 

 The whole inner structure of the human body, — the disposi- 

 tion of its various systems of organs, — the arrangement of 

 the bones, muscles, blood-vessels, and the like, — the coarser 

 and more minute structure of all these organs, corresponds so 

 well with that of all other Mammals, — such as Apes, Gnawing 

 animals (Rodentia), Hoofed animals (Ungiilata), Whales, 

 and Oppossums, — that the complete dissimilarity of the 

 outward form is as nothing in the balance aojainst it. We 

 learn also from Comparative Anatomy that the fundamental 

 characteristics of animal organization are so much alike, 

 even within the various classes, numbering from thirty 

 to fort}^ in all, that they may fittingly be arranged in from 

 six to eight principal groups. But even in these few groups, 

 which represent the lineages or types of the animal kingdom, 

 it can be shown that certain organs, especially the intestinal 

 canal, were originally uniform. 



. We can only explain this most essential uniformity in 

 all these various animals, notwithstanding their great ex- 

 ternal dissimilarity, by the aid of the Theory of Descent. 

 Only by considering the internal correspondence as the 

 result of Heredity from common ancestral forms, and the 

 external dissimilarity as the result of Adaptation to varied 

 conditions of life, can this wonderful fact be thoroughly 

 understood. 



The recognition of this truth raised Comparative 

 Anatomy itself to a higher rank, so that Gegenbaur,^'' the 



