16 GENERAL PRINCIPLES OF ZOOLOGY. 



vertebrate animals, viz., Mollusca, Articulata, and Radiata. Cuvier 

 called these grades standing above the classes, provinces or chief 

 branches (embranchements), for which later the name Types was 

 introduced by Blainville. But still more important are the differ- 

 ences which appear in the structural basis of the system. Instead 

 of, like the earlier systematists, using a few external character- 

 istics for the division, Cuvier built upon the totality of internal 

 organization, as expressed in the relative positions of the most 

 important organs, especially the position of the nervous system, 

 as determining the arrangement of the other organs. " The 

 type is the relative position of parts" (von Baer). Thus for the 

 first time comparative anatomy was employed in the formation of 

 a natural system of animals. 



Lastly the type theory established an entirely new conception 

 of the arrangement of animals. Cuvier found prevalent the theory 

 that all animals formed a single connected series ascending from 

 the lowest infusorian to man; within this series the position of 

 each animal was definitely determined by the degree of its organi- 

 zation. On the other hand Cuvier taught that the animal kingdom 

 consisted of several co-ordinated unities, the types, which exist 

 quite independently side by side, within which again there are 

 higher and lower forms. The position of an animal is determined 

 by two factors : first, by its conformity to a type, by the structural 

 plan which it represents; second, by its degree of organization, by 

 the stage to which it attains within its type. 



Comparative Embryology. Evolution vs. Epigenesis. The 

 same results which Cuvier reached by the way of comparative 

 anatomy were attained two decades later by C. E. von Baer by the 

 aid of embryology. Embryology is the youngest branch of 

 zoology. What Aristotle really knew, what was written by 

 Fabricius ab Aquapendente and Malpighi upon the embryology of 

 the chick, did not rise above the range of aphorisms, and were not 

 of sufficient value to make a science. The difficulties of observa- 

 tion, due to the delicacy and the minuteness of the developmental 

 stages, were lessened by the invention of the microscope and 

 microscopical technique. Further, the prevailing philosophical 

 conceptions placed hindrances in the way; there was no belief in 

 Embryology in the present sense of the word; each organism was 

 thought to be laid down from the first complete in all its parts, 

 and only needed growth to unfold its organs (evolutio *) ; eithei the 



* This original meaning of ' evolution ' is different from that prevailing 

 at present. 



