Chap. X. CLASSIFICATION. 99 



in the Animal Kingdom, it furnishes us at tlie same 

 time with the means, by the comparison of these various 

 forms_, of recognising the truly essential, the type of 

 these organs, and separating therefrom everything un- 

 essential. In this, developmental history serves it as a 

 check or test. Thus, as the idea of development is not 

 that of mere increase of size, but that of progress from 

 what is not yet distinguished, but which potentially 

 contains the distinction in itself, to the actually dis- 

 tinct, — it is clear, that the less an organ is developed, so 

 much the more does it approach the type, and that, 

 during its development, it more and more acquires 

 peculiarities. The types discovered by comparative 

 anatomy and developmental history must therefore 

 agree." 



Then, after Johannes Müller has combated the idea 

 of a graduated scale of animals, and of the passage 

 through several animal grades during development, he 

 continues : — " What is true in this idea is, that every 

 embryo at first bears only the type of its section, from 

 which the type of the Class, Order, &c., is only after- 

 wards developed." 



In 1856, in an elementary work,^ in which it is usual 

 to admit only what are regarded as the assured acquisi- 

 tions of science, Agassiz expresses himself as follows : — 



" The ovarian eggs of all animals are perfectly identi- 

 cal, small cells with a vitellus, germinal vesicle and 

 germinal spot" (§ 278). " T7ie organs of the lody are 



3 ' Principles of Zoology/ Part I. Comparative Physiology. By Louis 

 Agassiz and A. A. Gould., Eevised Edition. Boston, 1856. 



H 2 



