8 THE DOCTRINE OF DESCENT. 



life, we must begin with a survey of the animal forms 

 now spread over the earth. As astronomy begins with 

 the mere classification of the stars and constellations, 

 and the knowledge of their apparent motions, so do 

 we also range our material in large groups, and this 

 in the manner offered by the historical development of 

 science. 



What first strikes the observer of the animal world 

 is, that it consists of apparently innumerable forms. 

 The primary requirement is discrimination and arrange- 

 ment. In the first stages of their development, zoology, 

 as well as botany and mineralogy, necessarily consisted 

 of mere descriptions, of a knowledge of objects in a 

 state of completeness. Physics and chemistry, on the 

 other hand, deal with the investigation of phenomena 

 directly referring to their origin, that is to say, with series 

 of phenomena mutually connected as causes and effects, 

 the knowledge of which, therefore, leads at once to 

 results satisfactory and tranquillizing to the mind. This 

 description, at first limited to the exterior, was gradually 

 extended to the interior, because zootomy and com- 

 parative anatomy, even more than fifty years ago, had 

 advanced so far in the accumulation of endless details 

 that Cuvier then ventured to found the Natural System. 



But this delineation of the animal world required 

 completion on two sides, and, as the science proceeded 

 towards perfection, it received it almost simultaneously 

 on both. To the knowledge of the existence of an 

 animal belongs also the description of its origin. I 

 say emphatically, " the description," for the history of 

 animal development is not as yet in itself a natural 

 science in the same sense as the mathematico-physical 



