8 THE DOCTRINE OF DESCENT. 



continuity of 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 con- 

 stellations, 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 arrangement. 

 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 com- 

 pleteness. Physics and chemistry, on the other hand, 

 deal with the investigation of phenomena directly refer- 

 ring to their origin, that is to say, with series of phe- 

 nomena mutually connected as causes and effects, the 

 knowledge of which, therefore, leads at once to results 

 satisfactory and tranquillizing to the mind. This de- 

 scription, at first limited to the exterior, was gradually 

 extended to the interior, because zootomy and compara- 

 tive anatomy, even more than fifty years ago, had ad- 

 vanced 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 sciences; it 



