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 
