130 ON SYSTEMATIC ZOOLOGY. 
offer an explanation of these resemblances ; and if the 
theory by which this is done can be reduced to one sim- 
ple and universal law, we may feel assured that law 
forms part of the system of nature. 
(182.) This brings us, thirdly, to the principle of 
variation, which bas long since been pronounced an im- 
portant characteristic of the natural system. The variety 
in nature appears infinite. If we only contemplate 
those beings which have passed under our own exa- 
mination, and which everywhere surround us, we 
cannot fail to be struck with that divine skill which 
could imagine and produce such an extraordinary di- 
versity of forms under which living beings should 
exist. It is obvious, therefore, that these, as ema- 
nating from a divine Creator, must have been pro- 
duced upon some one uniform plan. Hence it follows, 
that no system can be natural which does not aim at the 
partial developement of this plan, so far, indeed, as its 
comprehension is permitted to finite beings. The im- 
mense difficulties of attaining such an insight have in.. 
duced many of the most profound philoscphers to 
relinquish the search in despair, and have tempted others 
to pronounce it hopeless: but we are yet to learn the 
limits which have been assigned to the human under- 
standing in matters of physical research ; nor are there 
valid grounds for supposing that the discovery of those 
-laws which regulate the variation of animals is unat- 
| tainable, when those which regulate the motion of the 
» heavenly bodies have been detected. It is not enough to 
tell us in what manner such and such animals vary 
from each other; for that is to communicate nothing 
more than a mere matter of fact: the question is, upon 
what general principle is this variation regulated? 
Why do we observe, for instance, that one peculiar 
division of every natural group is aquatic, and another 
furnished with long tails?* What is the principle, 
in short, of these variations ? and how far is it applicable 
.* See Preliminary Discourse, p. 255. 
