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 has 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 skiU which 

 could imagine and produce such an extraordinary di- 

 versity of forms under which Hving 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 philosophers 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 



I * See Freliminar; Discourie, p. 255. 



