6 INTRODUCTION. 



from them any such important generalizations as 

 may lead to a comprehensive theory, have been 

 the work of different individuals. In sciences, in- 

 deed, which have made any considerable advance, 

 such division of labour becomes unavoidable. The 

 observer, therefore, need not be discouraged, be- 

 cause he is not possessed himself of those attain- 

 ments necessary for proceeding to the investigation 

 of such principles as his observations may assist in 

 establishing. He may leave this to others, and 

 content himself, if he will, with a more subordi- 

 nate part. And in Natural History especially it is 

 almost necessarily the case that the observer and 

 the theorist should be in some measure separated. 

 For the facts here required are of such a kind as 

 cannot all be procured at will, or in any situation 

 we please. Many of them call for opportunities of 

 a very peculiar nature ; and those who enjoy such 

 opportunities are not unfrequently, by that very 

 circumstance, shut out from making any applica- 

 tion themselves of the knowledge they have ac- 

 quired to the furtherance of the general interests 

 of the science. 



(5.) That we may see this the more clearly, let 

 us stop to take a general view of the descriptions 

 of facts which are wanted by the naturalist to 

 enable him to proceed in his inquiries into the 

 general principles upon which Nature seems to 

 have based her system. For this purpose he must 

 have under his view all the different species and 

 varieties of animals with which this earth is peo- 

 pled ; and he ought to be able to inspect them, not 



