Preface. ix 



pure strain, and how, in the same way as we 

 have ascertained the behaviour of chemical 

 elements and compounds. This requires trouble 

 and patience little more. 



The next requisite, of even greater importance, 

 is vastly more difficult to obtain, for it involves 

 some knowledge on our part of the mode 

 of action, if not of the nature, of the forces by 

 which the formation and precise arrangement 

 of the several parts of the mechanism of an 

 organism are determined, as well as of those 

 laws under the influence of which one creature, 

 say, is caused to be of one colour, while another, 

 in other respects very closely allied, has a 

 different colour imposed upon its correspond- 

 ing textures, and so with regard to other 

 characters. 



Until such knowledge is obtained, it is difficult 

 to see how any intelligent conception of the 

 relative importance of the various zoological 

 characters of living forms can be reached. 



