GENERA. 249 



be characterized ? Is it not the special structure of some 

 part or other, wliich characterizes genera \ Is it not the 

 finish of the organization of the body, as worked out in 

 the ultimate details of structure, which distinguishes one 

 genus from another ? Latreille, in expressing the want 

 he felt with reference to the study of genera, has given us 

 the key-note of their harmonious relations to one another. 

 Genera are most closely allied groups of animals, differing 

 neither in form nor in complication of structure, but 

 simply in the ultimate structural peculiarities of some of 

 their parts ; and this is, I believe, the best definition wliich 

 can be given of genera. They are not characterized by 

 modifications of the features of the families, for we have 

 seen that the prominent trait of family difference is to be 

 found in a typical form; and genera of the same family 

 may not differ at all in form. Nor are genera merely a 

 more comprehensive mould than species embracing a 

 wide range of characteristics ; for species in a natural 

 genus should not present any structural differences, but 

 only such as express the most special relations of then- 

 representatives to the surrounding world and to each 

 other. Genera, in one word, are natural groups of a pecu- 

 liar kind; and their special distinction rests upon the 

 ultimate details of their structure. 



SECTION VI. 



SPECIES. 



It is generally believed that nothing is easier than to 

 determine species ; and that, of all the degrees of relation- 

 ship which animals exhibit, that which constitutes specific 

 identity is the most clearly defined. An unfailing crite- 

 rion of specific identity is even supposed to exist in the 



