170 ESSAY ON CLASSIFICATION 



of such writers as are celebrated in the annals of science for having 

 characterized with particular felicity any one kind of these groups, 

 and I have mentioned Latreille as prominent among zoologists for 

 the precision with which he has defined the genera of Crustacea and 

 Insects, upon which he has written the most extensive work extant.^^ 

 An anecdote which I have often heard repeated by entomologists who 

 knew Latreille well is very characteristic as to the meaning he con- 

 nected with the idea of genera. At the time he was preparing the 

 work just mentioned he lost no opportunity of obtaining specimens, 

 the better to ascertain from nature the generic peculiarities of these 

 animals, and he used to apply to the entomologists for contributions 

 to his collection. It was not show specimens he cared to obtain, any 

 would do, for he used to say he wanted them only "to examine their 

 parts." Have we not here a hint from a master to teach us what 

 genera are and how they should be characterized? Is it not the special 

 structure of some part or other which characterizes genera? Is it not 

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

 mate details of structure, which distinguishes one genus from an- 

 other? Latreille, in expressing the want he felt with reference to the 

 study of genera, has given us the key-note of their harmonious rela- 

 tions to one another. Genera are mosf closely allied groups of ani- 

 mals, 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 which can be given 

 of genera. They are not characterized by modifications of the fea- 

 tures 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 the 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 their representatives to the surrounding world 

 and to each other. Genera, in one word, are natural groups of a 

 peculiar kind, and their special distinction rests upon the ultimate 

 details of their structure. 



^Genera Crustaceorum et Insectorum (4 vols., Paris and Strasburg, 1806-1809). 



