232 ESSAY ON CLASSIFICATION. 



of the classes, families subdivisions of orders, genera sub- 

 divisions of families, and species subdivisions of the 

 genera ; but not in the sense that each type should 

 necessarily include the same number of classes, nor even 

 necessarily several classes, as this must depend upon the 

 manner in which, the type is carried out. A class, again, 

 might contain no orders, 1 if its representatives presented 

 no different degrees, characterized by the greater or less 

 complication of their structure ; or it may contain many 

 or few, as these gradations are more or less numerous 

 and well marked ; but as the representatives of any and 

 every class have of necessity a definite form, each class 

 must contain at least one family, or many families, in- 

 deed, as many as there are systems of forms under which 

 its representatives may be combined, if form can be 

 shown to be characteristic of families. The same is the 

 case with genera and species; and nothing is more 

 remote from the truth than the idea that a genus is 

 better defined in proportion as it contains a greater 

 number of species, or that it may be necessary to 

 know several species of a genus before its existence 

 can be fully ascertained. A genus may be more satis- 

 factorily characterized, its peculiarity more fully ascer- 

 tained, and its limits better defined, when we know all its 

 representatives ; but I am satisfied that any natural genus 

 may be at least pointed out, however numerous its spe- 

 cies may be, from the examination of any single one of 

 them. Moreover, the number of genera, both in the ani- 

 mal and vegetable kingdoms, which contain but a single 

 species, is so great, that it is a matter of necessity in all 

 these cases to ascertain their generic characteristics from 

 that one species. Again, such species require to be cha- 



1 See Chap. I, Sect. 1. 



