38 Introduction to Animal 



in all essential characters, leading to the belief that 

 they have descended directly from a common stock. 

 A group of species closely resembling each other in 

 their general characters is named a genus. A group 

 of genera bound together by natural affinities is a 

 family ', a group of allied families, an order* of allied 

 orders a class, of allied classes a sub-kingdom, which 

 is the highest category in the predicamental line 

 under the animal kingdom. The embodiment of the 

 special form-characters of any of the categories is 

 called its type. Thus \ve speak of the type of a genus, 

 family, &c. When one type form unites in itself the 

 leading special characters of two or three diverse 

 forms, it is called a synthetic type. Such forms were 

 common in earlier periods of the world's history. 

 Type-forms which retain as characters many of the 

 embryonic features of the entire superior category in- 

 cluding them, are called generalised types. These are 

 the poorest in species, and the most local in distri- 

 bution of animal forms. The arrangement of indivi- 

 duals in these categories we know as classification^ and 

 this is often a matter of difficulty, as no two individuals 

 are precisely alike. In categories of which we have 

 many individuals, connecting links between appa- 

 rently different forms constantly occur, so that a 

 systematist has either to make a large number of 

 small groups or the opposite (witness the family 

 Helicidae, and in botany the genera, Hieracium, Rubus, 

 and Salix). We cannot define the exact limits of any 

 species until we know its range of variation, which 

 has never yet been precisely ascertained for any single 



* When further divisions are required, we speak of orders beinjj divided 

 into sub-orders, and classes into sub-classes. 



