110 FAMILY CHARACTEEISTICS. 



mon parlance, the children or the descendants 

 of known parents ; they understand by this name 

 natural groups including different kinds of ani- 

 mals, having no genetic relations so far as we 

 know, but agreeing with one another closely 

 enough to leave the impression of a more or 

 less remote common parentage. The difficulty 

 here consists in determining the natural limits 

 of such groups, and in tracing the characteristic 

 features by which they may be defined ; for in- 

 dividual investigators differ greatly as to the de- 

 gree of resemblance existing between the mem- 

 bers of many Families, and there is no kind of 

 group which presents greater diversity of circum- 

 scription in the classifications of animals pro- 

 posed by different naturalists than these so-called 

 Families. 



It should be remembered, however, that, unless 

 a sound criterion be applied to the limitation of 

 Families, they, like all other groups introduced 

 into zoological systems, must forever remain arbi- 

 trary divisions, as they have been hitherto. A 

 retrospective glance at the progress of our sci- 

 ence during the past century, in this connection, 

 may perhaps help us to solve the difficulty. 

 Linnaeus, in his System of Nature, does not ad- 

 mit Families ; he has only four kinds of groups, 

 Classes, Orders, Genera, and Species. It was, 

 as I have stated in a previous chapter, among 



