THE CLASSIFICATION OF ANIMALS. 121 



such as sub-kingdonis, pliyla, divisions, classes, 

 orders, etc. ; but none of these names can be rigidly 

 applied throughout a classification, because the various 

 groups and sub-groups'are not of co-equnl value, and 

 to apply the same terms to them, is therefore often 

 misleading. But the following system of naming the 

 divisions is that generally followed. Each of the chief 

 divisions of the animal kingdom, sometimes called 

 sub-kingdoms, but now more usually spoken of as 

 phyla, is divided into sub-divisions called classes, 

 while the classes are again divided into orders, and 

 these again into families. The necessities of classifica- 

 tion often require also the introduction of intermediate 

 sub-divisions, which may receive various names, such 

 as groups, tribes, sub-orders, etc. Bach order usually 

 comprises a number of genera, and each genus a 

 number of species. It must be remembered, how- 

 ever, in mjikiug use of this system of nomenclature, 

 that it survives from a time when the animal kingdom 

 was supposed to have been created in a set of neat 

 little series, so to speak, each made up according to 

 a definite plan, whereas we now know that the differ- 

 ences between different animals are the expression 

 of the outward conditions under which the ancestors 

 of those animals have been placed, and are therefore 

 infinitely various in nature and degree. Therefore, 

 if we agree for the sake of convenience to speak of 

 "orders," or of "classes," we must remember that the 

 distinction between two " classes " in one group may 

 be much greater than the distinction between two 



