92 STUDIES IN ANIMAL LIFE. 



face and Tomkinson with the square forehead, we 

 simply say John and William Tomkinson. 



Linnaeus did more than this. He not only fixed 

 definite conceptions of species and genera, but in- 

 troduced those of orders and classes. Cuvier added 

 families to genera, and sub-kingdoms (embranche- 

 m&nts) to classes. Thus a scheme was elaborated 

 by which the whole animal kingdom was arranged 

 in subordinate groups : the sub-kingdoms were di- 

 vided into classes, the classes into orders, the orders 

 into families, the families into genera, the genera 

 into species, and the species into varieties. The 

 guiding principle of anatomical resemblance determ- 

 ined each of these divisions. Those largest groups, 

 which resemble each other only in having what is 

 called the typical character in common, are brought 

 together under the first head. Thus all the groups 

 which agree in possessing a backbone and internal 

 skeleton, although they differ widely in form, struc- 

 ture, and habitat, do nevertheless resemble each 

 other more than they resemble the groups which 

 have no backbone. This great division having been 

 formed, it is seen to arrange itself in very obvious 

 minor divisions or classes the mammalia, birds, 

 reptiles, and fishes. All mammals resemble each 

 other more than they resemble birds ; all reptiles 

 resemble each other more than they resemble fishes 

 (in spite of the superficial resemblance between ser- 

 pents and eels or lampreys). Each class, again, 

 falls into the minor groups of orders, and on the 



