PBIMABY DIVISIONS AND CLASSES. 



199 



modifying the localization of the functions and the division 

 of the physiological result. 



Although it is easy for the zoologist to distinguish the 

 four groups just mentioned, and to refer to one or the other 

 of them an animal under examination, yet there are some 

 beings which seem connected with two different types, like 

 border lands whose rights of possession have not yet been 

 determined. 



Fig. 138. Skeleton of the Ostrich. 



It sometimes also happens that it is difficult to define rigo- 

 rously these four groups ; but, to give an exact idea, it will 

 be sufficient to indicate the more prominent characters pecu- 

 liar to each type, and to remark, that the reunion of all these 

 characters is not always to be met with, sometimes one and 

 sometimes another being effaced as we descend to the 

 limits of the division. 



