130 THE CHARACTERS OF GENERA. 



urally bind all the representatives of a natural 

 Family in minor groups. 



Genera, then, are divisions of a more restricted 

 character than any of those we have examined 

 thus far. Some of them include only one Spe- 

 cies, while others comprise hundreds ; since cer- 

 tain definite combinations of characters may be 

 limited to a single Species, while other combiiiiv- 

 tions may be repeated in many. We have strik- 

 ing examples of this among Birds : the Ostrich 

 stands alone in its Genus, while the number of 

 Species among the Warblers is very great. 

 Among Mammalia the Giraffe also stands alone, 

 while Mice and Squirrels include many Species. 

 Genera are founded, not, as we have seen, on 

 general structural characters, but on the finish of 

 special parts, as, for instance, on the dentition. 

 The Cats have only four grinders in the upper 

 jaw and three in the lower, while the Hyenas 

 have one more above and below, and the Dogs 

 and Wolves have two more above and two more 

 below. In the last, some of the teeth have also 

 flat surfaces for crushing the food, adapted es- 

 pecially to their habits, since they live on vegeta- 

 ble as well as animal substances. The formation 

 of the claws is another generic feature. There 

 is a curious example with reference to this in the 

 Cheetah, which is again a Genus containing only 

 »ne Species. It belongs to the Cat Family, buj 



