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 combina- 
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 
one Species. It belongs to the Cat Family, but 
