; 
1884.] Growth, tts Conditions and Variations. 1095 
by considerable labor. Thus their food supply is greatly limited. 
The gnawing teeth of rodents are inefficient weapons as com- 
pared with the cutting teeth of Carnivora and the grinding teeth 
of the large Herbivora. The same remarks might be applied to 
the food and weapons of the Insectivora. Again, among rodents 
and in fact among all classes, the tree livers are smaller than the 
ground livers. They need great agility, their food is usually of 
reduced quantity, if vegetable food it is usually placed at the ex- ° 
tremity of the small branches, if animal it escapes to this exfrem- 
ity, so that great bulk would either reduce the food-getting 
power, or the safety from danger. The largest arboreal animals 
are the climbing cats, which obtain their prey by springing, 
and these are much smaller than the ground-lurking cats. On 
the contrary the coiling and crashing serpents attain their great- 
est size in trees, which seem to offer them a special advantage in 
the capture of large prey. 
In regard to those grass and leaf-eating Herbivora which have 
learned to defend themselves by weapons instead of by flight, and 
thus to avoid the excessive exertion of their timid relatives, their 
size is greatly influenced by the degree of exertion necessary to 
obtain a supply of food ;- that is, on the abundance of food native 
to their habitat. The camel, for instance, is native to sandy 
deserts dotted with occasional grassy oases, so that it needs 
swift motion and great powers of endurance of hunger and 
thirst, to enable it to shift from pasture to pasture. The horse 
also is native to broad, level plains, but sparsely provided with 
pasturage, and needs swift motion to obtain a sufficient food sup- 
ply. A similar argument applies to the American bison and the 
larger deer. These animals, while attaining a considerable bulk, 
_ are much smaller than the slugglish tropical Herbivora, the ele- 
Phant, rhinoceros and hippopotamus, which live among super- 
abundant food, and have little occasion to fear enemies. 
Again, as tree-living animals are necessarily small, so are 
mountain animals of reduced bulk. Though they are in no great 
danger from carnivorous foes, their food supply is small, and can 
only be obtained by great exertion. Reference may here also be 
Made to the tribe of birds. These are necessarily smaller than 
land animals. The element in which they live requires great 
Muscular activity, which increases in a rapid proportion with 
increase in bulk. Their food supply is also limited, and only to 
