4 Mammals of Eastern Asia 
hind feet to carry the weight of the body, while the little-used 
outer hind toes and the forelimbs become proportionately 
smaller. 
Aquatic life has had either moderate or profound effects upon 
the limbs. The amphibious existence of Water Rats, Water 
Shrews, Otter Civets, Otters, and Water Opossums produces 
only minor changes in the feet. The soles and tactile surfaces of 
the toes may become soft, naked, and rubbery and the pads 
blend together. Fringes of stiff hairs that become erect and firm 
during the swimming stroke may be developed. The skin web 
between the fingers may be enlarged or the claws may be re- 
duced as in the Small-clawed Otter. But in mammals as fully 
aquatic as the Seals, Whales, and Dugongs, much greater 
changes have taken place. Partial or total obsolescence has oc- 
curred in the hind limbs, and the forelimbs have been altered 
into paddles. 
To permit travel for considerable distances through the air, 
modification of the limbs has taken two distinct courses ; one is 
the development of gliding membranes attached to the arms and 
legs, and the other is the development of real wings and the 
power of true flight. Gliding membranes have developed in four 
distinct groups of mammals : the Flying Squirrels, the Flying 
Lemurs, the Phalangerine Flying Marsupials, and the Phasco- 
larctine Flying Marsupials. But the problem of true flight has 
been solved only by the Bats, through enormous enlargement of 
the arm, hand, and fingers, and expansion of the skin covering 
them. 
Teeth. The teeth, quite as variously modified as the limbs, 
have two associated functions — getting the food into the mouth, 
and cutting, crushing, or grinding it in preparation for swallow- 
ing. It is logical that the food-grasping teeth are at the front 
of the mouth and the food-treating teeth behind them. 
The generalized type of mammalian dentition is not to be 
sought among the Shrews, where we saw the generalized foot. 
