What Is a Mammal? 3 
especially the forelimbs, become short, thick, heavily muscled, 
with the hand broad and the claws lengthened, broadened, and 
flattened. Examples of this include the Badgers, the Moles, the 
Mole Rats. Ability in climbing comes about either by develop- 
ment of grasping, hand-like organs as in Tarsiers and Monkeys, 
in which the unused claws become nails, or by alteration of the 
normal claws into short, sharp, strong, hook-like structures as 
in the Martens, Cats, Squirrels, Dormice, Marmosets, and some 
Civets. Claws become modified for food-getting also, as in the 
great enlargement to be seen in Anteaters and Pangolins, used 
for tearing open termite nests, or the special device for retract- 
ing the claws into sheaths seen in Cats and Linsangs, both of 
which use their claws for seizing living prey as well as for 
climbing. 
Ability to run well leads to a tapered form of limb as in the 
Rabbits, and to a tendency to reduction of the number of digits 
as seen in the Dogs and many even-toed and odd-toed ungulates. 
But the evolution of the claws in these groups differs radically. 
In Rabbits and Dogs the claws remain essentially claw-like; 
they are blunt but still effective scratching organs. In the ungu- 
lates, the claws have become thickened, shortened, and hoof- 
like. 
The Elephants, in response to their enormous body weight, 
have grown pillar-like legs that are centered over a single axis 
and lack the ability to spring and flex that the legs of Dogs, 
Horses, and Rabbits show. Although the five toes in Elephants 
are retained, they are shortened and tipped with small thick 
hoofs, while the short, rounded foot rests on a thick, elastic, 
fatty sole. 
From the spring-like, narrowed, few-toed foot of the Dogs 
and rat-like rodents it is a rather short step to the hopping type 
of foot found in Jerboas, Jumping Mice, and Kangaroos. Con- 
siderable lengthening of the foot takes place, and at the same 
time a tendency appears for only one or two of the toes of the 
