126 
MAMMALS 
getting in the squirrel, for burrowing and locomotion in the 
moles, for flying in the bats, and for swimming in the seals. 
In all fore-limbs of mammals, even in those as different as 
the leg of the squirrel, the flipper of the seal, and the wing 
of the bat, the arrange¬ 
ment of the bones is the 
same. The hind-legs of 
mammals do not show 
so much variation as the 
fore-limbs. But in some 
cases, as in the whale, 
the hind-legs have almost 
disappeared through dis¬ 
use, and there is no ex¬ 
ternal evidence of them. 
Some animals, like the 
bears, walk on the soles 
of their feet, and some, 
like the cats and the 
dogs, walk on all their 
toes. In some mammals 
there is a variation in the 
number of the toes. For 
example, the cow walks 
on two toes and the horse 
on one toe, the hoof being 
a modified toe nail. In 
such cases the other toes 
are entirely lacking or rudimentary (not perfectly developed). 
Figure 115 . — Flying Squirrel. 
The skin is stretched between the fore- 
and hind-legs and acts like a parachute. 
Does the animal really fly ? 
103. The Horse. — The horse is interesting because it has 
been associated with man since the pre-historic period known 
as the Stone Age. It has been suggested that man “ first 
hunted horses for food, then drove them, and finally used 
them for riding and as beasts of burden.” The fine animals 
which we see to-day have gradually developed through this 
