VARIATION BY USE AND DISUSE. 
259 
tion in time produced a new organ or modified one. 
For instance, the giraffe, living upon the foliage of 
trees, developed its long neck and front legs by con¬ 
tinually reaching for its food. The American bison, 
when the white man first came to this country, was a 
short-legged, thick-bodied animal. But when he was 
hunted persistently on horseback by both Indians and 
whites, he developed longer legs and a more slender 
body, thus becoming much more capable of escaping 
by flight. 
Likewise, organs which were in the way or useless 
disappeared. The snake is supposed to have had 
limbs like the lizard, but living as it does in the under¬ 
brush and constantly forcing its way through between 
stems and roots, it found these limbs in the way, and 
by disuse gradually lost them, while the body grew 
to a disproportionate length. 
The wild duck has not only thicker muscles on 
the breast-bone than the tame duck, but also a pro¬ 
portionately larger brain. It needs to be more on 
the alert for danger and so exercises its brain more. 
The domestic duck is less on the wing and uses its 
feet more; hence it has weaker wings and longer, 
stronger legs than the wild one. 
The fiying fish, as mentioned in Book lY, has the 
pectoral fins much enlarged by exercise. The mole 
and the fishes in Mammoth Cave have lost their eyes 
by disuse. 
The kangaroo has long, strong hind legs and 
short, weak front limbs, for he uses the former con¬ 
stantly and the latter but seldom. The legs of the 
