228 Evolution and Adaptation 
not move their long bodies. Since the plan of organization 
limits the snakes to only. four legs, and since this number 
would be useless, they have disappeared. 
Many insects are destitute of wings, although wings are a 
part of the plan of organization of this group. They are 
absent only in those forms whose habits render wings useless, 
consequently they have disappeared through disuse. 
The preceding cases are those in which the disuse of an 
organ has led to its degeneration. The following cases 
are cited to show that by use an organ increases in size. 
The formation of the web in the feet of water-birds has 
already been given as a character which Lamarck supposes 
to have been acquired through use; also the case of shore- 
birds, which, by an effort to elongate their legs, have actually 
made them so in the course of time. The necks of water- 
birds are also long on account of their having been stretched 
in the efforts to catch fish. The long tongues of the ant-eater, 
of the woodpecker, and of humming-birds are the result of 
use, and the long, forked tongue of serpents has come from 
their using their tongue to feel objects in front of them. 
Fishes that have acquired the habit of living in shallow 
water, flounders, soles, etc., have been forced to swim on their 
sides in order to approach nearer to the shore. Since more 
light comes from above than from below, the eye on the 
under side, straining to turn to the light, has finally migrated 
to the upper side. 
The habit of eating great quantities of food, which distends 
the digestive organs, has caused the bodies of herbivorous 
quadrupeds to become large, as seen in the elephant, the 
rhinoceros, oxen, horses, and buffaloes. The habit of stand- 
ing for a long time on their feet has caused some animals to 
develop hard, thick hoofs. Herbivorous animals, that inhabit 
countries where they are constantly subjected to attack, as 
deer and antelopes for example, are forced to escape by rapid 
flight, and in consequence their bodies have become slen- 
