508 STRUCTURE AND LIFE HISTORIES 
preference to another, or to the total disuse of a part which 
is now useless. . . . The lack of use of an organ, made 
constant by acquired habits, weakens it gradually until 
it degenerates or even disappears entirely." Thus, "it 
is part of the plan of organization of reptiles, as well as of 
other vertebrates, that they have four legs attached to 
their skeleton . . . but snakes acquired the habit of glid- 
ing over the ground and concealing themselves in the grass; 
owing to their repeated efforts to elongate themselves, in 
order to pass through narrow spaces, their bodies have 
acquired a considerable length, not commensurate with 
their width. Under the circumstances, legs would serve 
no purpose and, consequently, would not be used, long 
legs would interfere with the snakes' desire for gliding, 
and short ones could not move their bodies, for they can 
only have four of them. Continued lack of use of the 
legs in snakes caused them to disappear, although they 
were really included in the plan of organization of those 
animals." 
On the other hand, "the frequent use of an organ, made 
constant by habit, increases the faculties of that organ, 
develops it and causes it to acquire a size and strength it 
does not possess in animals which exercise less. A bird, 
driven through want to water, to find the prey on which 
it feeds, will separate its toes whenever it strikes the water 
or wishes to displace itself on its surface. The skin uniting 
the bases of the toes acquires, through the repeated separ- 
ating of the toes, the habit of stretching; and in this way 
the broad membrane between the toes of ducks and geese 
has acquired the appearance we observe to-day." 
If such modifications are acquired by both sexes they 
are transmitted by heredity from generation to generation. 
