5o8 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. 



