of functional changes. Lamarck's explanation of the snake's 

 elongated legless body, and the duck's webbed feet is given 

 by himself as follows: 



"It is part of the plan of organization of reptiles, as 

 well as of other vertebrates, that they have four legs at- 

 tached to their skeleton .... but snakes acquired the habit 

 of gliding over the ground and concealing themsleves in 

 the grass; owing to their repeated efforts to elongate them- 

 selves, 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; 



Fig. 1. — LAMARCK 



long legs would interfere with the snakes' desire for gliding 

 and short ones could not move their bodies, for they can 

 pnly have four of them. Continued lack of use of the legs 

 m 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 or- 

 gan, develops it and causes it to acquire a size and strength 

 It does not possess in animals which exercise less. A bird, 



12 



