238 THE THEORIES OF EVOLUTION 



Generally speaking, modifications in plants are due 

 to changes in nutrition, while modifications in animals 

 are produced in a more indirect way by what Lamarck 

 calls "habits.'* "Important changes in conditions 

 bring about important changes in the animals' needs 

 and changes in their needs bring about changes in 

 their actions. If the new needs become constant or 

 durable, the animals acquire new habits which are as 

 durable as the needs which created them. . . . 

 Whenever new conditions, becoming constant, impart 

 new habits to a race of animals, that is, induce them 

 to perform new actions which become habitual, these 

 habitual actions lead to the use of a certain part in 

 preference to another, or to the total disuse of a part 

 which is now useless." 4 



This, in turn, leads to a modification in shape and 

 structure. "The lack of use of an organ, made con- 

 stant by acquired habits, weakens it gradually until 

 it degenerates or even disappears entirely. The ver- 

 tebrates whose general plan of organisation is almost 

 identical, although they present a great variety of de- 

 tails, are supposed to have their jaws armed with 

 teeth; in those, however, which, owing to special cir- 

 cumstances, acquired the habit of swallowing their 

 food without first masticating it, teeth have not at- 

 tained any development. Either the teeth have re- 



4 Vol. I, pp. 223-224. 



