LAMARCK'S THEORY OF DESCENT 



3" 



" We are not concerned here with the shrinkage 

 and shortening produced by a puckering of the parts, 

 which permit ordinary extension, if instead of a con- 

 tinued emptiness these viscera should be filled ; the 

 shrinkage and shortening in question are real, consider- 

 able, and such that these organs would burst open 

 rather than yield suddenly to the causes which would 

 require ordinary extension. 



" In circumstances of persons of the same age, com- 

 pare a man who, in order to devote himself to habitual 

 study and mental work, which have rendered his di- 

 gestion more difficult, has contracted the habit of 

 eating lightly, with another who habitually takes a 

 good deal of exercise, walks out often, and eats 

 heartily ; the stomach of the first will be weakened, 

 and a small quantity of food will fill it, while that of 

 the second will be not only maintained in its ordinary 

 health but even strengthened. 



" We have here the case of an organ much modi- 

 fied in its dimensions and in its faculties by the single 

 cause of a change in habits during the life of the 

 individual. 



" The frequent use of an organ become constant by 

 habit increases the faculties of this organ, even develops 

 it, and enables it to acquire dimensions and a power of 

 action which it does not possess in animals which exer- 

 cise less. 



" We have just said that the lack of employment 

 of an organ which necessarily exists modifies it, im- 

 poverishes it, and ends by its disappearing entirely. 



" I shall now demonstrate that the continued em- 

 ployment of an organ, with the efforts made to draw 

 out its powers under circumstances where it would 

 be of service, strengthens, extends, and enlarges this 

 organ, or creates a new one which can exercise the 

 necessary functions. 



" The bird which necessity drives to the watertofind 

 there prey fitted for its sustenance, opens the digits of 



