THE THEORY OF ORGANIC EVOLUTION 169 



perfect" describes the situation and no one disputes the 

 claim that such changes do occur in the individual. The con- 

 verse, of the disuse of a part, is no less familiar. The man 

 of sedentary life is painfully aware of the reduced capacity of 

 his little used muscles after a day of unusual exercise. The 

 religious fanatics of certain countries, by the voluntary and 

 persistent disuse of a limb, bring about not only a loss of 

 function in the part but its permanent reduction in size. In 

 animals under experimentation, similar atrophies can be 

 produced. Changes are also induced in the individual by 

 environment. A mammal taken into a cold climate is 

 stimulated to lay on fat or produce longer hair. An insect 

 may be modified in color by a change in the water-content 

 of the atmosphere. The lowland tree, when growing upon a 

 mountain, is modified in a manner peculiar to the new en- 

 vironment. The human skin is tanned by the sun, hence 

 the white man, after long residence in the tropics, may 

 never recover his whiteness of face and hand, even though 

 he returns to his northern home. 



Many examples of the effects of use and disuse and of 

 environment will occur to the reader. Animals and plants 

 under experimentation give similar results. There can be no 

 question regarding the effects produced by these Lamarckian 

 factors of use, disuse, and environment upon the individual. 

 New characteristics are thus acquired by the individual; and 

 the phrase acquired characteristics has become a technical 

 term in biology, having this restricted meaning. 



Thus far, we have spoken only of the individual animal or 

 plant, which is itself changed by the action of these Lamarck- 

 ian factors. But the crux of the theory is its claim that 

 characteristics, thus acquired by the individual, are inherited 

 by the next generation. It is for this reason that the La- 

 marckian hypothesis may be described as the theory of 

 inheritance of acquired characteristics, using the words ac- 

 quired characteristics in the sense above explained. La- 

 marck believed that the changes so produced hi animals and 



