206 THE PROBLEMS OF EVOLUTION 



the living substance in any form is able to respond 

 to conditions demanding the exercise of a given 

 function by the gradual increase of its capacity 

 for that function, and if the type of activity excited 

 by the environmental stimulus results in a visible 

 development, that too may be expected to increase. 

 Conversely, no part of the organism and none of 

 its functions may be expected to develop beyond 

 the degree favored by the conditions of its exist- 

 ence. The possible limits of the cumulative process 

 can only be estimated. When an organ is absolutely 

 unused its reduction may proceed in the individual 

 to a state of atrophy, but the body maintains it 

 in some degree of development. When excessive 

 activity is demanded of an organ, the individual 

 suffers; its capacity for response is apparently not 

 unlimited, but it may be extended to an unsus- 

 pected degree by the cumulative effects of response 

 to a long-continued or gradually increasing stimu- 

 lus. Exact information on this problem is greatly 

 to be desired. At present we can only rest on this 

 significant but incomplete evidence. 



Granting that cumulative change is possible, 

 even in the individual, we have found a possible 

 source of chromosomal change. Acquired charac- 

 ters are cytoplasmic in their expression, but so 

 also are inherited characters. Cytoplasm is the 

 material of construction in which all characters 

 are wrought. But we have seen that cytoplasm 



