SOURCES OF CHANGE 57 



nal environment, i.e., in the body of the individual, 

 without immediate cause in the other factors of 

 life, must be admitted. The body is exceedingly 

 complex and it is quite probable that reactions 

 may occur in it which are not immediately de- 

 pendent upon external conditions or upon the ac- 

 tion of hereditary units. Changes due to increas- 

 ing age are undoubtedly of this type in part. Since 

 the body is an intermediary between heritage and 

 environment, however, these changes seem less 

 significant in evolution than those correlated with 

 external stimuli. Whatever takes place in the body 

 is possible by virtue of properties received from 

 the heritage and developed in response to external 

 conditions, hence purely somatic actions are more 

 likely to be a result of, and in harmony with, 

 normal activity than to be a source of novelty. 



Fluctuations of the heritage occur, as we have 

 seen, by chance recombination within the limits 

 of the species, but they are of interest to us only 

 in their relations with other factors. 



Thus far the appearance of new unit characters 

 in the constitution of a species is limited to one 

 known process, mutation. It has been easy to 

 look upon mutations as fortuitous occurrences. 

 Indeed, they are a part of the heritage and have 

 their basis in the chromosomes, but they cannot be 

 referred to the action of the chromosomes alone 

 for these bodies, like all living matter, exist only 



