262 EVOLUTION AND GENETICS 



constantly in these regions, and minor adjustments are as con- 

 stantly taking place. In an area of less than a square mile the 

 writer once watched within a decade a transition from a beauti- 

 fully wooded valley to a weedy patch of dead and dying trees, 

 of prairie land to a field of sweet clover, and of a grassy meadow 

 to a slough, overgrown with coarse sedges and with water standing 

 in its lower spots, all without the interference of man. In the 

 same area it was often possible to see walnut trees defoliated by 

 larvae of a moth. The overpopulation of a tree resulted in the 

 destruction of the food supply, and the caterpillars were forced 

 to leave the tree in search for another, some perhaps successfully, 

 some perhaps not. Such cases are examples on a small scale of 

 the factors which bring about adaptation. 



Adaptations are the result of change, change in the organism 

 through influence of its internal environment, change brought 

 about by factors in the external inorganic environment, or change 

 due to competition within the species or with other organisms, 

 but always change. Static conditions do not demand fluctuating 

 response, and static organisms, if such could exist, would be 

 incapable of responding in more than one way. The organism 

 varies and everything about it varies. In varied responses, whether 

 migrations into new regions or reassociations within the same 

 limited region, lies the beginning of the many adaptations which 

 characterize the various creatures of the earth. Adaptation is a 

 process and that process is evolution. 



This much it is easy to say, but the fact remains that adaptation 

 as a process is visible to us only in the individual. In order to 

 accomplish the evolution of species it must be a process in the 

 entire aggregates of individuals that constitute species, and here 

 we face the complex problem of chronological as well as immediate 

 association of individuals. The unity of successive generations is 

 the field of heredity. It became evident many years ago that 

 the linldng of generation with generation was an important point 

 of attack for the solution of the problems of evolution and the 

 attention of many biologists has been turned upon it. Out of 

 their efforts has grown the science of genetics, whose importance, 

 both practical and purely scientific, is inestimable. 



Summary. The components of individual existence, viz., 

 heredity, environment and response, are variable. Change of 

 environment in the broad sense necessitates change of response 



