The Causes of Individual Differences 33 



by means of which, under the stress of circumstances, 

 they are able to live together. Plants have adapted 

 themselves to other plants as truly as to soil or climate ; 

 and if these latter environments are ever the sources or 

 causes of variation, then the first must be also. We must 

 look upon the struggle for existence, therefore, as itself 

 a cause of individual differences, since we know that any 

 continued pressure from without awakens an adaptive 

 response in the form of the vegetable organisms. 



