SECONDARY OR DIRECTIVE FACTORS. 



8n 



factor in evolution will vary according to stringency of the 

 eliminating process, and it must be noted that the " struggle 



FIG. 378. Diagram showing hypothetically the action of natural 

 selection in the evolution of a white race of mice from a 

 dark-coloured stock. The arrows indicate elimination ; the 

 successive levels indicate generations. After Schonichen. 



for existence" varies in intensity within wide limits, that 

 it requires to be investigated for each case, and cannot be 

 postulated as a force of nature. 



