Analysis of Heterogeneity in Complex Characters 269 



and of chance occurrence, dependent upon chance gametic agents, combinations, 

 and conditions present, as far as their appearance and frequency in nature are 

 concerned. 



The discussion and description of the foregoing relations in the pronotal pat- 

 tern are based upon materials held under uniform conditions of growth and 

 development and provided with optimum food, temperature, and other life rela- 

 tions. In nature the conditions of life are neither constant, uniform, nor 

 optimum, and probably no population ever exists that does not encounter more or 

 less in the way of adverse conditions at some time. It requires but little obser- 

 vation in nature to show that from generation to generation different aspects of 

 the population are presented, and these have usually been attributed to the effects 

 of external conditions. That this is true to some extent no one doubts, but the 

 exact role of external forces in any given case has yet to be determined. These 

 pronotal patterns are no exception to the rule, and this aspect of the problem will 

 be considered after a discussion of the conditions in nature have been presented. 



