xir 



XT 

 X 



W 



FIG. 36. Diagram showing the ineffectiveness of selection through twelve 

 generations within a homozygous strain in the case of the Colorado 

 potato-beetle (Leptinotarsa) . In each generation extreme dark speci- 

 mens were selected as the parents of the succeeding generation but the 

 progeny always swung back to the type. After Tower. 



