222 HEREDITY AND ENVIRONMENT 



is hereditary or environmental, but in the case 

 of animals and plants, where experiments may 

 be performed on a large scale, it is possible to 

 make such tests by (1) experiments in which 

 the environment is kept as uniform as possible 

 while the hereditary factors differ, and (2) ex- 

 periments in which, in a series of cases, the 

 hereditary factors are fairly constant while 

 the environment differs. In this way the dif- 

 ferential cause or causes of any character may 

 be located in heredity, in environment or in 

 both. 



The observational and statistical study of 

 inheritance helped to outline the problem but 

 did little to solve it. Certain phenomena of 

 hereditary resemblance between ascendants 

 and descendants were made intelligible, but 

 there were many peculiar and apparently ir- 

 regular or lawless phenomena which could not 

 be predicted before they occurred nor ex- 

 plained afterward. For example when Dar- 

 win crossed different breeds of domestic 

 pigeons, no one of which had a trace of blue 

 in its plumage, he sometimes obtained off- 

 spring with more or less of the blue color and 



