8o Heredity and Environment 



physiological, and averages are of less value in finding the causes 

 of such phenomena than is the intensive study of individual cases. 



By observation alone it is usually impossible to distinguish be- 

 tween inherited and environmental resemblances and differences, 

 and yet this distinction is essential to any study of inheritance. If 

 all sorts of likenesses and unlikenesses are lumped together, 

 whether inherited or not, our study of inheritance can only end 

 in confusion. The value of statistics depends upon a proper 

 classification of the things measured and enumerated, and if 

 things which are not commensurable are grouped together the 

 results may be quite misleading and worthless. 



Statistical Studies Insufficient. — Unfortunately Galton and 

 Pearson, as well as some of their followers, have not always 

 carefully distinguished between hereditary and environmental 

 characters. Furthermore much of their material was drawn 

 from a general population in which were many different fam- 

 ilies and lines not closely related genetically. Consequently their 

 statistical studies are of little value in discovering the physio- 

 logical principles or laws of heredity. Jennings (1910) well says, 

 "Galton's laws of regression and of ancestral inheritance are the 

 product mainly of a lack of distinction between two absolutely 

 diverse things, between non-inheritable fluctuations on the one 

 hand, and permanent genotypic differentiations on the other." 

 In the case of man we have few certain tests to determine whether 

 the differential cause of any character is hereditary or environ- 

 mental, 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) 

 experiments in which, in a series of cases, the hereditary factors 

 are fairly constant while the environment differs. In this way 

 the differential cause or causes of any character may be located 

 in heredity, in environment or in both. 



The observational and statistical study of inheritance helped 



