PHENOMENA OF INHERITANCE 325 



Furthermore much of their material was drawn from a general popula- 

 tion in which were many different families and lines not closely related 

 genetically. Consequently their statistical studies are of little value in 

 discovering the physiological 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 dif- 

 ferentiations, 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 environmental, 

 but in the case of animals and plants, where experiments may be per- 

 formed on a large scale it is possible to make such tests by (1) experi- 

 ments in which 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 to out- 

 line the problem but did little to solve it. Certain phenomena of hered- 

 itary resemblances between ascendants and descendants were made in- 

 telligible, but there were many peculiar and apparently irregular or law- 

 less phenomena which could not be predicted before they occurred nor 

 explained afterwards. For example when Darwin crossed different 

 breeds of domestic pigeons, no one of which had a trace of blue in its 

 plumage, he sometimes obtained offspring with more or less of the blue 

 color and markings of the wild rock pigeon from which domestic pi- 

 geons are presumably descended. He described many cases of dogs, 

 cattle and swine, as well as many cultivated plants, in which offspring 

 resembled distant ancestors and differed from nearer ones; such cases 

 had long been known and were spoken of as " reversion." He observed 

 many cases in which certain characters of one parent prevailed over cor- 

 responding characters of the other parent in the offspring, this being 

 known as " prepotency " ; but there was no satisfactory explanation of 

 these curious phenomena. They did not come under either of Galton's 

 laws, and their occurrence was apparently so irregular that every such 

 case seemed to be a law unto itself. 



C. EXPERIMENTAL STUDY OF INHERITANCE 



I. Mendelism 



The year 1900 marks the beginning of a new era in the study of in- 

 heritance. In the spring of that year three botanists, deVries, Correns 

 and Tschermak, discovered independently an important principle of 



