PREFATORY NOTE. 



In 1909 my pupils and I desciibed certain preliminary studies of size 

 characters in successive generations of rabbits bred chiefly for a study 

 of color inheritance. We reached the provisional conclusion that total 

 body-weight and skeletal dimensions are subject to intermediate or 

 blending inheritance and do not conform with Mendel's law, as this 

 law was then understood and as it was understood by Mendel himself. 

 This conclusion seems to me still sound. But almost immediately 

 after the publication of our paper the whole matter of blending inher- 

 itance was placed in a new light by the work of Nilsson-Ehle (which 

 then came to my notice for the first time) . Accordingly my experiments 

 with rabbits were straightway arranged to show if possible whether 

 or not blending inheritance is (as suggested by Nilsson-Ehle) really 

 Mendelian, but without the occurrence of dominance and with several 

 independent factors concerned in producing the observed results . These 

 experiments were intrusted to Mr. MacDowell in the fall of 1909 and 

 by him continued under my supervision until June 1912, when the 

 results which he had obtained were presented in the form of a disserta- 

 tion for the degree of doctor of science. 



A number of animals not then mature have since furnished material 

 which is incorporated in this publication. 



While these studies were in progress several different investigations 

 of size-inheritance in plants were published which have favored the 

 interpretation of Nilsson-Ehle (and also of my colleague, Professor 

 East) that in all inheritance, whether blending or not, Mendelian factors 

 are concerned. Very naturally Mr. MacDowell has been strongly 

 influenced by this idea, which seems to unify, if not to simplify, our 

 conceptions of the method of inheritance. While not entirely sharing 

 his views, I have tried not to bias his judgment either for or against 

 the multiple-factor hypothesis which he adopts in this paper. But to 

 avoid misunderstanding, I wish to say that in my own opinion the 

 theory of the purity of the gametes has not been established, and too 

 great definiteness and fixity is ascribed to Mendelian units and factors 

 in current descriptions of heredity; consequently, too great importance 

 is attached to hybridization and too little to selection, in explaining 

 evolution. 



But neither my views nor Dr. MacDowcll's should bias the judgment 

 of the reader. We wish to place before him clearly the results of experi- 

 ments which have entailed much painstaking observation; the correct 

 interpretation will become evident in due time. 



These experiments have been made possible by a grant from the 

 Carnegie Institution of Washington, for which grateful acknowledg- 



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