126 Shull. 



the ears of maize, but for all the characters, internal as well as exter- 

 nal, by which the parents had been differentiated. My investigations 

 on the effect of cross and self-fertilization in maize, had led me as 

 early as 1907 to the conclusion that the decreased vigor which appears 

 when a normally cross-bred plant is selfed, is a counterpart of the 

 increased vigor long known to result when species or varieties not too 

 remotely related to each other are hybridized (SHULL 1908). In other 

 words, hybridity itself, - - the union of unlike elements, the state of 

 being heterozygous, has, according to my view, a stimulating effect 

 upon the physiological activities of the organism, which effect disappears 

 as rapidly as continuous breeding reduces the progenies to homozygous 

 types. There can be little doubt of the general validity of my conclu- 

 sions in this regard, completely supported as the have been by my 

 own continued work (SHULL 1909 a, 1910, 1911a) and by the splendid 

 researches of EAST (1909, 1911), EAST and HAYES (1912) and others. 

 Here again as in the case of plural determiners, there is some 

 danger of misconception due to the fact that all discussions of the 

 stimulus of hybridity have taken as their starting point, for the sake 

 of simplicity, the typical Mendelian distribution of the germinal sub- 

 stances. The essential features of the hypothesis may be stated in 

 more general terms, as follows: The physiological vigor of an organism, 

 as manifested in its rapidity of growth, its height and general robust- 

 ness, is positively correlated with the degree of dissimilarity in the 

 gametes by whose union the organism has been formed; In other 

 words, the resultant heterogeneity and lack of balance produced by such 

 differences in the reacting and interacting elements of the germ-cells 

 act as a stimulus to increased cell-division, growth, &c. The more 

 numerous the differences between the uniting gametes, at least with- 

 in certain limits, the greater, on the whole, is the amount of stimu- 

 lation. These differences need not be Mendelian in their inheritance, 

 although in most organisms they probably are Mendelian to a prevailing 

 extent. It is not improbable that the same phenomenon is manifested 

 also in some cases as a result of interaction between the Mendelian 

 genes (and the non- Mendelian genes, if such be present) of the male 

 nucleus, and the elements of the egg-cytoplasm which it enters in the 

 process of fertilization, as emphasized by A. F. SHULL (1912), but it 

 seems unlikely that an initial stimulation of this kind can account for 

 any large part of the increased vigor which is maintained throughout 

 all subsequent development; if the continued stimulation which is mani- 

 fested by hundreds or even thousands of clonal generations, be attributable 



