36 BULLETIN 1121, V. S. DEPARTMENT OF AGRICULTUEE. 



the progressive one, while the recessive was retrogressive and often 

 lacking in vigor. He pointed out that such a relation helped in 

 understanding the degeneration sometimes but not always associated 

 with inbreeding. This is very close to our present view. At that 

 time, however, it was somewhat lacking in substance as a general 

 explanation of the effects of inbreeding. The earlier work on Mende- 

 lian inheritance had naturally been confined for the most part to big, 

 discontinuous variations. Thus the light which the Mendelian 

 mechanism throws on the appearance of abnormalities following in- 

 breeding was, as above stated, quickly recognized. It was not clear at 

 first that any light was thrown on the relatively slight decline in size, 

 fertility, and constitutional vigor which are more typical consequences 

 of inbreeding. It was necessary to reach the viewpoint that heredi- 

 tary differences may be due to a summation of the effects of numerous 

 individually insignificant Mendelian units and that, indeed, the 

 Mendelian mechanism is the universal mechanism of heredity under 

 sexual reproduction. 



The independent experiments of G. H. ShuU and East with corn 

 marked a big advance in adding substance to the general theory of 

 heredity along the lines indicated above, as well as to the problenr 

 of inbreeding. Shull found that on self-fertilization an ordinary, 

 seemingly homogeneous variety of corn broke up into strains, each 

 highly uniform and differentiated from the others in numerous minute 

 characteristics. There was more or less decline in size and produc- 

 tivity in all strains in the earlier generations of selfing, but stability 

 was soon reached. On crossing these strains with one another there 

 was in general a return to the original vigor. 



All of these things had been observed by Darwin. On the basis of 

 the new knowledge of heredity, however, Shull was able to show how 

 everything could be explained on the assumption that an ordinary 

 variety of maize is really a complex hybrid and that self-fertilization 

 automatically isolates the various pure biotypes or ''elementary 

 species" through the segregation of Mendelian homozygotes, with the 

 help of the additional assumption that the hybrids are more vigorous 

 than the pure strains. 



East obtained the same results and independently reached essen- 

 tially the same conclusions. He suggested that there was a physio- 

 logical stimulus to development in proportion to the degree of differ- 

 ence between the uniting germ plasms. This means in proportion 

 to the amount of heterozygosis in some or all of the factors. 



This view was contrasted with the older hypothesis which soon after 

 was brought up again by Bruce and by Keeble and Pellew, that the- 

 vigor of crossbreds is due to dominance of factors conducive to vigor . 

 Keeble and Pellew described experiments in which a cross between 

 two pure strains of the pea produced hybrids taller than either, while 



