36 BULLETIN 1121, U. S. DEPARTMENT OF AGRICULTURE. 
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 asummation 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. Shull 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 problem 
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 talier than either, while 
