EFFECTS OF INBREEDING AND CROSSBREEDING. 37 
the segregation in the second generation proved that each strain 
furnished a dominant factor lacking in the other. In this case the 
factors were visibly different in effect, in that one increased the num- 
ber of internodes, the other their length. 
The two hypotheses are not easy to distinguish. Under both of 
them the departure from the condition of the pure inbred strains is in 
direct proportion to the amount of heterozygosis in at least certain 
factors. Under the stimulation hypothesis, however, the differences 
in vigor may be due to factors which of themselves have nothing to 
do with vigor: i. e., there may be no difference between AA and aa in 
vigor, while Aa may be superior to both. Under the dominance 
hypothesis it is merely to be supposed that Aa is equal to the more 
vigorous of the two homozygous types. Dominance indeed need 
not be perfect. 
Under the dominance hypothesis it should be possible to isolate 
inbred strains, homozygous in all factors conducive to vigor and hence 
equal or superior to crossbreds. This should not be the case with the 
stimulation hypothesis. Again, under the dominance hypothesis, 
the crossing of two inbred strains, followed by random breeding, 
should result in a population with a skew distribution as regards 
measurable characters, according to the expansion of (?+ +4)" where 
nm is the number of factors. The distribution would be symmetrical 
under the stimulation hypothesis. 
The invariable degeneration following self-fertilization which East 
observed in corn. and the failure to find skew distributions as the 
rule in the second generation of crosses, were urged by him in favor 
of the stimulation hypothesis. These objections, however, were met 
by Jones, who continued East’s experiments with corn. He pointed 
out that owing to the phenomenon of genetic linkage, which was 
known to occur in corn as well as in several other plants and animals, 
the consequences of the dominance hypothesis of hybrid vigor would 
really be much closer to those of the stimulation hypothesis in the 
above respects than had been recognized. It should frequently 
happen that detrimental recessive factors would be linked with 
favorable dominant ones. In these cases the homozygotes would be 
of the types AbAb or aBaB, while the heterozygotes would be largely 
AbaB. The heterozygotes, containing both dominant factors, would 
be superior to both homozygous types, giving a situation almost 
indistinguishable from that which follows the stimulation hypothesis. 
In this suggestion Jones did not add a new hypothesis to the theory 
of inbreeding; he merely pointed out the logical consequences of a 
phenomenon, linkage, which had already been demonstrated. The 
dominance hypothesis was thus greatly strengthened. Collins (1921) 
has recently shown that even if linkage is disregarded, the objec- 
tions to the dominance hypothesis are not serious in cases in which 
