1918] CURRENT LITERATURE 71 
each character is represented by only a single dose in the hybrid and by a 
double dose in the pure race, so that mathematically the two situations are 
equivalent. This objection is valid only if we assume complete lack of domi- 
nance. We are certainly within our rights in assuming some slight degree of 
dominance, and if we do this it follows that hybrids have more in the way of 
active “desirable” characters than have pure races, and, having more “desir- 
able” characters, hybrids are more vigorous. They are vigorous, not because 
they contain more heterozygous sets, but because they contain more positive 
dominant characters. 
This is a rather obvious explanation of hybrid vigor, and one that has 
probably occurred to a number of geneticists, being commonly referred to as 
“the hypothesis of dominance” (accounting for hybrid vigor). It involves 3 
assumptions: (1) that there is such a thing as dominance; (2) that most domi- 
nant characters are “desirable” ones, that is, of survival value; this assumption 
is rendered easier if we accept the presence and absence hypothesis; (3) that 
these dominant “‘desirable” characters add more vigor than they detract from 
it, and add to vigor to the degree in which they are dominant. This last assump- 
tion is the critical one; but even that seems very reasonable. 
KEEBLE and PELLEW? suggested this explanation in 1910, and since then 
it has had some discussion in the literature. At first statement the theory 
seems sound, but actually it does not fit the facts. The two chief objections to 
this theory of dominance may be found in the publications of SHULL, EMERSON, 
and East (loc. cit.). 
1. If hybrid vigor were due to dominance, it would be possible in genera- 
tions subsequent to the F, to recombine in one race all of the dominant deter- 
miners. Thus there could be isolated a race that was “100 per cent vigorous,” 
and since it would be homozygous, its vigor would not be lost by inbreeding. 
Actually, though, hybrid vigor cannot be fixed in this way; ‘‘all maize varieties 
lose vigor when inbred.” 
2. Experience assures us that the distribution of individuals in the F. 
generation with reference to hybrid vigor is represented graphically by a sym- 
metrical curve similar to the normal probabilities curve; the class containing 
the greatest number of individuals is that which shows the medium amount of 
hybrid vigor, while on either side of this class the fall in the curve is regular, 
Teaching its lowest point in the two small extreme classes which show respec- 
tively greatest hybrid vigor and least hybrid vigor. According to the domi- 
nance hypothesis, however, the largest class of F, individuals would be that 
showing the greatest hybrid vigor, while the smallest class would be that show- 
ing least hybrid vigor. The curve representing such a situation would be 
unsymmetrical and strikingly different from that which actually occurs. For 
these two reasons the dominance hypothesis seems to have been discarded. 
* KEEBLE, F., and PELLEw, C., The mode of inheritance of stature and time of 
flowering of peas (Pisum sativum). Jour. Genetics 1:47-56. 1910. 
