On the Dominance Ratio. 
323 
1921-22.] 
Recent work in genetics (East and Jones, 1920) leads unavoidably to the 
conclusion that inbreeding is not harmful in itself, but is liable to appear 
harmful only through the emergence of harmful recessive characters. 
This raises the question as to why recessive factors should tend to be 
harmful, or why harmful factors should tend to be recessive: unless this 
association exist we should expect to obtain great improvements by 
inbreeding ordinarily crossbred species, as often as great deterioration. 
The statistical reason for this association is clear from the distribution 
of the ratio of allelomorph frequency which occurs under genotypic 
selection, for, if we assume that adaptation is the result of selection, the 
majority of large mutations must be harmful, and these can only be 
incorporated in the common stock in the sheltered region where the 
rare recessives accumulate (fig. 4).* Similarly there are many well- 
attested cases of the crossbred (heterozygous) individual showing sur- 
prising vigour ; but it is not obvious that there is any biological reason 
for the beterozygote to be more vigorous than the two homozygotes. 
From a consideration of the stability of the frequency ratios, however, 
it appears that there will only be stable equilibrium if the heterozygote 
is favoured by selection against both the homozygotes : naturally this 
will occur only in a minority of factors, but when it occurs such a factor 
will be conserved. In the opposite case it will certainly be eliminated. 
Cases in which the heterozygote is favoured by selection in preference 
to both homozygous forms have an additional interest in that these cases, 
when the selection is intense, may form the basis upon which is built up 
a system of balanced lethal factors. Muller (1918) has shown that such 
systems will tend to be built up when selection strongly favours the 
heterozygote, and has explained how in the light of such systems the 
majority of the phenomena, including the “ mutations,” of (Enothera, find 
a genetic explanation. 
The interesting speculation has recently been put forward that random 
survival is a more important factor in limiting the variability of species 
than preferential survival (Hagedoorn, 4). The ensuing investigation 
negatives this suggestion. The decay in the variance of a species breeding 
at random without selection, and without mutation, is almost inconceivably 
slow : a moderate supply of fresh mutations will be sufficient to maintain 
the variability. When selection is at work even to the most trifling 
extent, the new mutations must be much more numerous in order to 
* On the Lamarckian theory of evolution, on the other hand, wliere most, or all, 
mutations are assumed to be beneficial, we should expect by inbreeding, which uncovers 
the accumulated mutations in this region, to make great and immediate progress. 
