FACTOR RELATIONS IN QUANTITATIVE INHERITANCE 193 
occurred in relatively infrequent numbers in the gametes, then other 
combinations would result. A graded series running from light to dark 
would then be produced, but since the mid-products, hs and hs, would be 
by far most numerous, in small progenies most of the individuals would 
display a mid-condition of pigment development. On such an explana- 
tion any particular type of hooded pattern would be allelomorphic to the 
wholly pigmented condition or to the Irish condition, if these latter two 
with hooded be members of a system of triple allelomorphs. Also, the 
variability of the factor due to interaction with whichever other member 
of the allelomorphic pair it happened to be in contact would account 
for the variability in the expression of the hooded pattern following 
hybridization. 
The progress of selection in such a form on the basis of a single 
factor as determining not only the hooded pattern, but also the extent of 
pigmentation in the hooded condition requires us to assume an instability 
in the factor even when in the pure condition. We would, therefore, 
assume that, in an animal of the constitution h3hs with respect to the 
factor for hooded pattern, the instability of the factor leads at times to 
the production of gametes by such an animal bearing the factor h. 
on the one hand, or h, on the other hand. If such gametes were produced 
_ relatively infrequently, they would almost invariably mate with 
gametes of the genetic constitution h;. The resulting progeny would 
have the genetic constitutions h4hs and hshs and they would be slightly 
lighter and slightly darker respectively than the bulk of the animals 
of the genetic constitution hshs. Selection of such individuals would 
rapidly lead to the production of races of the genetic constitutions haha 
and hehs. Individuals of the genetic constitution h4h4 on account of the 
variability in the factor itself would produce some gametes bearing the 
factor h3 or the factor hs, and by continuing the same process of selection 
a still lighter race of the genetic constitution h3h3 might soon be estab- 
lished. Assuming, therefore, that factor variability of this type occurs it 
is not difficult to see how a continuous process of selection such as Castle 
has employed should finally result in the establishment of new races 
differing markedly in their character expressions and possessing a different 
but related genotype to that of the original type from which selection has 
been made. Moreover, such an hypothesis accounts for the observed fact 
that return selection is just as efficient but no more so than the original 
selection in changing the mean of the races, a fact which presents some 
difficulties for a strict multiple factor interpretation. 
It should be stated that this hypothesis of factor variability does 
no violence to our conception of the nature of factors, except with respect 
to a rather ill-established belief in factor constancy. The continuous 
change in a factor such as we have outlined above reminds us very forcibly 
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