154 GENETICS IN RELATION TO AGRICULTURE 
variations from complete dominance to a strict intermediacy may be ob- 
tained among hybrids. For cases of complete dominance, the presence 
and absence idea satisfies conditions very satisfactorily as far as formal 
relations are concerned, and intermediacy and even other conditions 
of the hybrid expression may be assumed to depend upon the quantitative 
difference in the amount of the factor present in the hybrid race as 
contrasted with the parent races. Difficulties, however, begin to arise 
when attempts are made to explain the origin of dominant mutations in 
terms of this hypothesis, for in such cases it is almost necessary to assume 
that a factor has been added to the hereditary material. It is usually 
considered easy enough to account for a recessive mutation as due to the 
dropping out of a factor from the hereditary material, but when a factor 
is added to that material, we must ask from whence it came, what its 
nature, etc. If we regard mutations as simply due to changes in a fac- 
tor this difficulty vanishes for then dominance or recessiveness of the 
mutations depends merely on the relations between the mutated factor 
and its unchanged condition and there is no particular reason for as- 
suming that all mutations should be of the nature of ‘‘loss’”’ mutations, 
z.e., mutations depending upon the loss of a factor from the hereditary 
material and resulting in the absence of some dominant character in 
the individuals concerned. There is no difficulty therefore, in account- 
ing for the four or five dominant mutations which have been observed in 
Drosophila, if we regard mutation as a change in a locus, for these par- 
ticular mutations simply happened to involve changes of such a type 
that the mutated locus was dominant to the unmutated condition. 
Obviously, also,.such a view conforms more closely with the facts ob- 
served in cases of the competitive action of factors such as is seen in bar 
eyes in Drosophila or in the factors for flinty and floury endosperm in 
maize. 
But there are more serious objections than these which can be raised 
against the presence and absence hypothesis. In Drosophila, for in- 
stance, a number of cases of return mutations have been observed, 
many of them in cultures so controlled that the possibility of explaining 
them by chance contamination is practically precluded. Thus in stock 
so controlled by the presence of other factors that it would practically 
have been impossible to have a contamination go unnoticed on account 
of the introduction of other factors, the bar-eyed race of Drosophila has 
been known to produce normal-eyed mutants (May) and eosin-eyed flies 
have been observed to give white-eyed flies on several occasions; while 
on the other hand eosin, although dominant to white, originally arose as 
a mutant in a stock of white-eyed flies. If we assume that the change 
from eosin to white involves a relatively unessential change in the W 
factor in Drosophila, in chemical terms perhaps a slight rearrangement in 
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