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, 

 i.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 



