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, h & and h 6 , 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 h b h & 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 6 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 hji 5 and h- h 6 and they would be slightly 

 lighter and slightly darker respectively than the bulk of the animals 

 of the genetic constitution /i 5 /i 5 . Selection of such individuals would 

 rapidly lead to the production of races of the genetic constitutions hji* 

 and h 6 h 6 . Individuals of the genetic constitution hdi* on account of the 

 variability in the factor itself would produce some gametes bearing the 

 factor h s or the factor h 5 , and by continuing the same process of selection 

 a still lighter race of the genetic constitution h 3 h s 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 



13 



