190 GENETICS IN RELATION TO AGRICULTURE 



definite with respect to the factors involved. Such an assumption is 

 obviously an hypothesis of extensive multiple allelomorphism in which the 

 members of the system A i} A 2 , At, . . . A n when in contact with each 

 other react to form new allelomorphs. On the basis of a chemical con- 

 stitution for the locus in the chromatin material, such an hypothesis would 

 appear not improbable, for if the nucleus of this locus were identical in all 

 the allelomorphs involving it and the changes in it were changes which took 

 place around the fringe of the molecule, inside and end chains perhaps, 

 then there appears to be no good reason for believing that two such similar 

 allelomorphs when in intimate association with each other should not 

 interact to form intermediate factors. While the very existence of the 

 second and third categories is not generally accepted by geneticists, it 

 must be admitted that, provided the assumptions of factorial alterability 

 be accepted, they can explain the known facts of size inheritance. 



That a factor as such may vary and that selections of variations in the 

 expression of such a factor may permanently alter its expression in any 

 desired direction, Castle has endeavored to show in extensive selection 

 studies with hooded rats. The selections were made in both plus and 

 minus directions and in each case selection did establish a permanent 

 variation in the direction in which it was made. The condensed results 

 of these selection experiments involving over 30,000 individuals are given 

 in Tables XXXIV and XXXV. These results show very clearly that 

 selection has definitely changed the mode around which the pattern 

 fluctuates and in opposite directions in the two series. Since the hooded 

 pattern has been shown to be a simple Mendelian recessive to self color, 

 does this evidence prove that selection has modified the factor for hooded 

 pattern? 



Those who hold to the multiple factor idea of size inheritance contend 

 that it does not, and for several reasons. In the first place the difference 

 between hooded and self-colored rats, while in the main due to a single 

 factor difference, may at the same time involve other minor differences in 

 a number of factors which influence the extent of pigmentation when the 

 factor for hooded pattern is present. According to this view selection 

 would result in attaining a homozygous condition for certain of these 

 modifiers and, therefore, in increasing or decreasing the area of pigmen- 

 tation in the direction of selection. There may be a number of such 

 modifiers and others may arise from time to time by mutation. It is 

 interesting to note that Castle records the appearance of two such 

 mutant individuals. These mutant individuals when tested with each 

 other and the forms from which they arose, displayed a type of inher- 

 itance which indicated that their origin involved a single factor differ- 

 ence for extent of pigmentation from the parental group. Might not 

 other mutations have arisen which, on account of their lesser magnitude 



