No. 608] MULTIPLE FACTORS IX MICE AM) EA TS 465 



to rats in which only small spots around the eyes remain 

 pigmented. The following diagram will show roughly 

 the limits of variation and some of the intermediate types 

 as graded by Castle. Throughout the course of his selec- 

 tion experiments, Castle has at times crossed the plus 

 variants with the minus variants or else crossed selected 

 animals from the plus or minus series with wild or with 

 Irish rats. Both the last mentioned varieties are essen- 

 tially self pigmented animals and are dominant in inheri- 

 tance to the hooded pattern. The race with which Castle 

 started his selection experiments showed within the first 

 two generations of selection rats varying from grade 

 minus two to grade plus three and three quarters. 



Castle and Phillips, 1914, have recorded the results of 

 two crosses between minus variants and plus variants. 

 The first of these crosses was between females of minus 

 two grade in the sixth selection generation and males of 

 plus three and one half or three and three quarters 

 grades in the fifth selection generation. It is possible to 

 use the range of variation among the progeny of animals 

 of these grades within the selection generation from 

 which they are chosen to control the results of the cross. 

 In Table IV^ the first line represents the range of varia- 

 tion in the progeny of minus two animals of the sixth 

 selection generation. The second line represents the 

 range of variation in the progeny of plus three and one 

 half and three and three quarters animals of the fifth 

 selection generation. It will be seen that there is a space 

 of seven grades between the two limits of variation of 

 the parent races. The cross between these two types 

 produced in the generation a range of variation shown 

 in the third line of Table IV. It will be seen that for the 

 most part, the young fall in the seven classes between the 

 two parent races. In these seven classes will be found 

 sixty-(M,o-lit or 7.^.1 ]mm- cont. of tlio ninety-three young. 

 Two -2:27 i-r •■-nt. tin- } ^'w.vj: show grades of pig- 



