HOODED CROSSED WITH WILD. 165 



(See table 10, Castle and Phillips.) The mean of the extracted 

 hooded grandchildren in this case (being 3.05) shows a regression of 

 0.79 from that expected for the uncrossed hooded race. From the 

 extracted hooded grandchildren of 9 5513, produced as just described 

 by a cross with a wild male, 7 individuals, 2 males and 5 females, were 

 selected for a second cross with the wild race. They ranged in grade 

 from +2 to +3|. (See table 142.) They produced several litters of 

 young of the same character as the first FI young, all being similar to 

 wild rats in appearance, except for the frequent occurrence of a white 

 spot on the belly. These second FI young were at weaning time mated, 

 brother with sister, in breeding-pens, precisely as had been done with the 

 first F/s. They produced 394 second F 2 young, of which 98 were hooded 

 and 296 non-hooded, a perfect 1 : 3 ratio. The hooded young varied 

 in grade from +2 to +4, as shown in table 142, the data there being 

 given for each family separately as well as for all combined in the totals. 

 One family was very like another as regards the character of the hooded 

 young, except that the higher-grade grandparents had grandchildren 

 of slightly higher grade. Thus the average of all the 98 hooded young 

 was +3.47, but the average of those descended from the 3 grandparents 

 of lowest grade was less than this, while the average of those descended 

 from the 3 grandparents of highest grade was greater. This is just what 

 had been observed throughout the entire selection experiments. (See 

 Castle and Phillips.) 



If we weight each of the grandparents in table 142 in proportion to 

 the number of its hooded grandchildren, then the mean grade of all the 

 grandparents is +2.95. Since the mean grade of all the 41 first F 2 

 hooded grandchildren, from which these 7 were chosen, was +3.05, it 

 will be seen that these 7 are, so far as grade is concerned, fair repre- 

 sentatives of the 41, being in fact of slightly lower mean grade. It is 

 therefore all the more striking that their grandchildren, the second F 2 

 hooded young (table 142) , are of higher grade. They regress in an oppo- 

 site direction to that taken by the first F 2 hooded young. Thus the 

 original hooded ancestor ( 9 5513) was of grade 4.25. The grade of 

 hooded young expected from such animals is 3.84. What she produced 

 in F 2 , following a cross with the wild male, was young of mean grade 

 3.05. Seven of these of mean grade 2.95 produced a second F 2 contain- 

 ing hooded young of mean grade 3.47. This is a reversed regression of 

 0.52 on the grade of their actual hooded grandparents, or of 0.42 on the 

 group from which their grandparents were chosen. Their mean lies 

 about midway 1 between that which would have been expected from 

 the original hooded female (5513) had no crossing with wild rats 

 occurred and that which was observed in the first F 2 . 



'In The Scientific Monthly (Jan. 1916) I have stated that a second cross showed "a return to 

 about what the selected race would have been had no crossing at all occurred." This is obviously 

 inaccurate and should be corrected. It rests on a comparison with the combined average of both 

 the older and the more recent experiments. 



