170 INHERITANCE IN RATS. 



PLUS AND MINUS-SELECTION SERIES. 



The plus-selection experiment, when described by Castle and Phil- 

 lips, had been carried through 13 generations, but the last 2 genera- 

 tions were incomplete. The number of offspring included in genera- 

 i ion 12 (table 140) has now been raised from 590 to 682 and the number 

 of offspring included in generation 13 (table 147) has been raised from 

 194 to 52!). The mean grade of the parents for generation 12 has 

 advanced from 4.09 to 4.10; that of the offspring has fallen from 3.94 

 to 3.93. Neither of these changes is of significant size. The correla- 

 tion is now found to be 0.168 instead of 0.161. 



In generation 13 (table 147) the changes are greater, as might be 

 expected from the greater change in the number of observations. The 

 mean of the parents is now 4.13 (formerly 4.22) ; that of the offspring is 

 3.94 (instead of 3.88). The correlation is 0.117, as compared with 

 0.132, the value previously obtained. 



feneration 14 (table 148) includes 1,359 offspring of mean grade 

 4.01 . They are descended mostly from parents of grade +4 or higher, 

 mean 4.14. 



( feneration 15 (table 149) includes 3,690 individuals, more than have 

 been produced in any other generation of the series. The mean grade 

 of the parents was in this generation advanced about a quarter grade to 

 4.38; that of the offspring advanced a little, to 4.07. 



Generation 16 (table 150) was also large, including 1,690 offspring. 

 The grade of the parents was again advanced a little to 4.45; that of 

 the offspring followed a similar amount, to 4.13. 



In the three generations (14 to 16) which have been added since the 

 last report, the grade of the selected parents has been advanced by 

 0.32, from 4.13 to 4.45; that of the offspring has advanced 0.19, from 

 3.94 to 4.13 (the mean grade of the parents three generations earlier). 



The upper limit of variation of the offspring has meanwhile advanced 

 from 5.25 to 5.87, the highest grade being found in a rat black all over 

 except for a few white hairs on the chest. This rat has produced a 

 few offspring of almost as high grade, though the most of his young 

 are of much lower grade. 



In the minus-selection series, generation 13, in our previous report, 

 contained 571 offspring. This number has now been raised to 1,006 

 (table 151), the mean grade of both parents and offspring being prac- 

 t ica lly unchanged by the additional young recorded. The parents are 

 of mean grade -2.49, the offspring of mean grade —2.40. 



In the next generation (14) the offspring number 717, their mean 

 grade being -2.48, that of the selected parents being -2.64. (See 

 table 152.) 



Generation 15 includes 1,438 young of mean grade -2.54. The 

 mean made of the parents is -2.65. (See table 153.) 



