46 INDIVIDUAL DIFFERENCES AND FAMILY RESEMBLANCES 



tained from the two mice that made the slow records, and the result- 

 ing litter of five females and one male composed the second genera- 

 tion of the white family. All the mice of this litter gave time records 

 that were better than the average, so that the family time was very 

 good, amounting to 23.3 P.E. 5.6 seconds. A glance at the chart 

 will show that the majority of the mice of this second generation 

 tended to resemble their grandparents rather than their parents. 

 No. 48, the male of the litter, was quick to learn and made an average 

 time of ten seconds. He mated with two of the females of the same 

 litter, namely, No. 50, that made an average of 20 seconds, and No. 

 51, with an average of 9 seconds. From these two matings two 

 litters were obtained that compose the third generation of the strain. 

 All the mice in these litters made poorer records than either of their 

 parents, and thus reversed the condition that occurred between first 

 generation parents and second generation offspring, that we have 

 just examined. The offspring of No. 48 X No. 50 were two females 

 and one male. The male, No. 66, made the very poor average of 113 

 seconds, while one female took 35 seconds, and the other 75 seconds. 

 Four offspring of this generation, three males and one female, re- 

 sulted from the mating of No. 48 X No. 51, while here again one of the 

 males, No. 78, made the poor record of 122 seconds, one other male, 

 No. 77, took 68 seconds, and the female, No. 76, made an average of 

 31 seconds. The remaining male died before it could be tested. It 

 will thus be seen that from the mating of No. 48 with two females, 

 two litters were obtained that gave almost identical family averages ; 

 the average for Nos. 65, 66, and 67 was 74.3 P.E. 15.9 seconds, 

 while the average for Nos. 76, 77, and 78 was 73.7 P.E. 19.5 sec- 

 onds. The only litter that was successfully raised to continue the 

 fourth generation of the strain was obtained by back crossing No. 66, 

 who made the poor average of 113 seconds, with his own parent No. 

 50, with the quick average of 20 seconds. Their offspring composed 

 a litter of two males and two females. One of the males died; the 

 other made an average record of 53 seconds ; one female made a good 

 record of 24 seconds ; while here again, as in the two previous litters, 

 a single mouse, No. 72, made a poor average of 121 seconds. The 

 family average for these three mice was 66 P.E. 22.3 seconds. No. 

 71 mated with No. 72, but only a single male, No. 91, was saved from 

 their litter to stand as the sole representative of the fifth generation. 

 No. 91 made an average time of 38 seconds, and was destined to 

 become the paternal parent of a considerable number of offspring. 

 As neither of the females of the fourth generation mated with No. 

 91, he was mated successfully with four unrelated white females, 



