56 INDIVIDUAL DIFFERENCES AND FAMILY RESEMBLANCES 



were both crossed with No. 55, and two litters resulted. No. 55 X No. 

 56 gave Nos. 68, 69 and 70 in the third generation, and No. 55 X No. 

 54 gave Nos. 61 to 64 inclusive. From a survey of the complete 

 records of these mice, it is seen that although the second and third 

 generations came from the female, No. 37, which made the excep- 

 tionally low record of 17, still, two of her young in the second gen- 

 eration made poor records, and Nos. 61, 65 and 70 in the following 

 generation did the same. 



The last family history that will here be discussed is composed of 

 another strain of colored mice that began from a mating of two yel- 

 low mice. The records for these individuals are given in Table VI. 

 and are graphically represented in Fig. 16. This family was started 

 in order to see whether there was any real correlation between 

 strains of colored mice and their ability to learn quickly and to 

 adapt themselves to varied conditions. It was especially desired to 

 test the learning ability of a greater number of yellow mice, and to 

 this purpose the two founders of the family, Nos. 98 'and 99, were 

 both yellow mice that came from the same yellow strain that had 

 previously exhibited individuals that were very slow to learn. It 

 was also desired that a further study of the effects of close inbreeding 

 be carried on independently of the other problem, and for this pur- 

 pose certain strains of the family are now being bred with this object 

 in view. The inbreeding in this investigation has not been carried 

 on close enough, or for a sufficient number of generations, to give 

 any definite results. It is to be noted that in the white family it 

 was necessary to outbreed in the fifth generation. To return to the 

 records of the family of colored mice, No. 99, of the parent genera- 

 tion, made a very good record of eight seconds, while the male died 

 before he was tested. The first generation from this mating gave 

 one yellow male and two gray females. The male and one of the fe- 

 males made good records, while the other female gave an -average 

 time of 63 seconds. The family average was 40 P.E. 9.0 seconds. 

 The male of this generation and the normal female, No. 102, were 

 mated and a litter of four males and three females resulted that com- 

 posed the second generation. All these mice were of an agouti color 

 in which a good deal of yellow pigment was added. All the males 

 and one female, No. 132, made good records; a female, No. 133, 

 made an average of 53 seconds, and No. 134 an average of 95 seconds. 

 Here the family average was 31.9 P.E. 8.5 seconds. Two matings 

 were obtained from the mice of this litter, and the resulting offspring 

 compose the third generation ; No. 131, with a very good average of 

 6 seconds, was mated with No. 133, with an average of 53, and gave 



