IN RATS AND GUINEA-PIGS. 



stripe of different extent, one group being of moderate size (12), the 



other extremely reduced (2). Twenty-eight individuals whose grades fell 



in the lower part of curve G (fig. 4), and averaged 7.8, were the parents of 



the next generation, group H. Individuals 



This generation consisted of 



6 1 individuals having an aver- 



age grade of 5.6. The varia- 



tion curve for this group (H, 



fig. 4), shows a nearly com- 



plete dropping out of individ- 



uals with the stripe of moder- 



ate size, such as occurred in 



group G. All the classes 



above 7 are small and the 



upward range stops at 22. A 



few individuals of lot H had 



no back-stripe at all (com- 



pare pi. i, fig. 3). These with 



some individuals having a 



much reduced stripe were se- 



lected as parents for the next 



generation, lot /. The aver- 



age grade of the parents was 



in this case 1.7; that of their 



34 young (lot 7, fig. 4) was 4. 



Tw T enty-four of the 34 young 



fell in the lowest class, mean 



grade 2, while none were 



above class 12. 



The effect of selection in 

 this series of experiments is 

 clear from a comparison of 

 the four curves, A, G, H, 

 and /, fig. 4. vSelection has 

 steadily lowered the average 

 amount of pigmentation in the Gr<?c/e 



race by reducing the Upward 



r . . . 



range of the variation curve, 

 thus eliminating the more abmodal variations, reducing the skewness of the 

 curve, and diminishing the distance between mean and mode. One of the 

 noteworthy features of the case is the absence of what may properly be 



Effect of selection for back-stripe of reduced size in 

 hooded rats, lots A, G, H, and 7. 



