IN RATS AND GUINEA-PIGS- 



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70 



60 



50 



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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 77. Individuals 

 This generation consisted of 

 6 1 individuals having an aver- 

 age grade of 5.6. The varia- 

 tion curve for this group (77, 

 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 77 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 7. The aver- 

 age grade of the parents was 

 in this case 1.7; that of their 

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

 Twenty-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, 77, 

 and 7, fig. 4. Selection has 

 steadily lowered the average 

 amount of pigmentation in the^ r<?c/e 



race by reducing the Upward I?IG- 4 -~Eff ect of selection for back-stripe of reduced size in 

 ^ - hooded rats, lots A,G,H, and /. 



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 



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