14 INHERITANCE OF COAT-PIGMENTS AND COAT-PATTERNS 



the hooded rats forming lot C (fig. 2) ; the albinos, when back-crossed with 

 the original hooded stock, produced hooded offspring, which are included 

 in lot D (fig. 2), as are also the hooded descendants of lot B bred inter se. 



Lot D, accordingly, is a group of individuals of ancestry more mixed than 

 lot B, but on the whole similar. It shows a similar variability (fig. 3, D), 

 but is of a higher average grade and shows a tendency to form a second 

 mode, a group of wide-striped individuals, with a modal grade of 54.5, i.e., 

 individuals in which the stripe is about half as wide as the entire dorsal 

 surface (compare pi. i, fig. 2). 



Lot C (fig. 2 and fig. 3, C) contained only 36 individuals. Its average 

 grade is almost identical with that of lot D, 23.5 as compared with 23.9. 

 Its range also is similar; but the curve itself is flatter, yet with the same 

 tendency to become subdivided into two groups, one with a wider stripe, 

 the other with a stripe like that of the original hooded stock (lot A). Two 

 other lots of hooded rats (£ and F, figs. 2 and 3) belonging to this series 

 have also been studied. Lot E consisted of the hooded young of certain 

 individuals of lot D, those having the lesser amount of Irish ancestry, while 

 lot F was derived from certain individuals of lot E bred inter se. The parents 

 of lot E had an average grade of 21.8, similar to that of lot B (21.2), or to 

 the lower modal group of lot D; the average grade of lot E itself was very 

 similar, namely 21.1. The parents of lot F had an average grade of 17.8; 

 lot F itself had an average grade of 12.9. The variation curves for lots E 

 and F are unimodal, like those for the original lot A, giving no evidence of 

 a tendency to form a wide-striped variety. 



/ MODIFICATION OF HOODED PATTERN BY SELECTION. 



v/ 



The close agreement between the grades of the parents and children in 



lot E, and measurably also in lot F, suggests that one can at will either in- 

 crease or decrease the width of the stripe in a stock of hooded rats. To 

 test this matter more fully, selection has been made both for reduced and for 

 increased size of stripe. 



SELECTION FOR REDUCED STRIPE. 



Eleven individuals of lot A, having narrow or interrupted dorsal stripes 

 (average grade 11), formed the beginning of a selection experiment for 

 reduced pigmentation (compare pi. i, fig. 4). They produced 83 young, 

 lot G* figure 4, with an average grade of 9.6, and a form of variation 

 curve suggesting the presence of two groups of individuals, each with a dorsal 



*0n account of the smaller range of variation in this series of experiments, the size of 

 the classes was made only half as great as in the case of lots 4-^, fig. 3. The mean value? 

 of the classes in lots G-I are 2, 7, 12, 17, etc. 



