12 INHERITANCE OF COAT-PIGMENTS AND COAT-PATTERNS 



interrupted, that is, the proportion of the total stripe length which was 

 pigmented. The product of these two ratios, relative width and relative 

 length of the back-stripe, when multiplied by 100, gives an approximation to 

 the per cent of dorsal surface (posterior to the hood) which was pigmented. 

 Such a valuation, or grade, was obtained for each hooded individual. 



The grades for 183 individuals (lot A, fig. 2), belonging to three successive 

 generations, yield the variation curve shown in fig. 3, A . In obtaining this 

 curve grades from o to 9 inclusive were grouped in a single class, the mean 

 value of which is 4.5; grades from 10 to 19 inclusive, in a class the mean 

 value of which is 14.5, and so on. The ordinates in fig. 3, A, show the fre- 

 quencies of these classes. The grades fall into four classes, the one with 

 greatest frequency (modal class) being the one with the lowest mean (4.5), 



the frequencies of the other classes 

 decreasing in order upward. The 

 average grade for the entire 183 in- 

 1 dividuals is 13.3 (see vertical line 



r-J in fig. 3, A). 



The hooded stock was now crossed 

 with an Irish stock, and the Irish 

 cross-breeds thus produced were in 

 j turn crossed back with the parental 



t hooded stock. From this last cross 



' , | ' ' were obtained both Irish and hooded 



tj.-~-(uvi>jj'/- r 4.j. H (JLtot C ) I ff . , , ,. f 



i offspring, in proportions sensibly 



H(LotE) equal. These hooded offspring form 



a group (lot B, fig. 2) derived through 

 H (Lot F) one parent directly from the original 



FIG. 2.- Ancestry of lots A to F of hooded rats. hooded Stock (lot A), but through 

 Read downward. the other parent from the Ifish cross . 



breeds. One of the two gametes, accordingly, entering into each zygote had 

 a chance to be modified by association with the Irish character. Lot B 

 includes 126 individuals, with back-stripes on the average nearly twice as 

 extensive as those of lot A, the average grades of the two lots being 13.3 

 for lot A, and 21.2 for lot B. The form of the variation curve for lot B is 

 shown in fig. 3, B. The modal class is higher than in lot A, and the range 

 of variation is extended upward so as to include seven classes, the highest 

 one having a mean grade of 64.5. Accordingly, we conclude that a cross 

 with the Irish stock raises considerably the average size of the dorsal stripe 

 in hooded rats, as well as the range of variation upward in size of stripe. 

 In the same litters with the hooded rats forming lot B, were born rats of 

 two other sorts, Irish and albino (fig. 2) . The former bred inter se produced 



