40 F. B. SUMNER AND R. R. HUESTIS. 



The values employed in the studies thus far made are the per- 

 centages of " black," " white " and " color " in the sample. For 

 reasons which we shall not justify here, the difference between the 

 highest reading (always red, in our material) and 100 is regarded 

 as the percentage of " black " ; the lowest reading (blue-violet, 

 in our material) is regarded as being likewise the percentage of 

 " white "; while the balance (difference between the sum of these 

 two and 100) is regarded as the percentage of " color." By "color" 

 is here meant the free color, i.e., that which remains of the two 

 higher readings after deducting the amount regarded as combining 

 to form white light. The spectral position of this " color " is not, 

 of course, given in any direct way by these readings. It reaches 

 its maximum, in all cases, somewhere between red and yellow, its 

 nature being indicated crudely by the ratio between " free " red 

 and green (R : G ratio). This ratio is high in reddish skins, low 

 in yellowish ones. 1 



In the present studies, two dorsally situated and symmetrical 

 areas of the pelage were selected, one being in the shoulder region, 

 the other a little anterior to the base of the tail. The means of the 

 values for the two areas were employed in the computations, and 

 each value was likewise based upon two readings, made upon dif- 

 ferent days. 



In Table I. are given the means and standard deviations for 

 each of these " characters," the two parent races and the two hy- 

 brid generations being considered separately. The number of indi- 

 viduals here given is in each case smaller here than that employed 

 by Huestis for the study of microscopic hair characters. This 

 is because certain pelts which could be used for the latter pur- 

 pose were not suitable for the former, owing to injury or to molt- 

 ing within the areas under examination. 



The salient facts shown in the foregoing table are as follows : 



i. As was evident from a casual inspection of skins, P. e. fra- 

 terculus is a considerably darker race than P. e. eremicus. This 

 is indicated by a higher percentage of " black " in the former and 

 by the lower percentages of " white " and " color." 



i To illustrate : suppose that the readings for red, green and blue-violet are 

 25, 15 and ii, respectively We have in this case, 75, n and 14 respectively, as 

 the percentages of black, white and color. The R :G ratio is 3.5. 



