60 INHERITANCE IN GUINEA-PIGS. 



on either. Yellow pigment, on the other hand, is acted upon very dif- 

 ferently from black and brown by many factors. Accordingly it will 

 be convenient to use a term to include both black and brown pigments, 

 as dark pigments. The fur colors fall naturally into two groups, the 

 dark and yellow colors, characterized by the predominant presence of 

 dark and yellow pigments respectively. 



YELLOW GROUP OF COLORS. 



In the yellow group of colors the one of highest intensity is a rich 

 yellow-orange, which matches quite well with ochraceous tawny in 

 Ridgway's color charts (1912). There are all gradations from this 

 ochraceous tawny through cinnamon buff and cartridge buff to white. 

 In this paper it will be more convenient to use the conventional names, 

 red, yellow, and cream, for these grades. In grading the guinea-pigs, 

 three samples of hair have been used as standards of grades called 

 redo, yellow 3 , and cream 6 , respectively. White is considered to be 

 cream 8 . All of the yellow colors in guinea-pigs fall into this series, as 

 far as known. In mice, however, Little (1911) has shown that two 

 dilution series between red and white can be distinguished. There is 

 a series from red to cream resembling in appearance (though not geneti- 

 cally) the guinea-pig series. Another series (the " dilute " reds, yellows, 

 and creams) has a peculiar streaky appearance. The physical relation 

 between these two series is probably similar to that between the sepia 

 and blue types of dilution among the dark colors, which is discussed 

 below. 



DARK GROUP OF COLORS. 



Among the dark colors there are at least three distinct series: 



(1) There is the series of neutral grays, passing from black to white. 

 Such colors are shown by the blue rabbits, blue mice, and maltese cats. 

 There are no tame guinea-pigs known whose colors fall distinctly into 

 this series ; but the dull black of the wild Cavia cutleri, especially on the 

 belly, is a neutral gray quite free from any brown. Examination of 

 the hair of the blue rabbit under the microscope shows dense black 

 pigment masses alternating with colorless spaces, a condition described 

 by Miss Sollas (1909) in the hair of the blue mouse and apparently 

 comparable to the clumped condition of the black pigment in the 

 feathers of blue pigeons, described by Cole (1914). 



(2) There is a series of grades from black through dull brown and 

 tow-color to white. This series is shown by dilute black guinea-pigs. 

 The various shades of human hair, from black through brown to tow- 

 color, match samples from this guinea-pig series very closely. The 

 increase in quantity of pigment in this series in passing up from the 

 lower grades is accompanied by a change in quality. Yellowish-brown 

 pigment gives way to black. Dilution of this sort is produced inde- 



