MORGAN, COAT COLORS IN MICE 107 



moy Island and on Marthas Vineyard. Experience had taught better 

 methods of keeping the mice, so that I have been able to rear these types 

 in confinement and have obtained crosses between them. The results of 

 these crosses may be described at another time ; here I wish to record cer- 

 tain facts connected with a change of color in confinement. The change 

 in color has been more marked in the Monomoy variety than in any other 

 form. This species lives on the sand hills of the small island of Monomoy 

 off the southern coast of Massachusetts. In its natural habitat this variety 

 is in general much lighter than the mainland form. The white hairs of 

 the belly are white to the base, while in the mainland type, P. novebora^ 

 censis, the inner end of each hair is dark. There is a good deal of varia- 

 tion in the degree of lightness of the upper surface of different individ- 

 uals, which Bangs suggests is the result of mixing caused by occasional 

 migrations, when the island has been for a time connected with the main- 

 land. Some of the specimens that I have collected and have obtained 

 alive from Monomoy are as dark above as the mainland form, while 

 others are quite pale. Similarly, in the laboratory, great variation in 

 color in these Monomoy mice exists, and the mice have shown themselves 

 more susceptible to change than the other species and varieties kept 

 under similar conditions. The most extreme change is shown in Plate 

 IX, fig. 2. For comparison, one of the lighter individuals found in 

 nature is also figured (Plate IX, fig. 1). The modified form has lost 

 even the light gray color of the island type, except for a patch on each 

 side of the body and another on the head, and has assumed a pale bluish- 

 pink color that is well shown without exaggeration in the figures. When 

 seen with other mice this mouse looks almost white. A microscopic ex- 

 amination of the light hairs shows that the outer two thirds of the hair 

 from the middle of the back is entirely devoid of pigment, but near the 

 base of the hair some black pigment is present. The color of the mouse 

 is due, therefore, to the darker base showing through the clear outer ends 

 of the hairs. The effect, as stated, is to give a faint bluish-pink color to 

 the mouse, when seen from above or from the sides. 



The stock in which this mouse appeared came from Monomoy in the 

 summer of 1909. It was kept in a large cage in the attic of the labora- 

 tory throughout the winter and in an upper room in the laboratory for a 

 part of the summer of 1910. The mouse was found in the condition 

 figured at the end of the summer and has remained in the same condition 

 for six months. The cage in which it was kept had produced young mice 

 in the spring which had not been removed, and, as I was absent during 

 the summer, I can not state whether this particular individual was one 

 -of the original mice from Monomoy or one of their young that had be- 



