No. 507] BREEDING EXPERIMENTS WITH RATS 183 



hybrids are black whichever way the cross is made. 

 Black is dominant to gray. In all I have thirty-two 

 such individuals. 2 These black hybrids inbred have pro- 

 duced one litter of four black 3 and one gray. The num- 

 bers are too small to furnish conclusive data, but as far 

 as they go they indicate that the two colors follow Men- 

 del's law. 



The roof rats are white below as is also the Norway 

 rat, but there is a peculiar difference. In the roof rat the 

 white hair is white to its very base, i. e., it is not ticked.. 

 In the Norway rat the ventral hair has a dark base as in 

 most other rodents. 



All attempts to cross the black rat or the roof rat with 

 the Norway rat, or with its domesticated varieties have 

 failed, although by putting young rats together I have 

 had them live in harmony for a year or more. It would 

 appear that the two species M. decumanus and M. vattus 

 are infertile. 



On the contrary as stated above M. vattus and M. alex- 

 andrinus are perfectly fertile inter se. 



In respect to the ticking of the ventral hair it is inter- 

 esting to note that spotted gray mice of the domesticated 

 breeds have the hair of the white spots uncolored all the 

 way to the base, as in the albino. I have found a wild 

 species of brown mouse that has a white belly and the 

 hair is ticked, i. e., it has a dark base. When such a 

 mouse is crossed with a spotted mouse the first genera- 

 tion is gray with a white belly— the hair being ticked. 

 Such mice inbred produce amongst other combinations 

 some spotted mice. In these mice wherever a white 

 spot extends down the sides and across the belly the 

 region of the spot on the belly has hairs white to the 

 base. In other words, the white spot dominates the white 

 belly in the same way that it dominates the gray hair of 

 the sides and back/ 



"There is some variation in the color of the black hybrids of the first 

 generation. One that was reddish black at first later became black. 



8 In the second generation the four black individuals, still young, show 

 some differences in the shades ot black. 



