20 LABORATORY MOUSE 



ties. Several pied mice were listed from Syria. Blue-dilute 

 specimens came from Esthonia and Syria, non-agouti (black) 

 specimens from Cape Colony and England. Cinnamons were 

 taken in South Africa and the Tigris Valley. There were 

 white-bellied agoutis from Syria and Persia. Several speci- 

 mens from West Africa probably contained extreme dilution. 

 The mice were all taken in the wild and not purchased from 

 fanciers. These facts refute the common belief that the 

 varietal characteristics found in fancy stocks are the results 

 of domestication. The natural cause which produces such 

 mutations in the germ cells is as yet undiscovered. Under 

 laboratory conditions /3-rays of radium and X-rays are able 

 to produce in the fruit fly (Drosophila) mutations identical 

 with those appearing in this species in nature (139). It is 

 possible that cosmic rays in nature are responsible for the 

 origin of some of the house-mouse variations. 



Myriads of mutations may arise and be lost in nature 

 without ever being seen by man. Where accidentally a 

 mutation has struck the fancy of man, he has secured and 

 propagated it in captivity, selecting in the following genera- 

 tions for the particular character in question. Where a 

 mutant form arises in any one of the thousands of labora- 

 tories breeding mice today {113, 71, 160, 146), it is quite 

 likely to come to the attention of man and be preserved. 

 Thus the number of mutants recorded as found in the wild 

 is not comparable with the number observed to have origi- 

 nated in laboratory stocks nor is it legitimate to conclude 

 that the mutation rate in captivity is greater than that in 

 the wild. 



Gray (normal wild coat). The gray coat (4 U 2) of a wild house 

 mouse (see Fig. 12) is produced by the deposition of two 

 kinds of pigment (yellow and black) in different portions of 

 the hairs. Yellow is present normally in an apical or sub- 

 apical band of many of the hairs. In general upon the 

 ventral pelage the band becomes wider and the black pig- 

 ment less, giving the belly a distinctly lighter appearance 

 than the back. These pigment differences may be due to 



