430 



ZOOLOGY 



dc4's law thus affords a sort of explanation of why a child 

 often resembles a grandparent rather than a parent. For a 

 recessive condition that appears in only one grandparent 

 on each side cannot appear in the following generation, but 

 only in the grandchildren. 



In addition to albinos breeders have obtained mice Tsathout 

 the l)laek pigment in the hair; the result is a golden yellow. 



By reducing the j-ellow at the 

 tips of the hair, the color be- 

 comes chocolate. When the red 

 and j'ellow are both absent from 

 the tips, a black race is produced. 

 In addition, some mice move in 

 circles, producing the race known 

 as waltzing mice. 



Mice are of interest not only 

 on account of the evolution of the 

 fanc3' races, but also on account 

 of the rapidity with which they 

 as well as their allies, the rats, 

 have spread o^'er the world. The 

 house mouse was originalh' lim- 

 ited in its distribution to Asia. It made its way into 

 Europe. By hiding in the cargoes of vessels, it has made 

 its way o\'er the whole world. America originally had not 

 a single representative of the genus INIus to which rats 

 and mice belong. Both rats and mice were imported to 

 America liy the earl}^ exi^lorers. Of the rats the roof rat 

 seems to have been imported by the early Spanish discoverers 

 to the Southern States, where it still persists. It originated 

 in Egypt. The second was the black rat, believed to have 



Fig. 39:?. — Grt.<riir .Tohann 

 Mendfl. 



