HEREDITY 



425 



Fig. 7. Two of the grown-up young of a Black and of an Albino Guinea-pig. 



Fig. 8. A Group of four Young, produced by the animals shown in Fig. 7. 



crosses coincides with a lost racial combination. Then the phenomenon 

 is called reversion or atavism. Thus when yellow rabbits are crossed 

 with black ones, gray offspring are obtained similar to wild rabbits in 

 coloration. There is no longer anything mysterious about the process ; 

 it is simple recombination of different unit-characters formerly asso- 

 ciated together in the same race, but since isolated in some of the 

 derived races. 



Very different in nature, apparently, from the Mendelian inher- 

 itance of unit-characters is the result obtained when races of animals 

 are crossed differing in size or in the proportions of their parts. In 

 such cases the children are intermediate in character, and the grand- 

 parental conditions do not reappear among the grandchildren. The 

 result may be described as a blend apparently permanent. Fig. 13 

 shows the skulls of three rabbits, all adult, father, mother and son. 

 The skull of the son is shown between that of his parents, the mother's 

 skull being at the right. Size and proportions of parts are clearly 

 intermediate in the son. No grandchildren were obtained like either 

 grandparent in size. The color of the coat in this same family of 

 rabbits clearly followed Mendel's law, although the size characters 

 blended. The practical result is that one may at will produce a race 



