l8 COAT CHARACTERS IN GUINEA-PIGS AND RABBITS. 



realized as perfectly as is possible in an odd number of young, there 

 being 30 albinos in a total of 121 young. In cases (2) and (3) Darbi- 

 shire gives us no evidence that the animals which he mated were 

 really hybrids in the sense that they contained recessive albinism. He 

 calls them " extracted hybrids" because they had dark eyes, as did all 

 his primary hybrids obtained by crossing pink-eyed with albino mice. 

 The latter unquestionably were hybrids in the sense that they con- 

 tained recessive albinism, for in every case one of the parents was an 

 albino. The former may or may not have contained albinism recessive ; 

 Darbishire's experiments indicate that in some cases they did, and in 

 other cases they did not. Darbishire himself has shown that there is 

 in his mice (contrary to an erroneous prediction of Castle & Allen, 

 : 03, p. 612) no necessary correlation on one hand between recessive 

 albinism and dark eyes, and on the other hand between freedom from 

 albinism and the pink-eyed pigmented-coat condition. He calls especial 

 attention to the fact (p. 22) that in five different families of his mice 

 there occurred altogether seven pink-eyed pigmented animals, which 

 in each case had one albino and one pigmented parent, and which 

 accordingly must have been hybrids. In an earlier paper (Darbishire, 

 : 03, p. 285) he showed that a pink-eyed mouse of this kind does pro- 

 duce albino offspring when mated to albinos, a thing which his original 

 (pure) stock of pink-eyed mice never did. If, then, pink-eyed pigmented 

 animals may contain recessive albinism, is it improbable that dark- 

 eyed animals may in some cases fail to contain it ? 



Accordingly, in what have been called Darbishire's matings (2) and 

 (3), we may reasonably ask for evidence that the animals mated were 

 really hybrids. A pair which has produced albino offspring consists 

 unmistakably of two hybrid animals. Pairs which fail to do this are 

 questionably hybrid and maybe provisionally left out of consideration. 

 In mating (i) where both parents were unquestionably hybrid, since 

 each had an albino parent, Darbishire gets the precise Mendelian 

 proportion (one-fourth) of albinos. The question is, are fewer albinos 

 produced by hybrids in matings (2) and (3), in which the albino ances- 

 try was less. 



Omitting only pairs which failed to produce any albinos in matings 

 (2) and (3), Darbishire's observations may be summarized as follows : 



