1903.] IN FANCY MICE AND RATS. 75 



was actually made. The first generation wei-e " agoutis " of very 

 large size. Later genei'ations gave amongst others a strain of 

 blue, and of black-eyed white. A strain of agouti has also been 

 saved from it. He tells me that this formerly had the vihite feet, 

 a character he carefully bred out. Such a ci-oss may have affected 

 the whole race of fancy mice at the pi-esent day. Oui' seai-ch foi- 

 sti'uctural characters referable to syhaticiis, however, has failed 

 to show any case of one j)air of pectoral mammaa (as in sylvaticvs) 

 ov any ca,se of long hind foot. All specimens examined were pure 

 musculus in these features. On the other hand, a featui-e some- 

 times seen in fancy mice, and gi-eatly valued by exhibitors, is a 

 large eye, much exceeding the size in an ordinary mtoscuhis. But 

 this eye, though large, is still smaller than that of sylvaticus. 

 Nevertheless the large eye is a modern feature in the fancy, and 

 I think it not impossible it may have been dei'ived from a 

 sylvaticus ancestor. Further expei-iment alone can decide this 

 question. 



In order to appreciate what follows, the I'eadei- must have some 

 acquaintance with at least the outline of the Mendelian principles 

 of heredity. In their simplest expression these principles, as they 

 are exhibited for instance in the experiments of Cuenot (12), are 

 easily comprehended ; but when we pass from these simplest 

 phenomena to the more complex facts elsewhere witnessed, we 

 soon reach difficulties which our experimental evidence is as yet 

 only adequate to elucidate tentatively and in part. 



Cuenot experimented by making reciprocal crosses between 

 albino, pink-eyed, fancy mice, and wild grey mice [M. muscuhts). 

 He was careful to use wild mice in order to be sure that his 

 coloured form was pure. As a result he obtained always and 

 without exception grey mice. In Mendelian terms, grey is 

 therefore dominaiit over albinism, which is called by contrast 

 recessive. The first filial generation thus produced, which we 

 may conveniently call F^, when bred inter se, gave a, total of 

 198 greys and 72 albinos, constituting the second filial genera- 

 tion, or F.,. The ratio of dominants (D) to recessives (R) is here 

 2*75 to 1, a faii'ly near approach to the ratio 3:1, which on 

 the simplest form of the Mendelian hypothesis is to be expected. 

 In other words, the facts are, as Cuenot stated*, in agreement 

 with the supposition that in the formation of the gametes of the 

 hybiid F^, thei-e is complete segregation of the grey colour from 

 albinism, and that in both male and female hybrids there are on 

 an average equal numbers of gametes produced bearing each of 

 these two characters. 



According to the same hypothesis, the gi'ey mice in F should 

 consist of pure or homozygous greys (DD) and of heterozygous 

 greys (DR) in the proportion of 1 : 2. 



Cuenot tested this to some extent by breeding the F, dominants 



* Cuenot's paper seems to be the earliest application of Mendelian principles to 

 animals. 



