EVIDENCE FROM EXPERIMENT 163 



mouse-colour. If this last factor should be elim- 

 inated, the result would be the production of black 

 mice, an explanation which is strongly supported by 

 crossing black mice with the normal animal and 

 making a mathematical analysis of the proportion- 

 ate numbers of hybrid forms. If the pigment- 

 factor, which determines the formation of the black 

 colour, should disappear in black mice, brown ani- 

 mals would be produced and, if the factor control- 

 ling the distribution of colour should be lost, then 

 blotched mice would appear. Finally, the suppres- 

 sion of the general colour-factor would lead to the 

 development of white or albino mice. We may 

 thus witness the formation of new varieties through 

 the loss of factors, but the addition of new factors 

 has not yet been observed, unless the additional toe 

 in Castle's four-toed race of guinea pigs should be 

 so interpreted. 



Interesting and profoundly important as are the 

 results of the Mendelian investigations, it must be 

 admitted that, so far, they have rendered but little 

 assistance in making the evolutionary processes 

 more intelligible and, instead of removing difficul- 

 ties, they have rather increased them. That the 

 Mendelian laws of inheritance are immensely valu- 

 able to breeders, is not to be questioned, but it re- 

 mains to be determined just how far they are oper- 

 ative in nature and what bearing they have upon 

 the problems of evolution. The experimenter can, 

 if he so desires, preserve all of his plants or animals 



