q8 Hagedoorn. 



same set of factors influencing coat-colour has originally been present, 

 only, in some of them there have occasionally been lost genetic factors 

 in some individuals, which factors have, as far as we know, never 

 been lost by individuals of the of the other groups. 



Thus, in the housemouse, we have been able to study a large 

 number of genetic factors, which each of them are present in some, 

 and absent from other individuals, and we know that many of these 

 factors have never yet been reported lost from individuals of the 

 rabbit, the cavy or the brown rat. This last animal, because of the 

 fact that only two or three factors of the original set are ever absent, 

 is the least rich in colourvarieties. 



My own experiments were begun in 1902 in Holland, and con- 

 tinued there for five years, after which I began another series of 

 experiments in California, which were concluded in 1909. The publi- 

 cation of this paper has been delayed to permit me to study one 

 more factor, which has been studied by nearly all of the authors on 

 the subject, but which hitherto had always been absent from all the 

 animals bred by me. By both these last experiments and by the 

 recent publication of Miss Durham's work, my doubts as to the 

 phenomena observed by several authors in the inheritance of yellow 

 colour have been completely removed. 



I can therefore say that mj' results, for so far as they are com- 

 parable, fully corroborate those of all of the other authors on the 

 subject. Among the factors studied by me there are two which until 

 now do not seem to have been studied by any one. 



Plate, in a criticism of Baur's book, observes even, that, as he 

 in his cultures, which comprised many individuals, never met such 

 factors as these two, B and F, they can not exist. I hardly know 

 what to answer to such criticism. 



Method. 



In the course of my experiments with mice, I bred somewhat 

 over six thousand animals. I have found it impossible to attach a 

 number to a mouse in any satisfactory way, as can be so easily done 

 in the case of birds or larger mammals. I therefore had to adopt a 

 system of labels, which followed the mouse from cage to cage. On 

 each label I marked the animal's number, its sex and its colour, and 

 whenever it was necessary to keep two animals of the same colour 

 and the same sex together in one cage, I took care to earmark one 



