The Genetic Factors in the Development of the Housemouse, etc. 103 



tens of thousands. The range of genetic factors which we can hope 

 to study, is necessarily very small for each organism, for we can 

 only study anyone of these factors by comparing such organisms to 

 whose development the factor has cooperated, to others which have 

 developed without it. And it stands to reason that this at once 

 limits the set of genetic factors which we can ever hope to study to 

 those whose presence is not indispensable for the life of the organism. 

 It has too Uttle been emphasized that in the special branch of 

 biomechanics which is Mendelism, we are concerned exclusively with 

 the study of the genetic factors in the development of the organisms. 

 As yet we can only detect these genetic factors by rather elaborate 

 breeding experiments, and we observe the influence which each genetic 

 factor lias on the development ans so on the final qualities of an 

 organism. The time may come when we will be able to state the 

 chemical formulae for many of these genetic factors. If so, it will 

 be logical to make use of these in the genetic formulae. In the 

 meantime we will do wise to use symbols for them, which are non- 

 committal, and which simply serve to distinguish certain genetic 

 factors about which we know that under definite circumstances, they 

 have an influence on the development which we can predict before- 

 hand, and that they are independantly segregated in gametogenesis. 



I will therefore, throughout this paper, use the first letters of 

 the alphabet to denote the different genetic factors. 



For each of these factors, I will try to show the influence it has 

 on the colour of the coat, that is to say, which is for several com- 

 binations of other genetic factors the difference in colour between 

 animals having and animals lacking it. To show the regular distri- 

 bution of each of these factors over half the number of gametes 

 produced by an animal heterozygous for it, I will, for each factor, 

 add the number of young with and without the factor produced from 

 parents of which one was heterozygous for it (Xx), the other lacking 

 it (XX). 



I will, as much as possible, try to bring into account the work 

 which has been done with the four domesticated rodents, mouse, 

 brown rat, rabbit and cavy, and in doing so will use the same set 

 of symbols as for the mice. I have refrained from giving extensive 

 tables of matings in the text, and will limit myself to some tables 

 at the end of the paper, necessary to prove for each of the factors 

 studied its independance (and distinctiveness) from all or several of 

 the others. 



