46 VARIATION. 



accepted by those people who have learned to look upon genes 

 as upon potential factors in the development rather than as 

 upon determinants for characters. 



We may not even say, that some genes are more important 

 than others, or that, of a certain number of genes one is the 

 determining one and the others are modifying factors. In many 

 animals we have come to know a certain gene, which is neces- 

 sary for any production of pigment. If this gene lacks, the ani- 

 mal growing from the germ will be an albinto. The difference 

 between a black and a grey animal on one hand, and that be- 

 tween a black and an albino is not of the same magnitude. But 

 we may never think, that the gene which distinguishes colour- 

 ed from albino is "stronger," than that which distinguishes 

 black from agouti or chocolate from cinnamon. 



The same gene which by its presence or absence "determ- 

 ines" the difference between a black and an albino mouse, will 

 in other cases bring about the difference between a yellow and 

 an albino, or between a pink-eyed lilac and an albino, a differ- 

 ence therefore, which is certainly not of the same magnitude 

 as that between black and chocolate, or black and agouti. 



We happen to possess pink-eyed white mice, which we took 

 for albinos for some time after they came in our possession. 

 They have, however, few minute spots of pale lilac-colour. A 

 close search is necessary to distinguish between such mice and 

 albinos, and we have one yoimg mouse produced by this family 

 whose status is undetermined. Only test-matings will show 

 whether this animal has, or lacks, the same gene which pro- 

 duces the difference between our common glossy blacks and 

 albinos. 



Therefore, if we start breeding-work on colour of mice with 

 some of these animals, we might declare that this gene was of 

 minor importance, and that the result of its coSperation to the 

 development, was not as great as that of the gene which dis- 

 tinguishes black from agouti. 



The same gene may have a decidedly different effect on two 

 divers combinations. In the brown rat, agouti animals are 



