VARIATION. 47 



lighter-coloured than black, and this difference is due to the 

 presence in agouti of one more gene. 



The presence of this gene in yellow animals however makes 

 the colour decidedly darker. In our house-rats the reverse is 

 true. Here black is dominant over agouti and decidedly dar- 

 ker. The same gene, however, which causes the difference, 

 makes the yellow animals, in which it is present, lighter. 



If we conceive of the action of a gene, as of an influence on 

 some process of development, part of a series of processes, we 

 understand how the dropping out of a single gene will act as 

 the breaking of a link in a chain. 



The lack of one gene may result in the inactivity of a series 

 of others or rather, we should say that at different moments 

 the development may proceed in one of two different direct- 

 ions. And the cooperation or otherwise of a gene may decide in 

 which direction. In such a case the resulting organism may in 

 one case come under the influence of a set of developmental 

 factors, both genes and non-hereditable influences, which is 

 different in one or several ways from the alternate set. We 

 think that this is the way, in which we must look for the ulti- 

 mate biomechanical explanation of the difference between the 

 sexes. 



The things which are responsible in one sex for a series of 

 reactions, which we do not see in the other sex, must be never- 

 theless common to both sexes. In animals, like pheasants, 

 where the sexes differ considerably, the result of a cross be- 

 tween two species is the same for reciprocal crossmatings, even 

 in later generations. The characters of the male of a certain 

 species will appear in the first or second generation of the 

 cross, even if a female of that species was used, and vice versa. 

 We need not at all accept the conclusion that, as the ultimate 

 origin of the difference in sex is due to the presence in the fe- 

 male of one gene, absent from the male, this same gene there- 

 fore directly causes the visible colour-difference and all the 

 other differences, that the same gene "determines" all these 

 things. At an early stage of the development of the chick the 



