CROSSING. 59 



But in a given combination of other genes, and other non- 

 genetic developmental factors, its influence will always be the 

 same specific influence. 



One and the same gene may have no effect at all upon the 

 development of an organism with a set of genes X in the 

 environment A , whereas it will exert an appreciable influence 

 upon the development of an other organism with a set of genes 

 y in the same environment A , or even upon the development 

 of the same organism with a genotype X in another environ- 

 ment B. 



Take the case of Primula sinensis. There is a gene which 

 makes the difference between certain white and red-flowered 

 plants. In a sufficiently low temperature the presence of this 

 gene in the make-up of an otherwise white-flowered plant, will 

 make the flowers red. In a high temperature it will not do this, 

 it has no influence upon flower-colour. In that high tempera- 

 ture however, another gene may determine red colour. Its 

 influence upon the development is manifestly different from 

 that of the first-named. Neither of these two genes should be 

 called a determinant for red. They determine red colour only in 

 certain special circumstances, in cooperation with a whole set 

 of other circumstances, other factors in the development. 

 There is a dominant yellow in mice, that is to say, certain yel- 

 low mice have that colour because of the fact, that they possess 

 a gene which in ordinary conditions of keeping laboratory 

 mice, has such an effect upon the development of the animals 

 that their preponderant colour is yellow. There are no wild 

 mice having a yellow colour. But it may not be said, that there 

 are no wild mice which possess this gene, which will turn the 

 ordinary laboratory mice yellow. Mus wagneri for all we know 

 it, may possess this gene, or Mus sylvaticus, and nothing forces 

 us to assume, that these mice should be yellow if they carried 

 it. Their remaining set of genes is different from that of Mus 

 musculus, and a gene which in ordinary circumstances turns 

 mice with the combinations of genes of musculus yellow may 

 not have that effect in wagneri or sylvaticus. Again, albinos may 



