INSEPARABLE EFFECTS 33 



his theory, mated his hybrids in two pairs of characters 

 with the parents carrying both the recessives, and equal 

 numbers of all the four second-generation groups were 

 produced. In this case the hybrid walnuts were mated 

 with singles, that is, with the parents carrying both 

 the recessives, and there resulted 644 walnuts, 705 roses, 

 664 peas, and 716 singles. 



Another case in which the effects of the individual 

 factors cannot be identified separately occurs in Miss 

 Durham's experiments on the colours of mice, whose 

 details are to be found in the fourth " Report to the 

 Evolution Committee " and in the first volume of the 

 Journal of Genetics. In Miss Durham's first experiment 

 agouti-coloured mice were mated with chocolates, and 

 their hybrids' progeny consisted of agoutis, cinnamon- 

 agoutis, blacks, and chocolates in the ratio 9:3:3:1. 

 Thus there are two pairs of characters concerned, and 

 each of the four colours is the result of two separate 

 factors. As it is impossible, however, to identify the 

 effects of each of the two factors producing any one of 

 the four colours, we must set out the set of four groups 

 with " unknown " symbols, thus : 



Agouti XY 9 



Cinnamon-agoviti Xy 3 



Black xY 3 



Chocolate x y 1 



But the next experiment showed that there were 

 still other factors concerned in the production of these 

 colours. Black was mated with a fifth colour, silver- 

 fawn, whose very mating with black proves the presence 

 of one other factor at least. But the hybrids' progeny, 

 which consisted of blacks, blues, chocolates, and silver- 



c 



