96 MENDELISM CHAP. 



females and of grossulariata males. And this, as we 

 have already seen, is the actual result of such a cross. 

 Before leaving the currant moth we may allude 

 to an interesting discovery which arose out of these 

 experiments. The lacticolor variety in Great Britain 

 is a southern form and is not known to occur in 

 Scotland. Matings were made between wild Scotch 

 females and lacticolor males. The families resulting 

 from such matings were precisely the same as those 

 from lacticolor males and F I females, viz. grossulariata 

 males and lacticolor females only. We are, therefore, 

 forced to regard the constitution of the wild grossu- 

 lariata female as identical with that of the F I female, 

 i.e. as heterozygous for the grossulariata factor as 

 well as for the factor for femaleness. Though from 

 a region where lacticolor is unknown, the " pure " wild 

 grossulariata female is nevertheless a permanent 

 mongrel, but it can never reveal its true colours 

 unless it is mated with a male which is either 

 heterozygous for G or pure lacticolor. And as all 

 the wild northern males are pure for the grossu- 

 lariata character this can never happen in a state 

 of nature. 



An essential feature of the case of the currant 

 moth lies in the different results given by reciprocal 

 crosses. Lacticolor female X grossulariata male gives 

 grossulariata alone of both sexes. But grossulariata 

 female x lacticolor male gives only grossulariata males 

 and lacticolor females. Such a difference between 

 reciprocal crosses has also been found in other 

 animals, and the experimental results, though some- 

 times more complicated, are explicable on the same 

 lines. An interesting case in which three factors 



