92 CORRELATED VARIATIONS. 



son suggests that this gradual replacement of a type by 

 its variety is probably due to success in the struggle for 

 existence of this particular dark strain, but it may 

 equally well be accounted for by supposing that it is the 

 result of a greater fertility. Intermediate strains are 

 not unknown, for in Belgium it seems clear that one 

 has succeeded in establishing itself, and in England it 

 is probable that they were once more common than they 

 are now. Intermediate forms are said to be plentiful 

 also in the Rhenish Provinces and Westphalia, and the 

 same is true of the black forms. Bateson says there is 

 no doubt that the black variety existed at an early stage 

 in the transformation, side by side with the light one. 

 The course of events has not been that the insects of 

 each successive district have become more and more 

 tinged with black, till they culminated in A. double- 

 dayaria, but rather that this variety, or less often one 

 of the intermediate forms, spread into or at least ap- 

 peared in the area, and either coexists with the type or 

 has replaced it. 



The few breeding experiments thus far made on the 

 moth show that there is an imperfect blending of type 

 and variety. Steinert raised from a black wild female 

 a brood of 75 typical and 90 varietal forms, but there 

 were no really intermediate ones, though two of the 

 examples classed as betularia were darker than the nor- 

 mal. That this female had paired with a typical male 

 the progeny thus resembling either the one parent or 

 the other, but not both is shown by the following ex j 

 periment, also recorded by Bateson.* W. H. B. 

 Fletcher tied out a black female, which had been reared 

 * Science Progress, vol. vii. p. 53, 1898. 



