PROGRESS IN THE STUDY OF VARIATION. 65 



mediate between glacial conditions and temperate condi- 

 tions would have a normal population of intermediates. 

 On such points the systematic treatises are not very reli- 

 able ; but I cannot discover that such a population exists 

 anywhere. With regard to Norwegian and Arctic speci- 

 mens the accounts are somewhat contradictory, and the 

 individuals I have seen are too few to warrant a definite 

 opinion. The same applies to Central Asian forms. 

 Certain it is that in the Alps on passing from the lowlands 

 to alpine conditions a population having an intermediate 

 as its normal form is not met with in June and July. At 

 this time in such localities the common forms are napi 

 of the second brood, bryonies, and occasional intermediates 

 which may be either pure-bred second brood bryonies or 

 cross-breds, or both. 



It is to be hoped that some who have the requisite 

 opportunities will make collections of large samples of these 

 forms, either in the North of Europe or in alpine regions, 

 and so contribute to the solution of this very attractive 

 problem. It will be understood that the account here given 

 is the merest outline of the facts. 



It is interesting to reflect that if we had known nothing of 

 the history of the origin of A. betularia, var. doubledayaria? 

 and if we had not actual historical evidence that it has re- 

 placed the type in the North of England we should most 

 certainly have been told that the one had been very gradu- 

 ally evolved from the other in the course of ages. The 

 same example shows moreover that there need be no 

 natural distinction between the case of a variety discontinu- 

 ously occurring with the type and the case of two local 



1 To the experimental evidence of discontinuity in this case I should 

 have added the following reference, E.R. Bankes, Ent. Rec, vii., p. 181. 

 Since the first part of this article was published, Mr. W. H. B. Fletcher 

 has kindly lent me the entire brood produced from a black female, reared 

 in captivity, which was tied out at Worthing where only the type is known. 

 There can thus be no doubt she was fertilised by a betularia male. The 

 offspring are sharply divided as follows : betularia, ten males, eight females ; 

 doubledayaria, six males, five females. Curiously enough one of the betularia 

 is an abnormally light specimen. The rest are either normal, or else com- 

 pletely black all over. 



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