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THE YOUNG NATURALIST, 



Betularia or true Doulledayaria. Intermediate forms of this species do, 

 however, exist, but they are so rare that I never took but one, and never 

 saw but two." I venture to ask Mr. Gregson from how many specimens 

 were his long series of intermediate forms selected. Every one knows that 

 a good collection is by no means a fair illustration of the natural condition of 

 a species. The ordinary forms of all but really rare insects can be obtained 

 in a comparatively few years, then the process of selection begins, and by 

 purchase, by exchange with beginners who care more for quantity than 

 peculiarity, and by occasional capture we' gradually accumulate a number of 

 abnormal forms out of all proportion to their ordinary occurrence. In my 

 series of about a score of Betularia I have two intermediate forms, but I have 

 had hundreds of the species through my hands, of which these two were the 

 only intermediates. I have bred Betularia from eggs deposited by a black 

 female, and also from a grey female that paired with a black male. In both 

 cases I obtained black insects and grey ones, but no intermediates. This is 

 the special point to which I call attention. Mr Gregson raises a question 

 of great importance that I have never seen discussed ; the food of the plants 

 on which the larvae feed, and he argues that the effect of the food plant in 

 producing variation, depends upon the soil from which the plants draw their 

 own food, to call it so. This is well worth following up, and will probably 

 help us to an understanding of some of the difficulties, but it cannot affect 

 the question I raise. Why should Papilia Turnus have a different female in 

 the Southern parts of North America to what it has in the North, while the 

 males remain alike ; and why, even in the intermediate zone, are there no 

 intermediate forms ? Why again should Betularia produce both sexes of 

 both forms from one batch of eggs, and intermediate forms so very rarely ? 

 Till these questions are replied to, I cannot see that Mr. Gregson has even 

 touched my argument. 



It may be that the nature of the soil on which plants grow has some- 

 thing to do with variation. I do not know the evidence on which Mr. 

 Gregson bases his assertion, but if I understand it right, it is that dark 

 varities are produced by plants grown on older geological formations. This, 

 however, is clear, that the food of the larvae, wherever grown, is not the only 

 agent at work in producing variation, and also that it has not always the 

 same effect. Two larvae from the same batch of eggs and fed on the same 

 plant will produce insects differing from each other in some cases, while with 

 other species, difference in food does not appear to have any effect. My en- 

 quiry was not in this direction, but admitting that some cause produced 

 variation, I tried to show that some other cause was at work to render 

 such variation permanent. This has been effectually accomplished in 



