THE YOUNG NATURALIST. 



27 



Gregson does not notice, yet this is a very important element in the case, 

 but it is one of such common occurrence that we seem to pass it by as a 

 matter of course, and never enquire as to the reason. There are really two 

 important elements in it, first the cause of the sexes differing, and second 

 the absence of intermediate forms. Why should there be more spots on the 

 wing of a male Pieris brassicce than on a female ; and how is it that the male 

 never approaches the markings of the female or vice versa. We have a common 

 butterfly Theclu rubi, in which the sexes are alike ; then we have another equally 

 common Zephyrus quercus, so closely allied to the first that they are generally 

 included in the same genus, but in the latter species the sexes differ, and 

 always differ. Surely there must be some reason for this, and in species so 

 nearly allied one might expect that sometimes a departure from the rule would 

 take place. Mr. Gregson is not the man to shirk an, argument because it 

 tells against him, and I cannot understand how he passed over the strong 

 point made of the absence of intermediate forms when the sexes differ. Our 

 own Papilio Mackaon, has the sexes alike, but I instanced others of the 

 same genus in which not only the sexes differ, but in which there was more 

 than one form of the female and still no intermediates. One of my illustra- 

 tions was the North American Papilio Tumus, and I pointed out that North 

 of Latitude 37° the sexes were alike ; that South of Latitude 42° the sexes 

 were so different that the female had long been considered a distinct species ; 

 and further that in the zone between these latitudes both forms occurred 

 and had been bred from one batch of eggs. Now surely an important 

 matter like this ought not to be passed over in silence when the 

 question of intermediate forms is discussed. Sexual variation is as 

 important, or perhaps more important than any other, and though there 

 may not be exact similarity between the cases of sexual and non-sexual 

 variation, to ignore sexual variation altogether will certainly not help 

 us to an understanding. I may be mistaken in asserting that where 

 the sexes differ, there are no intermediate forms, but I ask Mr. Gregson or 

 any one else, to point out a case to the contrary, and I ask further, if there 

 are no such intermediates, or if there are but very few, why is it so ? 



Leaving sexual variation I would point out that even with species Mr. 

 Gregson refers to, I specially named intermediate forms as existing. Of 

 Betularia for instance, Mr. Gregson says " Betularia has every intermediate 

 variety from nearly white to deep rich coloured brown or black ; in my two 

 long rows of it, they run imperceptibly into each other." My reference to 

 the species was as follows : " Black Betularia (Boubledayaria) are by no 

 means rare and are easily reared. But though I have bred a goodly number 

 of it, all that ever I obtained were either one form or the other, either true 



