Xviii INTRODUCTION. 



most common form. Lyccena icdrus and L. adonis present a range of 

 sexual variation identical in character. According to Darwin's theory, 

 we have in these species, to assume that the males and females were 

 all originally brown or black, that blue has been gradually assumed 

 by the male until such a point has been reached that it is positively 

 permanent and invariable, and that it has then transmitted the colour, 

 in part, to the female. Here we are at once met with this difficulty. 

 Brown or black being the original colour, there must always have 

 been and still be a strong hereditary tendency for this original colour 

 to be present in some way or other, but as a matter of fact it never 

 occurs in the male, and it is difficult to suppose a species, shown to be 

 in a transition state as regards the female, never to show traces of re- 

 version in the male. On the other hand, Wallace's hypothesis that 

 the more brilliant was the original colour receives almost certain con- 

 firmation. By his hypothesis, that sex ( $ ) which remains constant, is 

 the one which is assumed to be of the original colour, and the large 

 percentage of dark females is sufficient to show the power of " natural 

 selection," whilst the large number of females with blue in them is 

 satisfactorily explained by supposing simple hereditary influence and 

 reversion at work. Certainly the more conspicuous blue coloration is 

 a disadvantage to the female, and I altogether incline more to a general 

 acquiescence in Wallace's view than to that of Darwin. There is still 

 another point. The greater number of Lyccenid species, especially in 

 tropical countries, tends to have the dimorphism less marked, and the 

 males and females are then more or less equally blue. Such is also the 

 case with our own Lyccena drgiolus, L. arion, L. bcetica &c. There is 

 no opposite result as we go north, for although a few species, as Lyccena 

 astrarche, have both sexes of the characteristic female form, yet they 

 are so in every latitude, and it must be borne in mind that the males of 

 icarus etc., are as constantly blue in Shetland and Scandinavia, and the 

 females as variable as with us. In Argynnis, Colias &c., the same holds 

 good, the male is the permanent, the female the variable form. 

 It appears to me that colour in relation to the sexes may be classified 

 as follows: (1) Male smaller and darker (more strongly coloured 

 and scaled) than the female, e.g., Bombyx quercus. (2) Male larger 

 and paler than the female, e.g., Agrotis cinerea. (3) Male smaller and 

 paler than the female, e.g., Hepialus humuli. (4) Male larger and 

 darker than the female, e.g., Epione vespertaria. Of these, I take it 

 Nos. 1 and 4 are practically identical, and whether they belong to Nos. 

 1 and 4 depends on the habits of the female, the more active females 

 going to Class 1, the more lethargic to Class 4. These would I think 

 together comprise a very large percentage of the British fauna. In 

 Class 2 the female is smaller, and unlike the few females that fall in 

 Class 4, generally produce proportionally fewer eggs. They fall, too, 

 largely among the NOCTUJE and species that rest on or near the ground 

 and hence much of the prevailing dark tint in the females is probably 

 due to " natural selection ; " it must also be considered that the tint 

 of the male though paler is often richer than that of the female, in 

 which case it is of the same genetic value as the colour in Class 4. 

 Class 3 is generally due to some individual pecularity, explicable in the 

 case of H. Jmmuli by " sexual selection," in the case of Fidonia piniaria 

 by "natural selection, "most of the instances in this class probably coming 



