The Colouration of Insects. 79 



In some cases, he remarks, variations are met with which may 

 with equal propriety be classed in either section. 



Albinism he finds to be very rare in British species, the only 

 locality known to him being the Outer Hebrides. This reminds us 

 of Wallace's remark upon the tendency to albinism in islands. 

 Xanthism, he finds to be more plentiful, and quotes the common 

 Small Heath (Ccenonympha pamphilus) as an illustration. In these 

 varieties we have simply a bleaching of the colouring matter of the 

 wings, and therefore no departure from structural lines. Melanism 

 arises from the spreading of large black spots or bars, or, as in 

 Biston betularia, a white moth peppered with black, dots by the 

 confluence of small spots ; for this insect in the north is sometimes 

 entirely black. It is singular that insects have a tendency to 

 become melanic in northern and alpine places, and this is especially 

 the case with white or light coloured species. (See Plate IV. Fig. 17) 

 It has recently been suggested that this darkening of these delicate 

 membranous beings in cold regions is for the purpose of absorbing 

 heat, and this seems very probable.* 



Of ordinary spots it is merely necessary to remark, that they 

 are all cases in our favour. Thus, in Satyrus hyperanthus we have 

 " the ordinary round spots . . . changed into lanceolate markings " ; 

 this takes place also in C. davus. The other cases of aberration do 

 not concern us. 



When, however, we come to the cases in which a species has 

 two or more permanent forms, it is necessary to show that they are 

 in all cases founded on structure lines. The patterns, as shown in 

 Plate V., Figs. 1-13, are always arranged structurally, and the 

 fact that not only are intermediate forms known, as in Araschnia 

 porima, Plate V., Fig. 6, but that the various forms are con- 

 vertible into one another, would in itself be sufficient to show that 

 in these cases there is no departure from the general law. In 

 Grapta interrogationis, Plate V., Figs. 8-10, we see in the central 

 figure one large spot above the median nervure, in the left-hand 

 form this is surmounted by another spot above the lowest sub-costal 

 branch, and in the right-hand figure this latter spot is very 

 indistinct. We have here a perfect gradation, and the same may 

 be said of the colouration of the lower wings. Take again the 

 three forms of Papilio Ajax in the same plate, Figs. 11-13, and we 

 have again only modifications of the same type. 



* Nature. E. Meldola on Melanism, 188.5. 



