400 



That climate is a more powerful and a more frequent cause of 

 variation than food, would appear from several facts adduced by 

 Mr. MacLachan as well as others. In the above paper Mr. Mac- 

 Lachan gives instances of more than thirty species of Lepidoptera, 

 in addition to some whole genera, which become more or less 

 " melanised" when occurring in the North of Englaud or Scotland, 

 "the darkening becoming more marked the further we proceed 

 northwards." * On the other hand, he says "there are a few 

 species which become paler the further we proceed north." 

 A yet more remarkable fact has been noticed in the Shetland Isles, 

 where " locality actually changes and confuses the normal sexual 

 variation in the colour ;" a form of Hepialus humidi (ghost moth) 

 being met with there, in which the male is coloured the same as 

 the female, the fine white silvery hue, which characterises the male 

 in England, being entirely lost. Variation is found also to extend 

 to other particulars beside colouring. It has been observed that 

 some species " which are double brooded in the South of England 

 have only one brood in Scotland." It has been also said that 

 " many species in Scotland habitually remain there in the pupa 

 state for two or three years, although in the south this would form 

 quite the exception to the same species ;" and it is thought that 

 " this retardation of development may probably have some effect 

 in causing variation." 



Without reference to locality, it is a well-known fact that some 

 species which are variable in the larva state are constant in the 

 imago, and, on the contrary, that others which " are very variable 

 in the imago state, are constant, or nearly constant in the larval, "t 



Variation in insects prevails mostly in species which are widely dis- 

 persed. This we might expect ; as, when spi'ead over a far-extended 

 region, they are exposed to very varying conditions of life. But 

 Mr. Wallace teUs us that " what is commonly called variation 

 consists of several distinct phenomena which have been too often 

 confounded." These he enumerates as, " 1st, simple variability ; 



* A similar remark is made bya writer in the " Zoologist," (p. 1731), who 

 thinks, however, that " this deep colour is given them by the quantity of iron 

 in the soil, which is taken up by the vegetation on which they feed." 

 t Ent. Trans., 3rd Ser., vol. ii., pp. 460 and 465. 



