1890.] THE YOUNG NATURALIST. 



27 



quoted by Mr. Dale as against my theory, support it strongly from my 

 point of view. In order to make this clear I must quote from the 

 address above referred to. Contrasting insects in cold and wet 

 climates with birds and animals which require to retain heat rather 

 than to absorb and as quickly radiate, or loose it, I said : — " Insects 

 on the contrary require rapidly to take advantage of transient gleams 

 of sunshine during the short summer season, and may be content to 

 sink into a dormant condition so soon as they have secured the repro- 

 duction of their species." 



Mr. Dale calls attention to the fact that " more rain falls in the 

 North of England where melanism is most frequent, than in the 

 South." There is therefore less sunshine, and under such conditions 

 it becomes more important to the existence and reproduction of a 

 species that it should possess such advantages as can be gained by a 

 power of rapid absorption, that advantage is best secured by the darkest 

 varieties, and tjiere we find melanism " most frequent." 



Again Mr. Dale points out that " even in the South of England 

 darker varieties occur on cold wet lands than on light dry soils." Yes ! 

 and for the same reason, namely, the advantage they derive by rapid 

 absorption of such limited heat as is available. 



Again, " the spring brood (of Pievis napi) which occurs in May, is 

 much darker and more strongly marked than the summer brood which 

 occurs in the month of July when the days are longer and the sun 

 more powerful." Surely the same argument applies in this case — when 

 the sun is less frequently shining, or less powerful, there is more need 

 of heat-absorbing colour to secure the vitality necessary to enable 

 the insects to fly and pair. 



" The upper sides of butterflies are more highly coloured than the 

 under." Yes ! the under side is often darker, but when at rest the 

 under, not the upper side, is exposed and is the means of conveying 

 to the body the necessary heat. In such of the Geomctridce as rest with 

 expanded wings, the under side is not usually darker than the upper. 

 The obvious advantage of protective resemblance must of course be 

 not ignored. I need scarcely point out that the extreme heat ex- 

 perienced on exposed surfaces of chalk is produced by reflection, or 

 radiation, not by absorption. 



36, Lowndes Street, 

 S.W. 



