MELANISM AND MELANOCHROISM. 123 



adds : — " An abundance, scarcity, or change of food, and its 

 environment during the larval stage, are known to produce 

 great variations among certain species ; with A. antipodiini, the 

 finest marked and best developed forms are evolved in humid 



seasons, and such succeeding wet winters I am 



not in a position to say that the tendency to darkening in the 

 colours of the species is, in humid seasons, a case of atavism ; 

 but I incline to such an opinion, as the preceding geological 

 period in New Zealand was immensely more humid ^ than the 

 present, a fact which I think favours such a view." 



Here, then, is a positive proof of a primary exciting cause, 

 and, if nature develops dark varieties in wet seasons more 

 than in others, and, in wet seasons, species normally in- 

 variable, are more or less variable — vide Entoni., vol. xxii., pp. 

 37-39, as well as my own remarks on the sudden development 

 of a melanic tendency in wet seasons — I think it a fair deduc- 

 tion to assume that the variation is primarily the result of 

 meteorological influence, and that humidity is essentially the 

 exciting cause. 



There is no doubt that "natural selection" is almost entirely 

 the direct cause of the variation in colour (both melanochroic 

 and leucochroic) of species which rest upon the bare ground, 

 and that the insects which assimilate most completely to the 

 geological strata on which they rest, are those which are 

 preserved in the struggle for existence, and which finally 

 establish a particular form in the district. Nocture and many 

 Geometr.e rest on trees, fences, or the ground. Many rocks, 

 and almost all trees and exposed fences are darkened by rain,'^ 

 hence the assimilation becomes more complete in naturally 

 wet and humid districts. But to take a special case where the 

 direct influence of humidity can, compared with " natural 

 selection " have very little to do with the variation in colour. 

 Gnophos obsciirata, as is well-known, is one of the most striking 

 examples. I devote a drawer to the series of this species in 

 my collection, and what do I find ? I find that among the 

 specimens captured on the chalk a large percentage of pale 

 forms occur, assimilating most perfectly to the chalk on which 



^ It is more than probable that this was so in Britain, as the country was, in 

 comparatively recent times, much more covered with forests and woods than at 

 present, and this alone would malce tiie country more humid. — J.W.T. 



^ Dr. Chapman writes : — " In your paper {Ent. Record, p. 56) you criticise the 

 idea of rain pcrmatun'.ly darkening rocks, trees, etc., by pointing out that they dry 

 in the intervals, but that is precisely what they do not do. Such objects are often 

 wet, especially in some localities, for weeks together, even in the summer ; of course 

 they do dry sometimes, but generally they retain one lot of moisture, until they get 

 another during the greater part of the summer" (in litt.) — ^J.W.T. 



