J. W. H. Harrison 229 



of 0. dilutata, the insect here clings to the pale ground of the original 

 exponents of the species which the others have abandoned. 



We thus see brought into action on a minor scale that geographical 

 isolation, which in parallel ways throughout the world and in a specially 

 striking way in the volcanic valleys of the Sandwich Islands, has been 

 so prolific in the production of new species. 



(e) The question of Melanism. 



We now turn our attention to the vexed question of melanism in 

 the lepidoptera — a most striking genetical phenomenon and one that 

 has progressed and is progressing under our very eyes. Of this phase 

 of variation the genus Oporabia in all its species, but more especially in 

 0. dilutata, aflFords excellent examples. 



In our investigation, although primarily concerned with Oporabia, 

 we shall not restrict ourselves solely to that genus but shall treat the 

 matter from a broad standpoint, adducing instances from any genus or 

 species capable of illustrating any given point; to do otherwise would 

 be an ill-judged procedure, so productive are other genera in melanic 

 species. 



As to the inciting cause many theories have been devised ; cold 

 humidity, soil, light deficiency, food and smoke each having had its 

 advocate in the past either in the crude form of direct cause or as 

 indirect units in a more complex system. But the most plausible theory 

 yet advanced was that urged by Tutt^ in his paper on "Melanism and 

 Melanochroism," and based in the interaction of moisture, smoke and 

 natural selection. Despite the plausibility of this combination the 

 operation of some of the other possible agencies is not to be dismissed 

 lightly. Cold, for instance, so ofi"handedly cast to one side by Tutt, is 

 the direct agent in producing the melanism in two cases to be referred 

 to shortly. We may not agree with Walsingham that the necessity of 

 blackish tones because they are absorptive of heat has, helped by natural 

 selection, ended in their acquirement and fixation ; too many instances 

 of pale Arctic and Alpine lepidoptera can be cited to disprove its 

 universal occurrence. Nevertheless, the " cold " theory is the only one 

 explanatory of such cases as Dicranura vinula var. phantoma and 

 Poecilopsis isabellae. Both of these insects are derived from forms with 

 whitish ground colour, the former from the type D. vvnula and the latter 



1 Tutt, " Melanism and Melanochroism in British Lepidoptera," Entomologists' Record, 

 Vols. I. and ii. (1890, 1891). 



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