XIV INTRODUCTION. 



such an inherent tendency to vary exists is certainly beyond question, 

 for if it were not present, variation would apparently be an utter im- 

 possibility. This general hereditary tendency to vary will also be 

 more or less influenced in any particular brood by the character of the 

 parent moths ; whilst in seasonally dimorphic species, which, however, 

 are almost unknown in British NocTiLaa, but which are common among 

 the DIURNI, GEOMETRY, &c., the different seasonal forms appear to 

 vary indefinitely inter se, and also to vary towards each other, such 

 variation being undoubtedly due to heredity. 



Of the abnormal conditions mentioned above as tending to produce 

 variation, those connected with meteorological phenomena undoubtedly 

 stand first, and of these, moistures appears to me to be by far the most 

 important, and, in the production of melanic and melanochroic forms 

 of variation it appears to be the all-important factor, in developing the 

 inherent tendencies to vary in this direction. Our melanic or melan- 

 ochroic varieties abound in the most humid districts and become 

 generally less and less in number as the districts become drier and less 

 humid. Whether the humidity accompanies a high or low latitude or 

 high or low altitude, or whether it be produced by nearness to the 

 sea, by ocean currents (as in the Gulf Stream and Black Current of 

 Japan), or brought about by excessive condensation by mists, clouds 

 and fogs, the result appears to be always the same, the increase of 

 melanic or melanochroic varieties accompanies in a more or less direct 

 ratio, the areas of greatest humidity. This is not only so in the British 

 Islands, but the general principle is proven by reference to the fauna 

 of the Continents of Europe, Asia and North America and to that of New 

 Zealand, and there is no doubt that the theory is of general application, 

 subject only to local considerations and explicable disturbing causes. 

 Among our own extreme melanic forms, some of the most striking 

 are Xylophasia polyodon vars. infuscata and nigra, Bisulcia ligustri var. 

 nigra, Aparnea didyma vars. nigra, lugens, leucostigma and albistigma, 

 Miana strigilis vars. fasciata and cethiops, whilst Agrotis lunigera, A. 

 cinerea, A. corticea, A. segetum, A. nigricans, and A. pyrophila, have each 

 an absolutely black form. Epunda viminalis var. obscura from York- 

 shire is, in its extreme forms, perfectly black ; Dianthcecia conspersa 

 from the Shetlands, Hebrides and West Coast of Great Britain, Epunda 

 lutulenta from the West Coast of Ireland and Dianthcecia ccesia from Ire- 

 land and the Isle of Man also offer striking examples of intense melanic 

 variation, when compared with the normal European forms. These are all 

 extreme in the development of their melanic tendencies, but a very 

 large number of other species have similar melanochroic tendencies 

 developed in a lesser degree. Although moisture appears to act so 

 strongly in the development of these melanochroic tendencies, it un- 

 doubtedly obtains its greatest power by combination with the generally 

 applicable and broad features of "natural selection." 



It may be well here to point out that Lord Walsingham in his 

 Presidential Address to the Fellows of the Entomological Society of 

 London, January 1891, pointed out that moist areas are frequently 

 such as exclude a large proportion of the sun's rays, and that, there- 

 fore, the lessened number of chemical rays may tend to produce 

 melanism. This phase of the subject is fully dealt with in the 

 * Entomologist's Record and Journal of Variation/ vol, ii., No. 1, pp. 4-7. 



