INTRODUCTION. XV 



Next to moisture, heat, and cold, must be considered the most 

 important factors in producing variation. All excessively cold areas, 

 if not subjected to the primary influence of humidity, appear to produce 

 pallid varieties with ill-developed and suffused markings. Lord 

 Walsingham, in his Presidential address to the Yorkshire Naturalists' 

 Union, 1885, referred to the melanic tendencies of high latitudes as of 

 common occurrence. This was pointed out in * The Entomologist's 

 Record,' vol. i., pp. 232-233, as an error, and it was shown 

 that insects from such latitudes did not produce insects which 

 were generally melanic, but on the contrary more pallid varieties, 

 and that melanism rarely occurred in high latitudes unless 

 accompanied by excessive moisture. This was acknowledged 

 by Lord Walsingham in his Presidential address to the Fellows 

 of the Entomological Society of London, 1890, when he stated 

 that he had in mind the general suffusion of markings which species 

 from such latitudes undergo, rather than actual melanism. For a long 

 time, cold was considered as the prevailing factor in the development 

 of melanic forms, the idea probably originating in the fact that most of 

 the known Alpine forms tended to be melanic ; but there appears to be 

 but little doubt now, that the melanism of high altitudes is due to mois- 

 ture rather than cold, since cold apart from moisture, as I have already 

 pointed out, appears totally unable to develop melanic forms, as the 

 almost entire absence of this tendency in the lepidoptera of the cold 

 icy plains of the Old and New Worlds abundantly testifies. 



Whilst cold apparently tends to produce pallid ill-marked speci- 

 mens, heat on the other hand tends to produce brilliantly coloured and 

 well-marked ones, and we find that almost all our species, having a 

 wide geographical range, become gradually brighter as we pass from 

 high latitudes towards the tropics. Cold, too, by delaying larval and 

 pupal existence is supposed to have an effect in the direction of dar- 

 kening the colour of lepidoptera, whilst heat is assumed to have an 

 opposite effect. In the present state of our knowledge, however, it is 

 difficult to say that this is, or is not so, but Nature, in those polar dis- 

 tricts, where larval and pupal existence is frequently delayed, pro- 

 duces as I have already pointed out, rather pallid than dark insects. 



Cases are on record in which disease undoubtedly appears to have 

 caused variation, and there is but little doubt that this is frequently 

 the case. Every entomologist who has bred lepidoptera largely, knows 

 that crippled specimens frequently vary in a remarkable manner from 

 the type. One of the most remarkable examples of this kind that I 

 have seen among the NOCTU^E is a specimen (previously referred to) of 

 Orthosia upsilon, belonging to Mr. A. Robinson of Brettanby Manor, 

 Darlington. This example accompanying a deformity in shape, 

 exhibits a complete variation in markings, the typical markings being 

 practically absent, and the specimen being characterised by a series 

 of longitudinal black marks, in connexion with disease, Dr. T. A. 

 Chapman of Hereford records in " The Entomologist's Record,' &c., 

 vol. i., pp. 271-272, a brood of Cuspidia alni, which all varied more 

 or less from the normal form, chiefly in the direction of losing 

 their stigmata, and darkening of ground colour. This brood came 

 from a race, which was interbred for some time, and during the 

 time that they interbred freely, the specimens were normal, but the pro- 



