XIT INTRODUCTION. 



to take interest in the subject since the publication of Darwin's theory 

 of " natural selection," but whether this be so or not, it is beyond 

 question that the study of variation has become the favourite hobby 

 of a large percentage of entomologists. 



First and foremost of the causes which tend to develop variation, 

 my own observation places " natural selection." But " natural selection " 

 is based on such a complex combination of circumstances that it is 

 perhaps advisable to take some of these into consideration, isolating 

 each particular factor as much as possible, but bearing in mind, that 

 whilst each factor is working in its own particular groove towards a 

 certain end, it is modified in every possible way by other factors, the 

 sum total producing the final effect as seen in the varying character of 

 our insects, and which we term "natural selection." Meteorological 

 causes appear to be the most direct active factors in the production of 

 variation, and inaseries of papers entitled "Melanism and Melanochroism 

 in British Lepidoptera " which I am contributing to the pages of 

 ' The Entomologist's Record and Journal of Variation,' I am working 

 out what appear to me to be the chief active agents in the production 

 of this particular form of variation, and at the same time, I have 

 attempted to correlate the different suggested causes with each other 

 nnd with " natural selection." I have also freely criticised previously- 

 formed theories and attempted to prove or disprove them by the ma- 

 terial we now have at hand. Although meteorological causes appear 

 to me to be the more or less active agents in producing variation ; 

 there can be but little doubt that heredity, disease, food, &c., each adds 

 its share towards producing the sum total of variation. We will 

 therefore, now look briefly at each of these probable causes. 



That " natural selection " has a very great deal to do with varia- 

 tion, no one who has made observations on the subject can doubt. 

 Among the species in the group that we are considering, some striking 

 examples occur. Take for instance the imagines of the Bryophilidce, 

 clothed in various shades of green, grey and yellow, with black or 

 reddish marks in the various species. These assimilate so closely to 

 the lichens on which the larva? feed, that only a trained eye detects 

 them resting in their chosen habitation, on the stone walls where they 

 occur. Different walls have variously coloured lichens growing on 

 them, and it is found that the environment determines the prevailing 

 colour of the species in a given locality. The greyer the lichens on 

 the wall, the greyer will be BryopJiila glandifera, and the greener the 

 lichens, the greener will be the moth. Hence on the former walls, 

 var. par will preponderate, on the latter, typical glandifera. Certain walls 

 in Deal are covered with yellow lichens and there Bryophila perla var. 

 flavescens occurs, whilst on the old dark granite walls, and on the 

 dark damp walls of the west coast we find var. svffusa, and so on with 

 the different varieties in this genus. But in such localities, other forms 

 will, of course, occur with whatever may be the prevailing form ; these 

 other forms are due without doubt, to hereditary influence, but a large 

 percentage will be of a characteristic form. The same facts hold good 

 with the typical form, and with the vars. suffusa and olivacea of Polia 

 clii. These different forms are found to be prevalent in different 

 localities, each assimilating in its own particular direction with its own 

 special surroundings, the white, dark, and greenish forms all respond- 



