( xxxi ) 



when studying them closely. There is no constancy. How far the varial)ility 

 extends is a primary subject of investigation. Lepidoptera are the most convenient 

 group for the study of variation, as they can be comparatively easily reared and 

 experimented upon. Variability may be fairly equal in all organs, or may be 

 excessive iu one aud slight in others ; variability of one certain organ may 

 always be accompanied (within one group of animals) by variability in certain 

 dtlier organs, or the variability may be quite independent. Variability (= state 

 of being different) is to be accounted for by two kinds of variation (= process 

 of becoming different) : firstly, individual variation, pertaining to brothers and 

 sisters ; and, secondly, generator)/ variation, pertaining to parent and offspring. 



Individual variation is normalli/ such that there is a gradation from one 

 extreme to the other, the number of individuals becoming, how^ever, smaller 

 towards the ends : monomorphisrn, respectively — chromatism. Or the individuals 

 which are all the offspring of the same parents fall into two or more groups, 

 which may or may not be connected by iutergradations : di- or pohjmorpliism, 

 respectively — chromatism. In every case there appear occasionally specimens 

 which stand outside the ordinary range of variation. Such aberrations are 

 generally aberrant in one organ only, and otherwise quite normal. Aberrations 

 should not be confouuded with monstrosities, in which the deviation from the 

 normal is due to injury of the early stages. The dimorphism of commonest 

 occurrence is sexual, the jiurely sexual characters being nearly everywhere in 

 sexually separate animals accompanied by differences in size, colour, aud some 

 kind of structure. Marked dimorphism in both sexes is comparatively rare. 

 But where such di- or polymorphism exists, and where the similar individuals 

 of both sexes are referred to under one name, it must not be implied that the 

 similar specimens are biological entities. They constitute like aberrant indi- 

 viduals a morphological or anatomical unit named in contradistinction to the 

 other individuals. A black c? of Papilio machaon is not more nearly related to 

 a black ¥ than to its own normally coloured brothers and sisters, and the brown 

 cJcJ of Mimas tiliae are as much the Si of the green ? ? as of the brown ones, 

 in spite of corporeal similarities and dissimilarities, and in spite of similarly 

 coloured specimens bearing the same name in classification. In most groups of 

 Lepidoptera the variability of the males is inferior to that of the otiier sex, there 

 being numerous species in which the male is monomorpliic, while the female is 

 polymorphic. In such cases the females are generally protectively coloured or 

 otherwise adapted in several directions, and may occasionally become mono- 

 morphic again or more constant than the males, in consequence of the best- 

 atlapted form becoming prevalent and finally appearing alone. The habit of 

 naming varieties has been in vogue among entomologists and conchologists more 

 than among any other systematists on account of the great variability of insects 

 and shells ; and as the student of variation requires names for the sake of 

 brevity of reference, there is nothing to be said against the habit. But since 

 every individual deviates in some organ from the mean, there is some danger 

 of the naming turning out indiscriminate. Fortunately, Lepidopterists have so 



