( xxxi ) 



when studying tliem closely. There is no constancy. How far the variability 

 extends is a primary snbject of investigation. Lepidojitera are the most convenient 

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

 experimented npon. Variability may be fairly e(jnal in all organs, or may be 

 excessive in one and slight in others ; variability of one certain organ mav 

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

 other organs, or the varialjility may be quite independent. Variability (= state 

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

 of becoming diiferent) : firstly, indkidual variation, pertaining to brothers and 

 sisters ; and, secondly, generatory variation, pertaining to parent and oft'spring. 



Individual variation is normalhj such that there is a gradation from one 

 extreme to the other, the number of individnals becoming, however, smaller 

 towards the ends : monomoi'phism, respectively — chromatism. Or tiie individuals 

 which are all the oft'spring of the same parents fall into two or more groups, 

 which may or may not be connected by iutergradations : cli- or pohimflrphism, 

 respectively — chromntism. In every case there appear occasionally s]iecimens 

 which stand outside the ordinary range of variation. Such ahft-rafioiis are 

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

 should not be confounded with monstrosities, in which tlje deviation from the 

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

 occurrence is sexual, the ])urely sexual characters being nearly everywhere in 

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

 kind of structure. Marked dimorjihism in both sexes is comi)araiively 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 tliat the 

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

 vidnals a morphological or anatomical unit named in contradistinction to the 

 other individuals. A black c? of PapiUo mackaon is not more nearly related to 

 a black ? than to its own normally coloured brothers and sisters, and tlie brown 

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

 in spite of corporeal similaritiis and dissimilarities, and in s])ite of similarly 

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

 Lepidoptera the variability of the 7)i'(lrs is inferior to that of the other sex, there 

 being numerous species in which the male is monomoriihic, wjjile tiie female is 

 polymorjjhic. In such cases the females are generally jjrotectively 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- 

 adapted form becoming i)revaleut and finally apjieai'ing alone. The habit of 

 naming varieties has been in vogue among entomologists and concholou-ists more 

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

 and shells ; and as the student of variation re(piires names for the sake of 

 brevity of reference, there is notliiiig to be said against the habit. But since 

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

 of the rjaming turning out indiscriminate. Fortunate!}', Lepidopterists have so 



