INTRODUCTION. XI 



these species which have brightly coloured or white hind wings, that 

 the modifications of the typical markings are most distinct, but I be- 

 lieve that a large amount of variation exists in every species. 



Either in markings or colour or both combined, I find it usual for 

 species in the same genus to vary in the same direction and between the 

 same extremes of colour and markings ; or in other words, for species in 

 a genus to vary in the direction of normal forms of other species in the 

 same genus whenever that genus is natural and consists essentially of 

 closely allied species. I have already touched on this subject with 

 regard to certain Agrotidce in the paragraph on variation in 

 markings, but the following examples will perhaps explain more fully. 

 Apamea unanimis rarely varies, but has a fixed and constant appearance 

 (transverse lines), so also have A. ophiogramma (dark costa) and A. 

 pabidatricula (central band) in other directions. The polymorphic 

 Apamea didyma varies in the direction of all these species and has 

 well developed forms in each. A. gemma varies in the same directions, 

 as also does Miana strigilis which has no apparent structural difference 

 from Apamea. The genus Acronycta offers similar parallel varie- 

 ties in its species, both in the markings of the fore and hind wings 

 although perhaps, this parallelism of variation is better exhibited in the 

 way Agrotis tritici and A. cursoria follow in their variations the other 

 species of Agrotis, and the species of Tceniocampa each other. The 

 species of the latter genus are very striking in this particular, as 

 they have not only parallel varieties in colour but in markings. 

 Thus T. instabilis may be looked upon as the most polymorphic of the 

 group and varies from the palest grey to black. T. gracilis has a 

 parallel range, varying from greyish-white to black, some of the red 

 and brown forms being most remarkable. In T. opima we find the 

 same range in coloration, and dark varieties of this species and instabilis 

 are sometimes very difficult to separate without careful examination. 

 The typical grey coloration of gracilis is found not only in opima and 

 instabilis, but also in munda, stabilis and cruda. The typical red colour 

 of stabilis is found in instabilis, gracilis, munda and cruda. Some of the 

 specimens of T. instabilis are of the peculiar slaty-grey of populeti, 

 others red like typical stabilis, others grey like gracilis, and so on. In 

 markings, the typical central band in instabilis, becomes developed in 

 certain varieties of stabilis, munda, gracilis, opima, cruda, and populeti. 

 The typical row of dots forming the elbowed line just outside the 

 stigmata in populeti, finds parallel varieties in other species where such 

 dots are abnormal. In fact there appears to be in all natural and 

 closely allied genera a tendency for the species in such genera to be 

 formed on parallel lines. 



Different genera also frequently show parallel ranges of variation, 

 but it is doubtful whether such genera are not very frequently closely 

 allied. This is certainly the case between Apltcta, certain parts of 

 Hadena, Mamestra and Apamea. 



With regard to the probable causes of variation, it is almost im- 

 possible to speak of them in a few short paragraphs. So little is 

 positively known about the matter, and the connection between cause 

 and effect is frequently so indistinct and uncertain, that one appears 

 to be dogmatising when one really does not mean it. Perhaps the 

 subtlety in the working of Nature in this direction has led so many 



