288 THE CAUSES WHICH 



variation in almost every alar feature o£ the butterflies and moths. 

 The wing- is enlarged and. dwarfed, and in this process it becomes 

 prolonged, rounded, or scaleless ; the caudal appendages are found 

 to be occasionally eliminated in rich exotics, and they are fre- 

 quently abbreviated. Then, as regards colour, the upper surface 

 may be considered as diversified with shades of dai'k and light. 

 A dark confused and composite ground, under favourable con- 

 ditions, is resolved by an increase of light spots into clear generic 

 dark lines and blotches inaugurating the species, which then 

 fade away, and the wing is finall}^ reduced to a paler hue ; or 

 on a light ground arise dark spots which coalesce to form lines, 

 enlarge to bands, and finally overcast the wing with shade. 

 The paler tint in many species may be traced to a fundamental 

 white. Perhaps the earliest butterflies were white ones? And 

 this colour we may often ti*ace through its glandular stains of 

 yellow, orange, or the softest rosy, into the more compound tints 

 of slate colour and brown, or we may resolve these stains again 

 in other specimens in an inverse fashion. The darker tint 

 lastly passes on by a rougher transition into clayey browns and 

 funereal black. For instance, take our English Swift Moths. 

 We can plainly trace orange differentiated in a series of males 

 into white, or white forming into orange bands. These run 

 together into spots, and eventually give place to a uniform tawny, 

 which in the fleet-winged Northern Swift is afterwards treated 

 in identical fashion with brown, so as to evoke intermediate 

 patterns in three colours. Again, examples of the very common 

 garden Lackey Moth of either sex may be selected from a cabinet 

 row of an ochreous colour with two purple lines crossing the 

 fore-wingj and in others of the series we witness these lines 

 filled in with purplish, and forming the edging of a band. To 

 this central ribbon a purplish blush extends, leaving a distinct 

 marginal line of the primitive colour, which by gentle transition 

 of the purplish blush to a deep sandy-red stands out sharply 

 defined in many specimens. A similar chromo-lithographic 

 process may be observed recorded on the wings of the common 

 Drinker and many other moths. 



In butterflies that produce dimorphic and pol^Tuorphic females 

 we may proceed in like manner to unravel the vestiges of divergence. 

 But what is perhaps more surprising, it has been found, despite 



