SEXUAL SELECTION 235 



sexual selection that, even in those groups of the animal kingdom 

 which are in general sexually monomorphic, there always occur some 

 species in which male and female are quite different, and a host of 

 species in which both sexes are alike in the main, yet with differences 

 in certain minor points. Among the parrots similarity of colouring 

 prevails as a general rule, but in New Guinea there lives a parrot the 

 female of which is a gorgeous blood-red and the male a beautiful 

 light-green; minor differences occur in many species, for instance, the 

 female of the horned parrot (CyaTiorhamphus cornutus Gm.) lacks 

 the two long black and red feathers on the head, that of the grass-para- 

 keet (Melopsittacus undulatus) is a slightly paler green and has not 

 the beautiful blue spots on the cheeks which the male possesses. 

 Innumerable similar instances might be cited, serving to show that 

 all these distinguishing characters of the males have been acquired 

 step by step and piece by piece, and are slowly and independently 

 transferred to the females — if, indeed, at all. 



In yet another way the correctness of the Darwinian theory of 

 sexual selection may be deduced from the markings and coloration 

 of birds and buttei-flies. 



It has frequently struck me, during the long period in which 

 I have been studying brightly coloured birds and butterflies, that 

 those colour-patterns which are referable to sexual selection are much 

 simpler than those which must be referred to species-selection, espe- 

 cially in the case of what we call 'sympathetic coloration.' How 

 crude is the decorative pattern of most parrots, notwithstanding all 

 the brilliance of their colour. Large tracts of the body are red, 

 others green, yellow, blue, and occasionally one finds a red and blue 

 striped feather collar, a head which is red above and yellow under- 

 neath, but it is seldom that the colours vary enough in a small space 

 to give rise to a delicate decorative pattern. The gayest of parrots 

 are the Brush Tongues (Trichoglossus), and even among them subtlety 

 of coloration does not go further than the combination of three 

 colours on one of the long tail-feathers, or the production of a double 

 band round the neck, and so forth. If we compare with this the 

 complex markings of the inconspicuously coloured females of the 

 pheasants, of the partridges, or that of the upper surface of the 

 many birds in mingled grey, blackish-brown and white, which 

 resemble the ground or the dried leaves when they crouch, we find 

 that the colour-pattern in these cases is infinitely finer and more 

 complex. 



This seems to me quite intelligible when we remember, on the one 

 hand, that species-selection must operate far more intensively than 



