192 NATURAL SCIENCE [September 1898 
In no family of birds are the males more gorgeous or more 
different from the females than in the Birds of Paradise. Darwin says 
in his treatise on Descent of Man and Sexual Selection that, accord- 
ing to Lesson, these birds are polygamous, but that Mr Wallace doubts 
it. The sexual selection therefore is to this extent less probable or less 
severe, but there is no doubt whatever about the difference of habits 
to which I attribute the difference of plumage between the sexes. 
In another passage in the same book Darwin writes: “ With Birds 
of Paradise a dozen or more full-plumaged males congregate in a 
tree to hold a dancing party as it is called by the natives, and here 
they fly about, raise their wings, elevate their exquisite plumes, and 
make them vibrate, and the whole tree seems filled with waving 
plumes.” 
It may be objected that the mechanical stimulation which I 
have adduced as the cause of the hypertrophy of the feathers, will 
not explain their brilliant colouring or the beauty and symmetry of 
their markings. To which I would reply that stimulation of the 
growth probably causes also a more intense production of pigment ; 
that symmetry of marking is a universal character in organic growth 
throughout the animal kingdom; and, thirdly, that very possibly 
the different qualities of the light to which males and females are 
exposed have something to do with the dull colours of the female 
which sits close with her young in obscure retreats, and the bright 
colours of the male which keeps more in the open. 
I have already referred to the fact that in some species the 
relative characters of the sexes are reversed, and it is the females 
which are larger, more pugnacious, and more elaborately adorned 
than the males. Darwin, of course, attributes this to the reversal 
of sexual selection, but it seems to me more rational to hold that 
the differences are not merely selected but called into existence by 
the habits and conditions. In these cases the male alone performs 
the duties of incubation and nursing, and the female takes all the 
initiative in courtship. Here, as in the males in the usual case, 
the peculiarities of the female only begin to develop when she is 
approaching maturity, the young of both sexes being similar to the 
adult male. Species of Zurnix in India and Australia are instances 
of this condition. 
J. T, CUNNINGHAM. 
1 MorRAB TERRACE, 
PENZANCE, 
(To be continued.) 
