464 THE DESCENT OF MAN. 



the accumulated results of selection. Judging, however, from a 

 widespread analogy, when a species migrates into a new country 

 (and this must precede the formation of representative species), 

 the changed conditions to which they will almost always have 

 heen exposed will cause them to undergo a certain amount of 

 fluctuating variability. In this case sexual selection, which de- 

 pends on an element liable to change — the taste or admiration of 

 the female — will have had new shades of color or other differences 

 to act on and accumulate; and as sexual selection is always at 

 work, it would (from what we know of the results on domestic 

 animals of man's unintentional selection), be surprising if animals 

 inhabiting separate districts, which can never cross and thus 

 blend their newly-acquired characters, were not, after a sufficient 

 lapse of time, differently modified. These remarks likewise apply 

 to the nuptial or summer plumage, whether confined to the males 

 or common to both sexes. 



Although the females of the above closely-allied or representa- 

 tive species, together with their young, differ hardly at all from 

 one another, so that the males alone can be distinguished, yet the 

 females of most species within the same genus obviously dill'er 

 from each other. The differences, however, are rarely as great as 

 between the males. We see this clearly in the whole family of the 

 Galllnaceee: the females, for instance, of the common and Japan 

 pheasant, and especially of the Gold and Amherst pheasant — of the 

 silver pheasant and the wild fowl — resemble one another very 

 closely in color, whilst the males differ to an extraordinary de- 

 gree. So it is with the females of most of the Cotingidae, Frin- 

 gillidse, and many other families. There can indeed be no doubt 

 that, as a general rule, the females have been less modified than 

 the males. Some few birds, however, offer a singular and inex- 

 plicable exception; thus the females of Paradisea apoda and P. 

 papuana differ from each other more than do their respective 

 males;' the female of the latter species having the under surface 

 pure white, whilst the female P. apoda is deep brown beneath. So, 

 again, as I hear from Professor Newton, the males of two species 

 of Oxynotus (shrikes), which represent each other in the Islands 

 of Mauritius and Bourbon," differ but little in color, whilst the 

 females differ much. In the Bourbon species the female appears 

 to have partially retained an immature condition of plumage, for 

 at first sight she "might be taken for the young of the Mauritian 

 "species." These differences may be compared with those in- 

 explicable ones, which occur independently of man's selection in 



' Wallace, 'The Malay Archipelag-o,' vol. ii. 1869, p. 394. 

 " These species are described, with colored figures, by M. F. Pollen, 

 in 'Ibis,' 1866, p. 275. 



