BIRDS. 535 



time new conditions may produce some direct effect either 

 on both sexes, or from their constitutional differences 

 chiefly on one sex. We see only that this is subordinate in 

 importance to the accumulated results of selection. Judg- 

 ing, however, 'from a wide-spread analogy, when a species 

 migrates into a new country (and this must precede the 

 formation of representative species), the changed condi- 

 tions to which they will almost always have been exposed' 

 will cause them to undergo a certain amount of fluctuating 

 variability. In this case sexual selection, which depends 

 on an element liable to change the taste or admiration of 

 the female will have had new shades of color or other dif- 

 ferences 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 selec- 

 tion) 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, differ- 

 ently 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 rep- 

 resentative species, together with their young, differ hardly 

 at all from one another, so that the males alone can be dis- 

 tinguished, yet the females of most species within the same 

 genus obviously differ from each other. The differences, 

 however, are rarely as great as between the males. AVe see 

 this clearly in the whole family of the Gallinaceae ; 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, while the males differ to an extraor- 

 dinary degree. So it is with the females of most of the 

 Cotingidas, Fringillidse, 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 inexplicable 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, 

 while the female P. apoda is deep brown beneath. So, 



* Wallace, " The Malay Archipelago," vol. ii, 1809, p. 394, 



