494 THE DESCENT OF MAN. 



ored than the males. They have, also, become so quarrelsome 

 that they often fight together for the possession of the males, like 

 the males of other pugnacious species for the possession of the 

 females. If, as seems probable, such females habitually drive 

 away their rivals, and by the display of their bright colors or 

 other charms endeavor to attract the males, we can understand 

 how it is that they have gradually been rendered, by sexual selec- 

 tion and sexually-limited transmission, more beautiful than the 

 males — the latter being left unmodified or only slightly modi- 

 fied. 



Whenever the law of inheritance at corresponding ages prevails, 

 but not that of sexually-limited transmission, then if the parents 

 vary late in life — and we know that this constantly occurs with 

 our poultry, and occasionally with other birds — the young will be 

 left unaffected, whilst the adults of both sexes will be modified. 

 If both these laws of inheritance prevail and either sex varies 

 late in life, that sex alone will be modified, the other sex and the 

 young being unaffected. When variations in brightness or in 

 other conspicuous characters occur early in life, as no doubt often 

 happens, they will not be acted on through sexual selection until 

 the period of reproduction arrives; consequently if dangerous to 

 the young, they will be eliminated through natural selection. 

 Thus we can understand how it is that variations arising late in 

 life have so often been preserved for the ornamentation of the 

 males; the females and the young being left almost unaffected, and 

 therefore like each other. With species having a distinct sum- 

 mer and winter plumage, the males of which either resemble or 

 differ from the females during both seasons or during the summer 

 alone, the degrees and kinds of resemblance between the young 

 and the old are exceedingly complex; and this complexity ap- 

 parently depends on characters, first acquired by the males, being 

 transmitted in various ways and degrees, as limited by age, sex, 

 and season. 



As the young of so many species have been but little modified 

 in color and in other ornaments, we are enabled to form some 

 judgment with respect to the plumage of their early progenitors; 

 and we may infer that the beauty of our existing species, if we 

 look to the whole class, has been largely increased since that 

 period, of which the immature plumage gives us an indirect rec- 

 ord. Many birds, especially those which live much on the ground, 

 have undoubtedly been obscurely colored for the sake of protec- 

 tion. In some instances the upper exposed surface of the plumage 

 has been thus colored in both sexes, whilst the lower surface in 

 the males alone has been variously ornamented through sexual 

 selection. Finally, from the facts given in these four chapters, 

 we may conclude that weapons for battle, organs for producing 

 sound, ornaments of many kinds, bright and conspicuous colors. 



