668 THE DESCENT OF MA2T. 



prevails, but not that of sexually limited transmission, then 

 if the parents vary late in life and we know that this con- 

 stantly occurs with our poultry and occasionally with other 

 birds the young will be left unaffected, while 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 youDg 

 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; conse- 

 quently if dangerous to the young they will be eliminated 

 through natural selection. Thus we cau 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 summer 

 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 apparently 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 record. Many birds, espe- 

 cially those which live much on the ground, have undoubt- 

 edly been obscurely colored for the sake of protection. In 

 some instances the upper exposed surface of the plumage 

 has been thus colored in both sexes, while 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, have generally been acquired 

 by the males through variation and sexual selection and 

 have been transmitted in various ways according to the 



