644 THE DESCENT OF MAN 



to attract the males, we can understand how it is that they 

 have gradually been rendered, by sexual selection and sex- 

 ually limited transmission, more beautiful than the males — 

 the latter being left unmodified or only slightly modified. 



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 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 inher- 

 itance 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 con- 

 spicuous characters occur early in life, as no doubt oftea 

 happens, they will not be acted on through sexual selec- 

 tion 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 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 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 im- 

 mature plumage gives us an indirect record. Many birds, 

 especially those which live much on the ground, have un- 



