Chap. XVI. SUMMARY. 287 



mission, 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 left un- 

 affected. 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 they will, if dangerous to the young, be elimi- 

 nated through natural selection. Thus we can under- 

 stand 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 un- 

 affected, and therefore like each other. With species 

 having a distinct summer and \Ainter plumage, the males 

 of which either resemble or differ from tlie 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 com- 

 plexity 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 colour 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, especially those which live much 

 on the ground, have undoubtedly been obscurely co- 

 loured for the sake of protection. In some instances 



