Chap. XVI. Birds — Summary. 499 



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

 parents vary late in life — and we know that this constantly 

 occurs with our j^oultry, 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 ; con- 

 sequently 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 pre- 

 served 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 seasoDS 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 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 coloured for the sake 

 of protection. In some instances the upper exposed surface of 

 the plumage has been thus coloured 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 colours, have generally Jbeen acquired by the 

 males through variation and sexual selection, and have been 

 transmitted in various ways according to the several laws of 

 inheritance —the females and the young being left comparatively 

 but little modified. 57 



57 I am greatly indebted to the birds, and the two following ones 

 kindness of Mr. Sclater for having ou mammals. In this way I have 

 waoked over these four chapters on been saved from making mistakes 



