A THEORY OF BIRDS' NESTS. 210 



her to surrounding objects, as the earth or the foliage, 

 would, on the whole, survive the longest, and thus 

 lead to the attainment of those brown or green and 

 inconspicuous tints, which form the colouring (of the 

 upper surface at least), of the vast majority of female 

 birds which sit upon open nests. 



This does not imply, as some have thought, that all 

 female birds were once as brilliant as the males. The 

 change has been a very gradual one, generally dating 

 from the origin of genera or of larger groups, but 

 there can be no doubt that the remote ancestry of 

 birds having great sexual differences of colour, were 

 nearly or quite alike, sometimes (perhaps in most 

 cases) more nearly resembling the female, but occa- 

 sionally perhaps being nearer what the male is now. 

 The young birds (which usually resemble the females) 

 will probably give some idea of this ancestral type, 

 and it is well known that the young of allied species 

 and of different sexes are often undistinguishable. 



Colour more variable than Structure or Habits, and 

 therefore the Character which has generally been 

 Modified. 



At the commencement of this essay, I have endea- 

 voured to prove, that the characteristic differences and 

 the essential features of birds' nests, are dependent on 

 the structure of the species and upon the present and 

 past conditions of their existence. Both these factors 

 are more important and less variable than colour; and 

 we must therefore conclude that in most cases the mode 



