4 THEORY OF BIRDS’ NESTS. : 249 
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 
