8 NATURAL SELECTION tit 
build in holes in banks. Bee-eaters, trogons, motmots, and 
toucans all build in holes, and in none is there any difference 
in the sexes, although they are, without exception, showy 
birds. Parrots build in holes in trees, and in the majority of 
cases they present no marked sexual difference tending to 
concealment of the female. Woodpeckers are in the same 
category, since, though the sexes often differ in colour, the 
female is not generally less conspicuous than the male. 
Wagtails and titmice build concealed nests, and the females 
are nearly as gay as their mates. The female of the pretty 
Australian bird, Pardalotus punctatus, is very conspicuously 
spotted on the upper surface, and it builds in a hole in the 
ground. The gay-coloured hang-nests (Icterinz) and the 
equally brilliant tanagers may be well contrasted; for the 
former, concealed in their covered nests, present little or no 
sexual difference of colour—while the open-nested tanagers 
have the females dull-coloured and sometimes with almost 
protective tints. No doubt there are many individual 
exceptions to the rule here indicated, because many and 
various causes have combined to determine both the colora- 
tion and the habits of birds. These have no doubt acted and 
reacted on each other; and when conditions have changed 
one of these characters may often have become modified, 
while the other, though useless, may continue by hereditary 
descent an apparent exception to what otherwise seems a 
very general rule. The facts presented by the sexual differ- 
ences of colour in birds and their mode of nesting are on 
the whole in perfect harmony with that law of protective 
adaptation of colour and form, which appears to have checked 
to some extent the powerful action of sexual selection, and to 
have materially influenced the colouring of female birds, as it 
has undoubtedly done that of female insects. 
Use of the gaudy Colowrs of many Caterpillars 
Since this essay was first published a very curious difficulty 
has been cleared up by the application of the general principle 
of protective colouring. Great numbers of caterpillars are so 
brilliantly marked and coloured as to be very conspicuous even 
at a considerable distance, and it has been noticed that such 
caterpillars seldom hide themselves. Other species, however, 
