White Down of Nestlings 
which this method leads, we may cite the often- 
quoted theory which ascribes the green colouring 
of some arboreal fruit-eating pigeons to adapta- 
tion to an existence among tropical foliage, and 
ignores the fact that in America tree-haunting 
pigeons are never of this colour, and that it is not 
by any means universal even among the old- 
world pigeons. 
Similarly, a theory has been advanced (W. P. 
Pycraft, Knowledge, 1904, p. 275) that the white 
down of some nestling birds, is an adaptation 
to resisting the heat of the sun in open nests. 
This is at once negatived by the fact that young 
owls, usually hatched in shaded places, are also 
generally white, while young cormorants, living 
in open nests, are black; yet the allied darters, 
with the same breeding haunts in some cases, 
have white young. Lest it should be thought 
that black has some especial value in a nestling 
living exposed, we may mention that young 
petrels, which are born in holes, have black or 
dark down. 
As we have already pointed out, naturalists 
in too readily accepting the theory that varia- 
tion is minute in degree and indefinite in 
direction, have raised quite unnecessary diffi- 
culties, even for the selection hypothesis. We 
have cited certain facts, which seem to show that 
variations, as a rule, are not indefinite in direc- 
tion; of these the most striking is furnished by 
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