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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