Chap. YL] AFFECTED BY NATURAL SELECTION. 247 



compensation, of the pressure of one part on another, 

 &c., — and finally of sexual selection, by which characters 

 of use to one sex are often gained and then transmitted 

 more or less perfectly to the other sex, though of no use 

 to this sex. But structures thus indirectly gained, 

 although at first of no advantage to a species, may 

 subsequently have been taken advantage of by its 

 modified descendants, under new conditions of life and 

 newly acquired habits. 



If green woodpeckers alone had existed, and we did 

 not know that. there were many black and pied kinds, I 

 dare say that we should have thought that the green 

 colour was a beautiful adaptation to conceal this tree- 

 frequenting bird from its enemies ; and consequently 

 that it was a character of importance, and had been 

 acquired through natural selection ; as it is, the colour 

 is probably in chief part due to sexual selection. A 

 trailing palm in the Malay Archipelago climbs the 

 loftiest trees by the aid of exquisitely constructed hooks 

 clustered around the ends of the branches, and this 

 contrivance, no doubt, is of the highest service to the 

 plant ; but as we see nearly similar hooks on many 

 trees which are not climbers, and which, as there is 

 reason to believe from the distribution of the thorn- 

 bearing species in Africa and South America, serve as a 

 defence against browsing quadrupeds, so the spikes on 

 the palm may at first have been developed for this 

 object, and subsequently have been improved and 

 taken advantage of by the plant, as it underwent 

 further modification and became a climber. The naked 

 skin on the head of a vulture is generally considered as 

 a direct adaptation for wallowing in putridity; and so 

 it may be, or it may possibly be due to the direct action 



