DIFFICULTIES OF THE THEORY 273 



tions, which seem to depend in a quite subordinate degree 

 on the nature of the conditions — of the tendency to re- 

 version to long-lost characters — of the complex laws of 

 growth, such as of correlation, compensation, of the press- 

 ure of one part on another, etc. — 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 advan- 

 tage 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 color 

 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 color 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 modi- 

 fication and became a climber. The naked skin on the 



