DIFFICULTIES OF THE THEORY 161 



how important an organ of locomotion the tail is in most aquatic 

 animals, its general presence and use for many purposes in so many 

 land animals, which in their lungs or modified swim-bladders be- 

 tray their aquatic origin, may perhaps be thus accounted for. A 

 well-developed tail having been formed in an aquatic animal, it 

 might subsequently come to be worked in for all sorts of purposes, 

 as a fly-flapper, an organ of prehension, or as an aid in turning, 

 as in the case of the dog, though the aid in this latter respect must 

 be slight, for the hare, with hardly any tail, can double still more 

 quickly. 



In the second place, we may easily err in attributing impor- 

 tance to characters, and in believing that they have been devel- 

 oped through natural selection. We must by no means overlook 

 the effects of the definite action of changed conditions of life, of 

 so-called spontaneous variations, which seem to depend in a quite 

 subordinate degree on the nature of the conditions, of the tend- 

 ency to reversion to long-lost characters, of the complex laws of 

 growth, such as of correlation, comprehension, of the pressure 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 the 

 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 adapta- 

 tion 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 con- 

 structed 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 climb- 

 ers, 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 subse- 

 quently 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 



