158 ORGANS OF LITTLE IMPORTANCE [Chap. VI. 



through, natural selection. We must hy 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 ten- 

 dency to reversion to long-lost characters, — of the complex laws of 

 growth, such as of correlation, 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 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 colour 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 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 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 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 of putrid matter ; but we should be very 

 cautious in drawing any such inference, when we see that the skin 

 on the head of the clean-feeding male Turkey is likewise naked. 

 The sutures in the skulls of young mammals have been advanced as 

 a beautiful adaptation for aiding parturition, and no doubt they 

 facilitate, or may be indispensable for this act; but as sutures 

 occur in the skulls of young birds and reptiles, which have only to 

 escape from a broken egg, we may infer that this structure has 

 arisen from the laws of growth, and has been taken advantage of in 

 the parturition of the higher animals. 



We are profoundly ignorant of the cause of each slight variation 



