148 ORGANS OF LITTLE IMPORTANCE [CHAP. VL 



subordinate degree on the nature of the conditions, of the 

 tendency 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, 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 

 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 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 dis- 

 tribution 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 advan- 

 tage 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 

 or individual difference ; and we are immediately made conscious 

 cf this by reflecting on the differences between the breeds of our 

 domesticated animals in different countries, more especially in 

 the less civilised countries where there has been but little 

 methodical selection. Animals kept by savages in different 



