208 ORIGIN OF SPECIES 



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 selec- 

 tion; 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 quadru- 

 peds, so the spikes on the palm may at first have been de- 

 veloped 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 adapta- 

 tion 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 of this by reflecting on the differences be- 

 tween 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 countries often have to struggle 

 for their own subsistence, and are exposed to a certain extent 

 to natural selection, and individuals with slightly different 



