188 ORGANS OF LITTLE IMPORTANCE 



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 Archipel- 

 ago 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 defense 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 con- 

 sidered 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 indis- 

 23ensable for this act: but as sutures occur in the skulls of 

 young birds and reptiles, which have only to escape from a 

 broken ^gg, 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 

 between the breeds of our domesticated animals in differ- 

 ent countries, more especially in the less civilized coun- 

 tries, where there has been but little methodical selection. 

 Animals kept by savages in different countries often have 

 to sti'uggle for their own subsistence, and are exposed to a 

 certain extent to natural selection, and individuals with 

 slightly different constitutions would succeed best under 



