AFFECTED BY NATURAL SELECTION. 177 



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 brows- 

 ing quadrupeds, so the spikes on the palm may at first 

 nave 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 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 m 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 different 

 countries, more especially in the less civilized 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 constitutions would succeed best under 

 different climates. With cattle susceptibility to the attacks 

 of flies is correlated with color, as is the liability to be pois- 

 oned by certain plants ; so that even color would be thus 

 subjected to the action of natural selection. Some observ- 

 ers are convinced that a damp climate affects the growth of 

 the hair, and that with the hair the horns are correlated. 

 Mountain breeds always differ from lowland breeds : and a 

 mountainous country would probably affect the hind limbs 

 from exercising them more, and possibly even the form of 



