274 THE ORIGIN OF SPECIES 



head of a vulture is generally cousidered 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 ad- 

 vanced 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 

 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 straggle 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 at- 

 tacks of flies is correlated with color, as is the liability 

 to be poisoned by certain plants; so that even color would 

 be thus subjected to the action of natural selection. Some 

 observers 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 affectl 



