162 THE ORIGIN OF SPECIES 



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 parturi- 

 tion, 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 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 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 moun- 

 tainous country would probably affect the hind limbs from exer- 

 cising them more, and possibly even the form of the pelvis; and 

 then by the law of homologous variation, the front limbs and the 

 head would probably be affected. The shape, also, of the pelvis 

 might affect by pressure the shape of certain parts of the young in 

 the womb. The laborious breathing necessary in high regions tends, 

 as we have good reason to believe, to increase the size of the chest; 

 and again correlation would come into play. The effects of les- 

 sened exercise, together with abundant food, on the whole organi- 

 zation is probably still more important; and this, as H. von 

 Nathusius has lately shown in his excellent Treatise, is apparently 

 one chief cause of the great modification which the breeds of swine 

 have undergone. But we are far too ignorant to speculate on the 

 relative importance of the several known and unknown causes of 

 variation; and I have made these remarks only to show that, if 

 we are unable to account for the characteristic differences of our 

 several domestic breeds, which nevertheless are generally admitted 



