248 ORGANS OP LITTLE IMPORTANCE [Chap. VI 



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 in- 

 dispensable 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 struc- 

 ture 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 ani- 

 mals in different countries, — more especially in the less 

 civilised countries where there has been but little 

 methodical selection. Animals kept by savages in dif- 

 ferent countries often have to struggle for their own 

 subsistence, and are exposed to a certain extent to natu- 

 ral selection, and individuals with slightly different 

 constitutions would succeed best under different cli- 

 mates. With cattle susceptibility to the attacks of 

 flies is correlated with colour, as is the liability to be 

 poisoned by certain plants; so that even colour would 

 be thus subjected to the action of natural selection. 

 Some observers are convinced that a damp climate af' 

 fects the growth of the hair, and that with the hair 

 the horns are correlated. Mountain breeds always dif- 

 fer from lowland breeds; and a mountainous country 

 would probably affect the hind limbs from exercising 

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



