178 ON THE ORIGIN OF SPECIES 



taken advantage of in the parturition of the higher 

 animals. 



We are profoundly ignorant of the causes producing 

 slight and unimportant variations ; and we are immedi- 

 ately made conscious of this by reflecting on the differ- 

 ences in the breeds of our domesticated animals in 

 different countries, — more especially in the less civil- 

 ised countries where there has been but little artificial 

 selection. Careful 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 affect the hind 

 limbs from exercising them more, and possibly even 

 the form of the pelvis ; and then by the law of homo- 

 logous variation, the front limbs and even the head 

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

 pelvis might affect by pressure the shape of the head 

 of the young in the womb. The laborious breathing 

 necessary in high regions would, we have some reason 

 to believe, increase the size of the chest ; and again 

 correlation would come into play. Animals kept by 

 savages in different countries often have to struggle 

 for their own subsistence, and would be exposed to a 

 certain extent to natural selection, and individuals 

 with slightly different constitutions would succeed best 

 under different climates ; and there is reason to believe 

 that constitution and colour are correlated. A good 

 observer, also, states that in cattle susceptibility to the 

 attacks of flies is correlated with colour, as is the 

 liability to be poisoned by certain plants ; so that 

 colour would be thus subjected to the action of natural 

 selection. But we are far too ignorant to speculate 

 on the relative importance of the several known and 

 unknown laws of variation ; and I have here alluded 

 to them only to show that, if we are unable to account 

 for the characteristic differences of our domestic 

 breeds, which nevertheless we generally admit to 

 have arisen through ordinary generation, we ought 

 not to lay too much stress on our ignorance of the 



