198 
DIFFICULTIES ON THEOEY. 
Chap. YI. 
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 ciyilised 
countries where there has been but little artificial selec¬ 
tion. 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 homologous 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 cor¬ 
related. 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 precise cause 
