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 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
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 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 lessened exercise, together with abundant food, on the whole
organisation 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