gone. These modifications occur chiefly in the pure and crossed races
of the _S. Indica_ type; but their commencement may be clearly detected in
the slightly improved breeds of the _S. scrofa_ type.[160] Nathusius states
positively (s. 99, 103), as the result of common experience and of his
experiments, that rich and abundant food, given during youth, tends by some
direct action to make the head broader and shorter; and that poor food
works a contrary result. He lays much stress on the fact that all wild and
semi-domesticated pigs, in ploughing up the ground with their muzzles,
have; whilst young, to exert the powerful muscles fixed to the hinder part
of the head. In highly cultivated races this habit is no longer followed,
and consequently the back of the skull becomes modified in shape, entailing
other changes in other parts. There can hardly be a doubt that so great a
change in habits would {73} affect the skull; but it seems rather doubtful
how far this will account for the greatly reduced length of the skull and
for its concave front. It is well known (Nathusius himself advancing many
cases, s. 104) that there is a strong tendency in many domestic animals--in
bull- and pug-dogs, in the niata cattle, in sheep, in Polish fowls,
short-faced tumbler pigeons, and in one variety of the carp--for the bones
of the face to become greatly shortened. In the case of the dog, as H.
Mueller has shown, this seems caused by an abnormal state of the primordial
cartilage. We may, however, readily admit that abundant and rich food
supplied during many generations would give an inherited tendency to
increased size of body, and that, from disuse, the limbs would become finer
and shorter.[161] We shall in a future chapter also see that the skull and
limbs are apparently in some manner correlated, so that any change in the
one tends to affect the other.

Nathusius has remarked, and the observation is an interesting one, that the
peculiar form of the skull and body in the most highly cultivated races is
not characteristic of any one race, but is common to all when improved up
to the same standard. Thus the large-bodied, long-eared, English breeds
with a convex back, and the small-bodied, short-eared, Chinese breeds with
a concave back, when bred to the same state of perfection, nearly resemble
each other in the form of the head and body. This result, it appears, is
partly due to similar causes of change acting on the several races, and
part