ly-cultivated races do not travel in search of food,
nor root up the ground with their ringed muzzles. These modifications of
structure, which are all strictly inherited, characterise several improved
breeds, so that they cannot have been derived from any single domestic or
wild stock.[737] With respect to cattle, Professor Tanner has remarked that
the lungs and liver in the improved breeds "are found to be considerably
reduced in size when compared with those possessed by animals having
perfect liberty;"[738] and the reduction of these organs affects the
general shape of the body. The cause of the reduced lungs in highly-bred
animals which take little exercise is {300} obvious; and perhaps the liver
may be affected by the nutritious and artificial food on which they largely
subsist.

It is well known that, when an artery is tied, the anastomosing
branches, from being forced to transmit more blood, increase in
diameter; and this increase cannot be accounted for by mere extension,
as their coats gain in strength. Mr. Herbert Spencer[739] has argued
that with plants the flow of sap from the point of supply to the
growing part first elongates the cells in this line; and that the cells
then become confluent, thus forming the ducts; so that, on this view,
the vessels in plants are formed by the mutual reaction of the flowing
sap and cellular tissue. Dr. W. Turner has remarked,[740] with respect
to the branches of arteries, and likewise to a certain extent with
nerves, that the great principle of compensation frequently comes into
play; for "when two nerves pass to adjacent cutaneous areas, an inverse
relation as regards size may subsist between them; a deficiency in one
may be supplied by an increase in the other, and thus the area of the
former may be trespassed on by the latter nerve." But how far in these
cases the difference in size in the nerves and arteries is due to
original variation, and how far to increased use or action, is not
clear.

In reference to glands, Mr. Paget observes that "when one kidney is
destroyed the other often becomes much larger, and does double
work."[741] If we compare the size of the udders and their power of
secretion in cows which have been long domesticated, and in certain
goats in which the udders nearly touch the ground, with the size and
power of secretion of these organs in wild or half-dome