duced. The domesticated hog differs in many important characters
from the wild boar. In South America and the West Indies it has
returned, in three centuries or less, to its original form.[162] The
horse is probably not known in a state originally wild, but it has run
wild in America and in Siberia. In the prairies of North America,
according to Catlin[163] they still show great varieties of color. The
same is the case in Sable Island, off the coast of Nova Scotia[164]
where herds of wild horses have existed since an early period in the
settlement of America. In South America and Siberia they have assumed
a uniform chestnut or bay color. In the plains of Western America they
retain the dimensions and vigor of the better breeds of domesticated
horses. In Sable Island they have already degenerated to the level of
Highland ponies; but in all countries where they have run wild, the
elongated and arched head, high shoulders, straight back, and other
structural characters probably of the original wild horse, have
appeared. We also learn from such instances that, while races among
domesticated animals may appear suddenly, they revert to the original
type, when unmixed, comparatively slowly; and this especially when the
variation is in the nature of degeneracy.

4. Some characters are more subject to variation than others. In the
higher animals variation takes place very readily in the color and
texture of the skin and its appendages. This, from its direct relation
to the external world, and ready sympathy with the condition of the
digestive organs, might be expected to take the lead. In those
domesticated animals which are little liable to vary in other
respects, as the cat and duck, the color very readily changes. Next
may be placed the stature and external proportions, and the form of
such appendages as the external ear and tail. All these characters are
very variable in domestic animals. Next we may place the form of the
skull, which, though little variable in the wild state, is nearly
always changed by domestication. Psychological functions, as the
so-called instincts of animals, are also very liable to change, and to
have these changes perpetuated in races. Very remarkable instances of
this have been collected by Sir C. Lyell[165] and Dr. Prichard.
Lastly, important physiological characters, as the period of
gestation, etc., and the structure of the internal organs connected
with the functions of nutrition, respiration, e