ve been exposed to more sudden
changes and to less continuously uniform conditions. As man has
domesticated so many animals and plants belonging to widely different
classes, and as he certainly did not with prophetic instinct choose those
species which would vary most, we may infer that all natural species, if
subjected to analogous conditions, would, on an average, vary to the same
degree. Few men at the present day will maintain that animals and plants
were created with a tendency to vary, which long remained dormant, in order
that fanciers in after ages might {407} rear, for instance, curious breeds
of the fowl, pigeon, or canary-bird.

From several causes it is difficult to judge of the amount of modification
which our domestic productions have undergone. In some cases the primitive
parent-stock has become extinct, or cannot be recognised with certainty
owing to its supposed descendants having been so much modified. In other
cases two or more closely allied forms, after being domesticated, have
crossed; and then it is difficult to estimate how much of the change ought
to be attributed to variation. But the degree to which our domestic breeds
have been modified by the crossing of distinct natural forms has probably
been exaggerated by some authors. A few individuals of one form would
seldom permanently affect another form existing in much greater numbers;
for, without careful selection, the stain of the foreign blood would soon
be obliterated, and during early and barbarous times, when our animals were
first domesticated, such care would seldom have been taken.

There is good reason to believe that several of the breeds of the dog, ox,
pig, and of some other animals, are respectively descended from distinct
wild prototypes; nevertheless the belief in the multiple origin of our
domesticated animals has been extended by some few naturalists and by many
breeders to an unauthorised extent. Breeders refuse to look at the whole
subject under a single point of view; I have heard one, who maintained that
our fowls were the descendants of at least half-a-dozen aboriginal species,
protest that he was in no way concerned with the origin of pigeons, ducks,
rabbits, horses, or any other animal. They overlook the improbability of
many species having been domesticated at an early and barbarous period.
They do not consider the improbability of species having existed in a state
of nature which, if like our present domestic breeds, wou