onscious
selection, and partly by natural selection, for animals kept in
semi-civilised countries have to provide largely for their own wants. Such
natural breeds will also, it may be presumed, have been directly acted on
to some extent by the differences, though slight, in the surrounding
physical conditions.

It is a much more important distinction that some breeds have been from
their first origin modified in so slow and insensible a manner, that if we
could see their early progenitors we should hardly be able to say when or
how the breed first arose; whilst other breeds have originated from a
strongly-marked or semi-monstrous deviation of structure, which, however,
may subsequently have been augmented by selection. From what we know of the
history of the racehorse, greyhound, gamecock, &c., and from their general
appearance, we may feel nearly confident that they were formed by a slow
process of improvement: and with the carrier-pigeon, as well as with some
other pigeons, we know that this has been the case. On the other hand, it
is certain that the ancon and mauchamp breeds of sheep, and almost certain
that the niata cattle, turnspit and pug-dogs, jumper and frizzled fowls,
short-faced tumbler pigeons, hook-billed ducks, &c., and with plants a
multitude of varieties, suddenly appeared in nearly the same state as we
now see them. The frequency of these cases is likely to lead to the false
belief that natural species have often originated in the same abrupt
manner. But we have no evidence of the appearance, or at least of the
continued procreation, under nature, of abrupt modifications of structure;
and various general reasons could be assigned against such a belief: for
instance, without separation a single monstrous variation would almost
certainly be soon obliterated by crossing.

On the other hand, we have abundant evidence of the constant occurrence
under nature of slight individual differences of the most diversified
kinds; and thus we are led to conclude that species have generally
originated by the natural selection, not of abrupt modifications, but of
extremely slight differences. This process may be strictly compared with
the slow and gradual improvement of the racehorse, greyhound, and gamecock.
As every detail of structure in each species is closely adapted to its
general {415} habits of life, it will rarely happen that one part alone
will be modified; but the co-adapted modifications, as formerly shown,