ds, between
species,--are highly injurious, as far as the reproductive system is
concerned, and in some few instances as far as constitutional vigour is
concerned. Can this parallelism be accidental? Does it not rather indicate
some real bond of connection? As a fire goes out unless it be stirred up,
so the vital forces are always tending, according to Mr. Herbert Spencer,
to a state of equilibrium, unless disturbed and renovated through the
action of other forces.

In some few cases varieties tend to keep distinct, by breeding at different
periods, by great differences in size, or by sexual preference,--in this
latter respect more especially resembling species in a state of nature. But
the actual crossing of varieties, far from diminishing, generally adds to
the fertility of both the first union and the mongrel offspring. Whether
all {178} the most widely distinct domestic varieties are invariably quite
fertile when crossed, we do not positively know; much time and trouble
would be requisite for the necessary experiments, and many difficulties
occur, such as the descent of the various races from aboriginally distinct
species, and the doubts whether certain forms ought to be ranked as species
or varieties. Nevertheless, the wide experience of practical breeders
proves that the great majority of varieties, even if some should hereafter
prove not to be indefinitely fertile _inter se_, are far more fertile when
crossed, than the vast majority of closely allied natural species. A few
remarkable cases have, however, been given on the authority of excellent
observers, showing that with plants certain forms, which undoubtedly must
be ranked as varieties, yield fewer seeds when crossed than is natural to
the parent-species. Other varieties have had their reproductive powers so
far modified that they are either more or less fertile than are their
parents, when crossed with a distinct species.

Nevertheless, the fact remains indisputable that domesticated varieties of
animals and of plants, which differ greatly from each other in structure,
but which are certainly descended from the same aboriginal species, such as
the races of the fowl, pigeon, many vegetables, and a host of other
productions, are extremely fertile when crossed; and this seems to make a
broad and impassable barrier between domestic varieties and natural
species. But, as I will now attempt to show, the distinction is not so
great and overwhelmingly important as i