HYBRIDISM BY THE ILLEGITIMATE PROGENY OF DIMORPHIC AND TRIMORPHIC
PLANTS--STERILITY OF CROSSED SPECIES DUE TO DIFFERENCES CONFINED TO THE
REPRODUCTIVE SYSTEM--NOT ACCUMULATED THROUGH NATURAL SELECTION--REASONS
WHY DOMESTIC VARIETIES ARE NOT MUTUALLY STERILE--TOO MUCH STRESS HAS
BEEN LAID ON THE DIFFERENCE IN FERTILITY BETWEEN CROSSED SPECIES AND
CROSSED VARIETIES--CONCLUSION.

It was shown in the fifteenth chapter that when individuals of the same
variety, or even of a distinct variety, are allowed freely to intercross,
uniformity of character is ultimately acquired. Some few characters,
however, are incapable of fusion, but these are unimportant, as they are
almost always of a semi-monstrous nature, and have suddenly appeared.
Hence, to preserve our domesticated breeds true, or to improve them by
methodical selection, it is obviously necessary that they should be kept
separate. Nevertheless, through unconscious selection, a whole body of
individuals may be slowly modified, as we shall see in a future chapter,
without separating them into distinct lots. Domestic races have often been
intentionally modified by one or two crosses, made with some allied race,
and occasionally even by repeated crosses with very distinct races; but in
almost all such cases, long-continued and careful selection has been
absolutely necessary, owing to the excessive variability of the crossed
offspring, due to the principle of reversion. In a few instances, however,
mongrels have retained a uniform character from their first production.

When two varieties are allowed to cross freely, and one is {174} much more
numerous than the other, the former will ultimately absorb the latter.
Should both varieties exist in nearly equal numbers, it is probable that a
considerable period would elapse before the acquirement of a uniform
character; and the character ultimately acquired would largely depend on
prepotency of transmission, and on the conditions of life; for the nature
of these conditions would generally favour one variety more than another,
so that a kind of natural selection would come into play. Unless the
crossed offspring were slaughtered by man without the least discrimination,
some degree of unmethodical selection would likewise come into action. From
these several considerations we may infer, that when two or more closely
allied species first came into the possession of the same tribe, their
crossing will not have influ