istics:

"Crossing is generally understood to refer to the alliance of
animals of different breeds, such as between a thorough-bred and
a half-bred among horses or a South Down and Leicester among
sheep. Now the advantages or disadvantages of this system depend
entirely on the object we have in view, whether merely to beget
an animal for the butcher, or for the purpose of perpetuating
the species. If the latter is the object, then crossing should
be adopted gradually and with care, and by no means between
distant or antagonistic qualities, as for example a
thorough-bred and a cart-horse. The result of the latter
connection is generally an ill-assorted and unfavorable animal,
too heavy perhaps for one purpose, and too light for another. If
we wish to instil more activity into the cart-horse breed, it is
better to do so by means of some half-bred animal, whilst the
latter can be improved by means of the three-parts-bred horse
and this again by the thorough-bred. There is a remarkable
tendency, in breeding, for both good qualities and bad to
disappear for one or two generations, and to reappear in the
second and third; thus an animal often resembles the grand dam
more than the dam. This peculiarity is itself an objection to
the practice of crossing, as it tends to prevent uniformity and
to encourage contrarieties; and thus we find in many flocks and
herds that the hopes of the breeders have been entirely baffled
and a race of mongrels established.

The first cross is generally successful--a tolerable degree of
uniformity is produced, resembling in external conformation the
sire, which is usually of a superior breed; and thus the
offspring are superior to the dams. These cross-bred animals are
now paired amongst each other, and what is the consequence?
Uniformity at once disappears; some of the offspring resemble
the grandsire, and others the grandams, and some possess the
disposition and constitution of the one and some of the other;
and consequently a race of mongrels is perpetuated. If, however,
the cross is really a good and desirable one, then, by means of
rigorous and continued selection, pursued for several
generations, that is, by casting aside, as regards breeding
purposes, every animal that does not exhibit uniformity, or
possess the qualifications we are desiro