CONDITIONS OF LIFE, AND BY NATURAL SELECTION.
ALL ORGANIC BEINGS OCCASIONALLY INTERCROSS; APPARENT EXCEPTIONS.
ON CERTAIN CHARACTERS INCAPABLE OF FUSION; CHIEFLY OR EXCLUSIVELY THOSE WHICH
HAVE SUDDENLY APPEARED IN THE INDIVIDUAL.
ON THE MODIFICATION OF OLD RACES, AND THE FORMATION OF NEW RACES BY CROSSING.
SOME CROSSED RACES HAVE BRED TRUE FROM THEIR FIRST PRODUCTION.
ON THE CROSSING OF DISTINCT SPECIES IN RELATION TO THE FORMATION OF DOMESTIC
RACES.

In the two previous chapters, when discussing reversion and prepotency, I was
necessarily led to give many facts on crossing. In the present chapter I shall
consider the part which crossing plays in two opposed directions,--firstly, in
obliterating characters, and consequently in preventing the formation of new
races; and secondly, in the modification of old races, or in the formation of
new and intermediate races, by a combination of characters. I shall also show
that certain characters are incapable of fusion.

The effects of free or uncontrolled breeding between the members of the same
variety or of closely allied varieties are important; but are so obvious that
they need not be discussed at much length. It is free intercrossing which
chiefly gives uniformity, both under nature and under domestication, to the
individuals of the same species or variety, when they live mingled together
and are not exposed to any cause inducing excessive variability. The
prevention of free crossing, and the intentional matching of individual
animals, are the corner-stones of the breeder's art. No man in his senses
would expect to improve or modify a breed in any particular manner, or keep an
old breed true and distinct, unless he separated his animals. The killing of
inferior animals in each generation comes to the same thing as their
separation. In savage and semi-civilised countries, where the inhabitants have
not the means of separating their animals, more than a single breed of the