and this was the opinion of that
most experienced observer Sir J. Sebright,[245] that the evil effects of
close interbreeding may be checked by the related individuals {116} being
separated during a few generations and exposed to different conditions of
life.

That evil directly follows from any degree of close interbreeding has been
denied by many persons; but rarely by any practical breeder; and never, as
far as I know, by one who has largely bred animals which propagate their
kind quickly. Many physiologists attribute the evil exclusively to the
combination and consequent increase of morbid tendencies common to both
parents: that this is an active source of mischief there can be no doubt.
It is unfortunately too notorious that men and various domestic animals
endowed with a wretched constitution, and with a strong hereditary
disposition to disease, if not actually ill, are fully capable of
procreating their kind. Close interbreeding, on the other hand, induces
sterility; and this indicates something quite distinct from the
augmentation of morbid tendencies common to both parents. The evidence
immediately to be given convinces me that it is a great law of nature, that
all organic beings profit from an occasional cross with individuals not
closely related to them in blood; and that, on the other hand,
long-continued close interbreeding is injurious.

Various general considerations have had much influence in leading me to
this conclusion; but the reader will probably rely more on special facts
and opinions. The authority of experienced observers, even when they do not
advance the grounds of their belief, is of some little value. Now almost
all men who have bred many kinds of animals and have written on the
subject, such as Sir J. Sebright, Andrew Knight, &c.,[246] have expressed
the strongest conviction on the impossibility of long-continued close
interbreeding. Those who have compiled works on agriculture, and have
associated much with breeders, such as the sagacious Youatt, Low, &c., have
strongly declared their opinion to the same effect. Prosper Lucas, trusting
largely to French authorities, has come to a similar conclusion. The
distinguished German agriculturist Hermann von Nathusius, who has written
the most able treatise on this subject which I have met with, concurs; and
as I shall have to quote from {117} this treatise, I may state that
Nathusius is not only intimately acquainted with works on agriculture in
a