most experienced observer Sir J. Sebright (17/1. 'The Art of Improving the
Breed, etc.' 1809 page 16.), that the evil effects of close interbreeding may
be checked or quite prevented by the related individuals being separated for a
few generations and exposed to different conditions of life. This conclusion
is now held by many breeders; for instance Mr. Carr (17/2. 'The History of the
Rise and Progress of the Killerby, etc. Herds' page 41.) remarks, it is a
well-known "fact that a change of soil and climate effects perhaps almost as
great a change in the constitution as would result from an infusion of fresh
blood." I hope to show in a future work that consanguinity by itself counts
for nothing, but acts solely from related organisms generally having a similar
constitution, and having been exposed in most cases to similar conditions.

That any evil directly follows from the closest 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; and 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, often 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