; and this, no doubt, applies
exclusively to adult animals fit for consumption. Hence the half-wild
British cattle which have long interbred within the limits of the same
herd are relatively far less fertile. Although in an unenclosed country
like Paraguay there must be some crossing between the different herds,
yet even there the inhabitants believe that the occasional introduction
of animals from distant localities is necessary to prevent
"degeneration in size and diminution of fertility."[253] The decrease
in size from ancient times in the Chillingham and Hamilton cattle must
have been prodigious, for Professor Ruetimeyer has shown that they are
almost certainly the descendants of the gigantic _Bos primigenius_. No
doubt this decrease in size may be largely attributed to less
favourable conditions of life; yet animals roaming over large parks,
and fed during severe winters, can hardly be considered as placed under
very unfavourable conditions.

With _Sheep_ there has often been long-continued interbreeding within
the limits of the same flock; but whether the nearest relations have
been matched so frequently as in the case of Shorthorn cattle, I do not
know. The Messrs. Brown during fifty years have never infused fresh
blood into their excellent flock of Leicesters. Since 1810 Mr. Barford
has acted on the same principle with the Foscote flock. He asserts that
half a century {120} of experience has convinced him that when two
nearly related animals are quite sound in constitution, in-and-in
breeding does not induce degeneracy; but he adds that he "does not
pride himself on breeding from the nearest affinities." In France the
Naz flock has been bred for sixty years without the introduction of a
single strange ram.[254] Nevertheless, most great breeders of sheep
have protested against close interbreeding prolonged for too great a
length of time.[255] The most celebrated of recent breeders, Jonas
Webb, kept five separate families to work on, thus "retaining the
requisite distance of relationship between the sexes."[256]

Although by the aid of careful selection the near interbreeding of
sheep may be long continued without any manifest evil, yet it has often
been the practice with farmers to cross distinct breeds to obtain
animals for the butcher, which plainly shows that good is derived fr