reason to believe that man is highly sensitive to the evil effects of
inter-breeding, especially in areas so large as New Zealand, and the
Sandwich archipelago with its diversified stations. On the contrary, it
is known that the present inhabitants of Norfolk Island are nearly all
cousins or near relations, as are the Todas in India, and the
inhabitants of some of the Western Islands of Scotland; and yet they
seem not to have suffered in fertility. (45. On the close relationship
of the Norfolk Islanders, Sir W. Denison, Varieties of Vice-Regal
Life, vol. i. 1870, p. 410. For the Todas, see Col. Marshall s work
1873, p. 110. For the Western Islands of Scotland, Dr. Mitchell,
Edinburgh Medical Journal, March to June, 1865.)
A much more probable view is suggested by the analogy of the lower
animals. The reproductive system can be shewn to be susceptible to an
extraordinary degree (though why we know not) to changed conditions of
life; and this susceptibility leads both to beneficial and to evil
results. A large collection of facts on this subject is given in chap.
xviii. of vol. ii. of my Variation of Animals and Plants under
Domestication. I can here give only the briefest abstract; and every
one interested in the subject may consult the above work. Very slight
changes increase the health, vigour, and fertility of most or all
organic beings, whilst other changes are known to render a large number
of animals sterile. One of the most familiar cases, is that of tamed
elephants not breeding in India; though they often breed in Ava, where
the females are allowed to roam about the forests to some extent, and
are thus placed under more natural conditions. The case of various
American monkeys, both sexes of which have been kept for many years
together in their own countries, and yet have very rarely or never
bred, is a more apposite instance, because of their relationship to
man. It is remarkable how slight a change in the conditions often
induces sterility in a wild animal when captured; and this is the more
strange as all our domesticated animals have become more fertile than
they were in a state of nature; and some of them can resist the most
unnatural conditions with undiminished fertility. (46. For the evidence
on this head, see Variation of Animals, etc., vol. ii. p. 111.)
Certain groups of animals are much more liable than others to be
affected by captivity; and generally all the species of the same group
are affected in the same manner. But sometimes a single species in a
group is rendered sterile, whilst the others are not so; on the other
hand, a single species may retain its fertility whilst most of the
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