ese organs, though not diseased,
are thus rendered incapable of performing their proper functions, or
perform them imperfectly.

_Sterility of Domesticated Animals from changed conditions._--With
respect to domesticated animals, as their domestication mainly depends
on the accident of their breeding freely under captivity, we ought not
to expect that their reproductive system would be affected by any
moderate degree of change. Those orders of quadrupeds and birds, of
which the wild species breed most readily in our menageries, have
afforded us the greatest number of domesticated productions. Savages in
most parts of the world are fond of taming animals;[387] and if any of
these regularly produced {161} young, and were at the same time useful,
they would be at once domesticated. If, when their masters migrated
into other countries, they were in addition found capable of
withstanding various climates, they would be still more valuable; and
it appears that the animals which breed readily in captivity can
generally withstand different climates. Some few domesticated animals,
such as the reindeer and camel, offer an exception to this rule. Many
of our domesticated animals can bear with undiminished fertility the
most unnatural conditions; for instance, rabbits, guinea-pigs, and
ferrets breed in miserably confined hutches. Few European dogs of any
kind withstand without degeneration the climate of India; but as long
as they survive, they retain, as I hear from Mr. Falconer, their
fertility; so it is, according to Dr. Daniell, with English dogs taken
to Sierra Leone. The fowl, a native of the hot jungles of India,
becomes more fertile than its parent-stock in every quarter of the
world, until we advance as far north as Greenland and Northern Siberia,
where this bird will not breed. Both fowls and pigeons, which I
received during the autumn direct from Sierra Leone, were at once ready
to couple.[388] I have, also, seen pigeons breeding as freely as the
common kinds within a year after their importation from the Upper Nile.
The guinea-fowl, an aboriginal of the hot and dry deserts of Africa,
whilst living under our damp and cool climate, produces a large supply
of eggs.

Nevertheless, our domesticated animals under new conditions
occasionally show signs of lessened fertility. Roulin asserts that in