; and this occurs even when the conditions have not been
greatly changed. This conclusion is not necessarily opposed to that at
which we have just arrived, namely, that lesser changes of other kinds are
advantageous to organic beings. Our present subject is of some importance,
from having an intimate connexion with the causes of variability.
Indirectly it perhaps bears on the sterility of species when crossed: for
as, on the one hand, slight changes in the conditions of life are
favourable to plants and animals, and the crossing of varieties adds to the
size, vigour, and fertility of their offspring; so, on the other hand,
certain other changes in the conditions of life cause sterility; and as
this likewise ensues from crossing much-modified forms or species, we have
a parallel and double series of facts, which apparently stand in close
relation to each other.

It is notorious that many animals, though perfectly tamed, {149} refuse to
breed in captivity. Isidore Geoffroy St. Hilaire[331] consequently has
drawn a broad distinction between tamed animals which will not breed under
captivity, and truly domesticated animals which breed freely--generally
more freely, as shown in the sixteenth chapter, than in a state of nature.
It is possible and generally easy to tame most animals; but experience has
shown that it is difficult to get them to breed regularly, or even at all.
I shall discuss this subject in detail; but will give only those cases
which seem most illustrative. My materials are derived from notices
scattered through various works, and especially from a Report, drawn up for
me by the kindness of the officers of the Zoological Society of London,
which has especial value, as it records all the cases, during nine years
from 1838-46, in which the animals were seen to couple but produced no
offspring, as well as the cases in which they never, as far as known,
coupled. This MS. Report I have corrected by the annual Reports
subsequently published. Many facts are given on the breeding of the animals
in that magnificent work, 'Gleanings from the Menageries of Knowsley Hall,'
by Dr. Gray. I made, also, particular inquiries from the experienced keeper
of the birds in the old Surrey Zoological Gardens. I should premise that a
slight change in the treatment of animals sometimes makes a great
difference in their fertility; and it is probable that the results observed
in different menageries would differ. Indeed some animals in ou