whether there are any analogous facts which will throw any
light on this subject, and will tend to explain why the offspring of
certain species, when crossed, should be sterile, and not others,
without requiring a distinct law connected with their creation to that
effect. Great numbers, probably a large majority of animals when caught
by man and removed from their natural conditions, although taken very
young, rendered quite tame, living to a good old age, and apparently
quite healthy, seem incapable under these circumstances of
breeding{248}. I do not refer to animals kept in menageries, such as at
the Zoological Gardens, many of which, however, appear healthy and live
long and unite but do not produce; but to animals caught and left partly
at liberty in their native country. Rengger{249} enumerates several
caught young and rendered tame, which he kept in Paraguay, and which
would not breed: the hunting leopard or cheetah and elephant offer other
instances; as do bears in Europe, and the 25 species of hawks, belonging
to different genera, thousands of which have been kept for hawking and
have lived for long periods in perfect vigour. When the expense and
trouble of procuring a succession of young animals in a wild state be
borne in mind, one may feel sure that no trouble has been spared in
endeavours to make them breed. So clearly marked is this difference in
different kinds of animals, when captured by man, that St Hilaire makes
two great classes of animals useful to man:--the _tame_, which will not
breed, and the _domestic_ which will breed in domestication. From
certain singular facts we might have supposed that the non-breeding of
animals was owing to some perversion of instinct. But we meet with
exactly the same class of facts in plants: I do not refer to the large
number of cases where the climate does not permit the seed or fruit to
ripen, but where the flowers do not "set," owing to some imperfection of
the ovule or pollen. The latter, which alone can be distinctly examined,
is often manifestly imperfect, as any one with a microscope can observe
by comparing the pollen of the Persian and Chinese lilacs{250} with the
common lilac; the two former species (I may add) are equally sterile in
Italy as in this country. Many of the American bog plants here produce
little or no pollen, whilst the Indian species of the same genera freely
produce it. Lindley observes that sterility is the bane of the
horticulturist{251}: L