level of the sea, allowing equal quantities of time, or an
equal number of generations, for habituating the species to successive
increments of cold. But every husbandman and gardener is aware that such
experiments will fail; and we are more likely to succeed in making some
plants, in the course of the first two generations, support a
considerable degree of difference of temperature, than a very small
difference afterwards, though we persevere for many centuries.

It is the same if we take any other cause instead of temperature; such
as the quality of the food, or the kind of dangers to which an animal is
exposed, or the soil in which a plant lives. The alteration in habits,
form, or organization, is often rapid during a short period; but when
the circumstances are made to vary farther, though in ever so slight a
degree, all modification ceases, and the individual perishes. Thus some
herbivorous quadrupeds may be made to feed partially on fish or flesh;
but even these can never be taught to live on some herbs which they
reject, and which would even poison them, although the same may be very
nutritious to other species of the same natural order. So when man uses
force or stratagem against wild animals, the persecuted race soon
becomes more cautious, watchful, and cunning; new instincts seem often
to be developed, and to become hereditary in the first two or three
generations: but let the skill and address of man increase, however
gradually, no farther variation can take place, no new qualities are
elicited by the increasing dangers. The alteration of the habits of the
species has reached a point beyond which no ulterior modification is
possible, however indefinite the lapse of ages during which the new
circumstances operate. Extirpation then follows, rather than such a
transformation as could alone enable the species to perpetuate itself
under the new state of things.

_Animals now subject to man had originally an aptitude to
domesticity._--It has been well observed by M. F. Cuvier and M. Dureau
de la Malle, that unless some animals had manifested in a wild state an
aptitude to second the efforts of man, their domestication would never
have been attempted. If they had all resembled the wolf, the fox, and
the hyaena, the patience of the experimentalist would have been exhausted
by innumerable failures before he at last succeeded in obtaining some
imperfect results; so if the first advantages derived from the
cultivation