may give rise to such a disparity in size, and, consequently, there will
be a multitude of distinct species, of which no two adult individuals
can ever depart so widely from a certain standard of dimensions as the
mere varieties of certain other species--the dog, for instance. Now, we
have only to suppose that what is true of size, may also hold in regard
to color and many other attributes; and it will at once follow, that the
degree of possible discordance between varieties of the same species
may, in certain cases, exceed the utmost disparity which can arise
between two individuals of many distinct species.

The same remarks may hold true in regard to instincts; for, if it be
foreseen that one species will have to encounter a great variety of
foes, it may be necessary to arm it with great cunning and
circumspection, or with courage or other qualities capable of developing
themselves on certain occasions; such, for example, as those migratory
instincts which are so remarkably exhibited at particular periods, after
they have remained dormant for many generations. The history and habits
of one variety of such a species may often differ more considerably from
some other than those of many distinct species which have no such
latitude of accommodation to circumstances.

_Extent of known variability in species._--Lamarck has somewhat
mis-stated the idea commonly entertained of a species; for it is not
true that naturalists in general assume that the organisation of an
animal or plant remains absolutely constant, and that it can never vary
in any of its parts.[802] All must be aware that circumstances influence
the habits, and that the habits may alter the state of the parts and
organs; but the difference of opinion relates to the extent to which
these modifications of the habits and organs of a particular species may
be carried.

Now, let us first inquire what positive facts can be adduced in the
history of known species, to establish a great and permanent amount of
change in the form, structure, or instinct of individuals descending
from some common stock. The best authenticated examples of the extent to
which species can be made to vary may be looked for in the history of
domesticated animals and cultivated plants. It usually happens, that
those species, both of the animal and vegetable kingdom, which have the
greatest pliability of organisation, those which are most capable of
accommodating themselves to a great varie