ering it more difficult to procure a regular supply
of food and to provide for their personal safety in some cases than in
others, can only be balanced by a difference in the population which
have to exist in a given area_--we shall be in a condition to proceed to
the consideration of _varieties_, to which the preceding remarks have a
direct and very important application.

Most or perhaps all the variations from the typical form of a species
must have some definite effect, however slight, on the habits or
capacities of the individuals. Even a change of colour might, by
rendering them more or less distinguishable, affect their safety; a
greater or less development of hair might modify their habits. More
important changes, such as an increase in the power or dimensions of the
limbs or any of the external organs, would more or less affect their
mode of procuring food or the range of country which they inhabit. It
is also evident that most changes would affect, either favourably or
adversely, the powers of prolonging existence. An antelope with shorter
or weaker legs must necessarily suffer more from the attacks of the
feline carnivora; the passenger pigeon with less powerful wings would
sooner or later be affected in its powers of procuring a regular supply
of food; and in both cases the result must necessarily be a diminution
of the population of the modified species. If, on the other hand, any
species should produce a variety having slightly increased powers of
preserving existence, that variety must inevitably in time acquire a
superiority in numbers. These results must follow as surely as old age,
intemperance, or scarcity of food produce an increased mortality. In
both cases there may be many individual exceptions; but on the average
the rule will invariably be found to hold good. All varieties will
therefore fall into two classes--those which under the same conditions
would never reach the population of the parent species, and those which
would in time obtain and keep a numerical superiority. Now, let some
alteration of physical conditions occur in the district--a long period
of drought, a destruction of vegetation by locusts, the irruption of
some new carnivorous animal seeking "pastures new"--any change in fact
tending to render existence more difficult to the species in question,
and tasking its utmost powers to avoid complete extermination; it is
evident that, of all the individuals composing the species, those