re so defective, the most acute naturalist must expect to
be sometimes at fault, and, like the novice, to overlook essential
points of difference, passing unconsciously from one species to another,
until, like one who is borne along in a current, he is astonished on
looking back, at observing that he has reached a point so remote from
that whence he set out.

It is by no means improbable, that, when the series of species of
certain genera is very full, they may be found to differ less widely
from each other than do the mere varieties or races of certain species.
If such a fact could be established, it would, undoubtedly, diminish the
chance of our obtaining certainty in our results; but it would by no
means overthrow our confidence in the reality of species.

_Some mere varieties possibly more distinct than certain individuals of
distinct species._--It is almost necessary, indeed, to suppose that
varieties will differ in some cases more decidedly than some species, if
we admit that there is a graduated scale of being, and assume that the
following laws prevail in the economy of the animate creation:--first,
that the organization of individuals is capable of being modified to a
limited extent, by the force of external causes; secondly, that these
modifications are, to a certain extent, transmissible to their
offspring; thirdly, that there are fixed limits, beyond which the
descendants from common parents can never deviate from a certain type;
fourthly, that each species springs from one original stock, and can
never be permanently confounded by intermixing with the progeny of any
other stock; fifthly, that each species shall endure for a considerable
period of time. Now, let us assume, for the present, these rules
hypothetically, and see what consequences may naturally be expected to
result from them.

We must suppose that when the Author of Nature creates an animal or
plant, all the possible circumstances in which its descendants are
destined to live are foreseen, and that an organization is conferred
upon it which will enable the species to perpetuate itself and survive
under all the varying circumstances to which it must be inevitably
exposed. Now, the range of variation of circumstances will differ
essentially in almost every case. Let us take, for example, any one of
the most influential conditions of existence, such as temperature. In
some extensive districts near the equator, the thermometer might never
vary, t