instances be
highly serviceable--as is the case. He would admit that general
resemblances scarcely definable by language might sometimes serve to
allocate a species by its nearest relation. He would be able to assign a
clear reason why the close similarity of the fruit in two varieties of
pine-apple, and of the so-called root in the common and Swedish turnips,
and why the similar gracefulness of form in the greyhound and
racehorse, are characters of little value in classification; namely,
because they are the result, not of community of descent, but either of
selection for a common end, or of the effects of similar external
conditions.

{447} _Origin_, Ed. i. p. 423, vi. p. 579. In the margin Marshall
is given as the authority.

_Classification of "races" and species similar._

Thus seeing that both the classifiers of species and of varieties{448}
work by the same means, make similar distinctions in the value of the
characters, and meet with similar difficulties, and that both seem to
have in their classification an ulterior object in view; I cannot avoid
strongly suspecting that the same cause, which has made amongst our
domestic varieties groups and sub-groups, has made similar groups (but
of higher values) amongst species; and that this cause is the greater or
less propinquity of actual descent. The simple fact of species, both
those long since extinct and those now living, being divisible into
genera, families, orders &c.--divisions analogous to those into which
varieties are divisible--is otherwise an inexplicable fact, and only not
remarkable from its familiarity.

{448} _Origin_, Ed. i. p. 423, vi. p. 579.

_Origin of genera and families._

Let us suppose{449} for example that a species spreads and arrives at
six or more different regions, or being already diffused over one wide
area, let this area be divided into six distinct regions, exposed to
different conditions, and with stations slightly different, not fully
occupied with other species, so that six different races or species
were formed by selection, each best fitted to its new habits and
station. I must remark that in every case, if a species becomes modified
in any one sub-region, it is probable that it will become modified in
some other of the sub-regions over which it is diffused, for its
organization is shown to be capable of being rendered plastic; its
diffusion proves that it is able to struggle with the other inhabitants