e classification,
even in a small and perfect group;--in the actual state of nature it is
almost impossible, the species being so numerous and the modifications
of form and structure so varied, arising probably from the immense
number of species which have served as antitypes for the existing
species, and thus produced a complicated branching of the lines of
affinity, as intricate as the twigs of a gnarled oak or the vascular
system of the human body. Again, if we consider that we have only
fragments of this vast system, the stem and main branches being
represented by extinct species of which we have no knowledge, while a
vast mass of limbs and boughs and minute twigs and scattered leaves is
what we have to place in order, and determine the true position each
originally occupied with regard to the others, the whole difficulty of
the true Natural System of classification becomes apparent to us.

We shall thus find ourselves obliged to reject all these systems of
classification which arrange species or groups in circles, as well as
these which fix a definite number for the divisions of each group. The
latter class have been very generally rejected by naturalists, as
contrary to nature, notwithstanding the ability with which they have
been advocated; but the circular system of affinities seems to have
obtained a deeper hold, many eminent naturalists having to some extent
adopted it. We have, however, never been able to find a case in which
the circle has been closed by a direct and close affinity. In most cases
a palpable analogy has been substituted, in others the affinity is very
obscure or altogether doubtful. The complicated branching of the lines
of affinities in extensive groups must also afford great facilities for
giving a show of probability to any such purely artificial arrangements.
Their death-blow was given by the admirable paper of the lamented Mr.
Strickland, published in the "Annals of Natural History," in which he so
clearly showed the true synthetical method of discovering the Natural
System.

_Geographical Distribution of Organisms._

If we now consider the geographical distribution of animals and plants
upon the earth, we shall find all the facts beautifully in accordance
with, and readily explained by, the present hypothesis. A country having
species, genera, and whole families peculiar to it, will be the
necessary result of its having been isolated for a long period,
sufficient for many series of