makes
directly against the opposite theory of special creation. For we have
recently seen that when we restrict our view to the case of species and
genera, the theory of special creation is obliged to suppose that for
some inscrutable reason the Deity had regard to systematic affinity
while determining on what large areas to create his species[20]. But
now we see that he must be held to have neglected this inscrutable
reason (whatever it was) when he passed beyond the range of genera--and
this always in proportion to the remoteness of systematic affinity on
the part of the species concerned.

[20] I say "_large_ areas" for the sake of argument; but the same
correlation between distribution and affinity extends likewise to
_small_ areas where only _small_ differences of affinity are
concerned. Thus, for instance, speaking of smaller areas, Moritz
Wagner says:--"The broader and more rapid the river, the higher and
more regular the mountain-chain, the calmer and more extensive the
sea, the more considerable, as a general rule, will be the taxonomic
separation between the populations"; and he shows that, in
correlation with such differences in the _degrees_ of separation,
are the _degrees_ of diversification--i. e., the _numbers_ of
species, and even of varieties, which these topographical barriers
determine.

I cannot well conceive a _reductio ad absurdum_ more complete than this.
But, having now presented these most general facts of geographical
distribution in their relation to the issue before us, we may next
proceed to consider a few illustrations of them in detail, for in this
way I think that their overwhelming weight may become yet more
abundantly apparent.

* * * * *

It will assist us in dealing with these detailed illustrations if we
begin by considering the means of dispersal of organisms from one place
to another. Of course the most ordinary means is that of continuous
wandering, or emigration; but where geographical barriers of any kind
have to be surmounted, organisms may only be able to pass them by more
exceptional and accidental means. The principal barriers of a
geographical kind are oceans, rivers, mountain-chains, and
desert-tracts, in the case of terrestrial organisms; and, in the case
of aquatic organisms, the presence of land. But it is to be observed
that, as regards marine organisms, any considerable difference in t