a, where about 30 per cent, of the species are
identical. But it is possible enough that at some previous time this
narrow isthmus may have been even narrower than at present, if not
actually open. At all events, the fact that this partial exception
occurs just where the land-barrier is so narrow, is more suggestive
of migration than of independent creation.

Although the geographical distribution of fresh-water fish and
fresh-water shells is often surprisingly extensive and apparently
capricious, this may be explained by the means of dispersal being here
so varied--not only aquatic birds, floods, and whirlwinds, but also
geographical changes of water-shed having all assisted in the process.
Moreover, in some cases it is possible that the habits of more widely
distributed fresh-water fish may have originally been wholly or partly
marine--which, of course, would explain the existing discontinuity of
their existing fresh-water distribution. But, be this as it may (and it
is not a question that affects the issue between special creation and
gradual evolution, since it is only a question as to how a given species
has been dispersed from its original home, whether or not in that home
it was specially created), the point I desire to bring forward is, that
where we find a barrier to the emigration of fresh-water forms which is
more formidable than a thousand miles of ocean--a barrier over which
neither water-fowl nor whirlwinds are likely to pass, and which is above
the reach of any geological changes of water-shed,--where we find such a
barrier, we always find a marked difference in the fresh-water faunas on
either side of it. The kind of barrier to which I allude is a high
mountain-chain. It may be only a few miles wide; yet it exercises a
greater influence on the diversification of specific types, where
fresh-water faunas are concerned, than almost any other. But why should
this be the case on any intelligible theory of special creation? Why, in
the depositing of species of newly created fresh-water fish, should the
presence of an impassable mountain-chain have determined so uniformly a
difference of specific affinity on either side of it? The question, so
far as I can see, does not admit of an answer from any reasonable
opponent.

* * * * *

Turning now from aquatic organisms to terrestrial, the body of facts
from which to draw is so large, that I think the space at my d