carry seeds for very great distances; for seeds do not retain their
vitality when exposed for a great length of time to the action of
seawater; nor could they be long carried in the crops or intestines of
birds. These means, however, would suffice for occasional transport
across tracts of sea some hundred miles in breadth, or from island to
island, or from a continent to a neighbouring island, but not from one
distant continent to another. The floras of distant continents would
not by such means become mingled in any great degree; but would remain
as distinct as we now see them to be. The currents, from their course,
would never bring seeds from North America to Britain, though they
might and do bring seeds from the West Indies to our western shores,
where, if not killed by so long an immersion in salt-water, they could
not endure our climate. Almost every year, one or two land-birds are
blown across the whole Atlantic Ocean, from North America to the
western shores of Ireland and England; but seeds could be transported
by these wanderers only by one means, namely, in dirt sticking to their
feet, which is in itself a rare accident. Even in this case, how small
would the chance be of a seed falling on favourable soil, and coming to
maturity! But it would be a great error to argue that because a
well-stocked island, like Great Britain, has not, as far as is known
(and it would be very difficult to prove this), received within the
last few centuries, through occasional means
of transport, immigrants from Europe or any other continent, that a
poorly-stocked island, though standing more remote from the mainland,
would not receive colonists by similar means. I do not doubt that out
of twenty seeds or animals transported to an island, even if far less
well-stocked than Britain, scarcely more than one would be so well
fitted to its new home, as to become naturalised. But this, as it seems
to me, is no valid argument against what would be effected by
occasional means of transport, during the long lapse of geological
time, whilst an island was being upheaved and formed, and before it had
become fully stocked with inhabitants. On almost bare land, with few or
no destructive insects or birds living there, nearly every seed, which
chanced to arrive, would be sure to germinate and survive.
_Dispersal during the Glacial period_. The identity of many plants and
animals, on mountain-summits, separated from each other by hundreds of
miles of lowlands, where the Alpine species could not possibly exist,
is one of the most striking cases known of the same species living at
distant points, without the apparent possibility of their having
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