these rare wanderers only by one means, namely, by dirt
adhering to their feet or beaks, which is in itself a rare accident.
Even in this case, how small would be the chance 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. Out of a hundred kinds of
seeds or animals transported to an island, even if far less well-stocked
than Britain, perhaps not more than one would be so well fitted to
its new home, as to become naturalised. But this is no valid argument
against what would be effected by occasional means of transport, during
the long lapse of geological time, whilst the island was being upheaved,
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, if fitted for the climate, would
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 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 migrated from one point to the other. It is indeed a
remarkable fact to see so many plants of the same species living on the
snowy regions of the Alps or Pyrenees, and in the extreme northern parts
of Europe; but it is far more remarkable, that the plants on the White
Mountains, in the United States of America, are all the same with those
of Labrador, and nearly all the same, as we hear from Asa Gray, with
those on the loftiest mountains of Europe. Even as long ago as 1747,
such facts led Gmelin to conclude that the same species must have
been independently created at many distinct points; and we might have
remained in this same belief, had not Agassiz and others called vivid
attention to the Glacial period, which, as we shall immediately see,
affords a simple explanation of these facts. We have evidence of almost
every conceivable kin