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 {365} (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, if fitted
for the climate, 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 migrated from one
to the other. It is indeed a remarkable fact to see so many of the same
plants 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 {366} same species must
have been independently created at several distinct points; and we might
have remained in this same belief, had not Agassiz and oth