o adapt the species, which originally came from the lowlands
of the nearest continent, to the mountain-summits than to the lower
districts of our islands. For the lowland species from the continent
would have first to struggle against other species and other conditions
on the coast-land of the island, and so probably become modified by the
selection of its best fitted varieties, then to undergo the same process
when the land had attained a moderate elevation; and then lastly when it
had become Alpine. Hence we can understand why the faunas of insular
mountain-summits are, as in the case of Teneriffe, eminently peculiar.
Putting on one side the case of a widely extended flora being driven up
the mountain-summits, during a change of climate from cold to temperate,
we can see why in other cases the floras of mountain-summits (or as I
have called them islands in a sea of land) should be tenanted by
peculiar species, but related to those of the surrounding lowlands, as
are the inhabitants of a real island in the sea to those of the nearest
continent{408}.

{408} See _Origin_, Ed. i. p. 403, vi. p. 558, where the author
speaks of Alpine humming birds, rodents, plants, &c. in S. America,
all of strictly American forms. In the MS. the author has added
between the lines "As world has been getting hotter, there has been
radiation from high-lands,--old view?--curious; I presume Diluvian
in origin."

Let us now consider the effect of a change of climate or of other
conditions on the inhabitants of a continent and of an isolated island
without any great change of level. On a continent the chief effects
would be changes in the numerical proportion of the individuals of the
different species; for whether the climate became warmer or colder,
drier or damper, more uniform or extreme, some species are at present
adapted to its diversified districts; if for instance it became cooler,
species would migrate from its more temperate parts and from its higher
land; if damper, from its damper regions, &c. On a small and isolated
island, however, with few species, and these not adapted to much
diversified conditions, such changes instead of merely increasing the
number of certain species already adapted to such conditions, and
decreasing the number of other species, would be apt to affect the
constitutions of some of the insular species: thus if the island became
damper it might well happen that there were no speci