less in the
former case than in the latter, and this is still more prominent a
characteristic of the insect and the bird faunas. This difference has been
shown to depend, almost entirely, on the one archipelago being situated in
a stormy, the other in a calm portion of the ocean; and it demonstrates the
preponderating importance of the atmosphere as an agent in the dispersal of
birds, insects, and plants. Yet ocean-currents and surface-drifts are
undoubtedly efficient carriers of plants, and, with plants, of insects and
shells, especially in the tropics; and it is probably to this agency that
we may impute the recent introduction of a number of common Peruvian and
Chilian littoral species, and also of several West Indian types at a more
remote period when the Isthmus of Panama was submerged.

In the case of these islands we see the importance of taking account of
past conditions of sea and land and past changes of climate, in order to
explain the relations of the peculiar or endemic species of their fauna and
flora; and we may even see an indication of the effects of climatal changes
in the northern hemisphere, in the north {291} temperate or alpine
affinities of many of the plants, and even of some of the birds. The
relation between the migratory habits of the birds and the amount of
difference from continental types is strikingly accordant with the fact
that it is almost exclusively migratory birds that annually reach the
Azores and Bermuda; while the corresponding fact that the seeds of those
plants, which are common to the Galapagos and the adjacent continent, have
all--as Sir Joseph Hooker states--some special means of dispersal, is
equally intelligible. The reason why the Galapagos possess four times as
many peculiar species of plants as the Azores is clearly a result of the
less constant introduction of seeds, owing to the absence of storms; the
greater antiquity of the group, allowing more time for specific change; and
the influence of cold epochs and of alterations of sea and land, in
bringing somewhat different sets of plants at different times within the
influence of such modified winds and currents as might convey them to the
islands.

On the whole, then, we have no difficulty in explaining the probable origin
of the flora and fauna of the Galapagos, by means of the illustrative facts
and general principles already adduced.

* * * * *

{292}

CHAPTER XIV

ST. HELENA