os, have
thus remained uninfluenced by later migrations, and have, in consequence,
been developed into a variety of distinct types adapted to the peculiar
conditions of existence under which they have been placed. Sometimes the
different species thus formed are confined to one or two of the islands
only, as the three species of Certhidea, which are divided between the
islands but do not appear ever to occur together. _Nesomimus parvulus_ is
confined to Albemarle Island, and _N. trifasciatus_ to Charles Island;
_Cactornis pallida_ to Indefatigable Island, _C. brevirostris_ to Chatham
Island, and _C. abingdoni_ to Abingdon Island.

Now all these phenomena are strictly consistent with the theory of the
peopling of the islands by accidental migrations, if we only allow them to
have existed for a sufficiently long period; and the fact that volcanic
action has ceased on many of the islands, as well as their great extent,
would certainly indicate a considerable antiquity.

The great difference presented by the birds of these islands as compared
with those of the equally remote Azores and Bermudas, is sufficiently
explained by the difference of climatal conditions. At the Galapagos there
are none of those periodic storms, gales, and hurricanes which prevail in
the North Atlantic, and which every year carry some straggling birds of
Europe or North America to the former islands; while, at the same time, the
majority of the tropical American birds are {284} nonmigratory, and thus
afford none of the opportunities presented by the countless hosts of
migrants which pass annually northward and southward along the European,
and especially along the North American coasts. It is strictly in
accordance with these different conditions that we find in one case an
almost perfect identity with, and in the other an almost equally complete
diversity from, the continental species of birds.

_Insects and Land-shells._--The other groups of land-animals add little of
importance to the facts already referred to. The insects are very scanty;
the most plentiful group, the beetles, only furnishing about forty species
belonging to thirty-two genera and nineteen families. The species are
almost all peculiar, as are some of the genera. They are mostly small and
obscure insects, allied either to American or to world-wide groups. The
Carabidae and the Heteromera are the most abundant groups, the former
furnishing six and the latter nine species.[61]

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