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CHAP. XIV.] THE NEOTROPICAL REGION. 35
the sea, because we find existing species with an enormous
ranoe. The ancestors of the Amblyrhyiichi must have come as
early, probably, as the earliest birds ; and the same powers of
dispersal have spread them over every island. The two American
genera of lizards, and the tortoises, are perhaps later immigrants.
Latest of all were the snakes, which hardly differ from continental
forms ; but it is not at all improbable that these latter, as well as
the peculiar American mouse, have been early human importa-
tions. Snakes are continually found on board native canoes
whose cabins are thatched with palm leaves ; and a few cen-
turies would probably suffice to produce some modification of
a sj)ecies completely isolated, under conditions widely different
from those of its native country. Land-shells, being so few and
small, and almost all modifications of one type, are a clear indi-
cation of how rare are the conditions which lead to their dispersal
over a wide extent of ocean ; since two or three individuals, ar-
riving on two or three occasions only during the whole period
of the existence of the islands, would suffice to account for the
present fauna. Insects have arrived much more frequently ; and
this is in accordance with their habits, their lower specific gravity,
their power of flight, and theiv capacity for resisting for some
time the effects of salt water.
We learn, then, from the fauna of these islands, some very im-
portant facts. We are taught that tropical land-birds, unless
blown out of their usual course by storms, rarely or never venture
out to sea, or if they do so, can seldom pass safely over a distance
of 500 miles. The immigrants to the Galapagos can hardly have
averaged a bird in a thousand years. We learn, that of all reptiles
lizards alone have some tolerably effective mode of transmission
across the sea ; and this is probably by means of currents, and
in connection with floating vegetation. Yet their transmission
is a far rarer event than that of land-birds ; for, whereas three
female immigrants will account for the lizard population, at least
eight or ten ancestors are required for the birds. Land serpents
can pass over still more rarely, as two such transmissions would
have sufficed to stock the islands with their snakes ;
and it is not
certain that either of these occurred without the aid of man.