CHAP. XIV.] 
THE NEOTROPICAL REGION. 
35 
the sea, because we find existing species with an enormous 
range. The ancestors of the AmblyrhynrM 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 species 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 their 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. 
D 2 
