402 
GEOGEAPHICAL DISTEIBUTION. 
Chap. XII. 
species, botli those found in other parts of the world 
and those confined to the archipelago, are common to 
the several islands, and we may infer from certain facts 
that these have probably spread from some one island 
to the others. But we often take, I think, an erro¬ 
neous view of the probability of closely-allied species 
invading each other's territory, when put into free 
intercommunication. Undoubtedly if one species has 
any advantage whatever over another, it will in a 
very brief time wholly or in part supplant it; but if 
both are equally well fitted for their own places in 
nature, both probably will hold their own places and 
keep separate for almost any length of time. Being 
familiar with the fact that many species, naturalised 
through man’s agency, have spread with astonishing 
rapidity over new countries, we are apt to infer that 
most species would thus spread; but we should remem¬ 
ber that the forms which become naturalised in new 
countries are not generally closely allied to the aboriginal 
inhabitants, but are very distinct species, belonging in a 
large proportion of cases, as shown by Alph. de Candolle, 
to distinct genera. In the Galapagos Archipelago, many 
even of the birds, though so well adapted for flying 
from island to island, are distinct on each ; thus there 
are three closely-allied species of mocking-thrush, each 
confined to its own island. Now let us suppose the 
mocking-thrush of Chatham Island to be blown to 
Charles Island, which has its own mocking-thrush : why 
should it succeed in establishing itself there ? We may 
safely infer that Charles Island is well stocked with its 
own species, for annually more eggs are laid there 
than can possibly be reared; and we may infer that the 
mocking-thrush peculiar to Charles Island is at least as 
well fitted for its home as is the species peculiar to 
Chatham Island. Sir C. Lyell and Mr. Wollaston have 
communicated to me a remarkable fact bearing on this 
