Chap. XII. 
OCEANIC ISLANDS. 
403 
subject; namely, that Maderia and the adjoining islet 
of Porto Santo possess many distinct but representative 
land-shells, some of which live in crevices of stone; and 
although large quantities of stone are annually trans¬ 
ported from Porto Santo to Maderia, yet this latter 
island has not become colonised by the Porto Santo 
species: nevertheless both islands have been colonised 
by some European land-shells, which no doubt had some 
advantage over the indigenous species. From these 
considerations I think we need not greatly marvel at 
the endemic and representative species, which inhabit 
the several islands of the Galapagos Archipelago, not 
having universally spread from island to island. In 
many other instances, as in the several districts of the 
same continent, pre-occupation has probably played an 
important part in checking the commingling of species 
under the same conditions of life. Thus, the south-east 
and south-west corners of Australia have nearly the 
same physical conditions, and are united by continuous 
land, yet they are inhabited by a vast number of distinct 
mammals, birds, and plants. 
The principle which determines the general character 
of the fauna and flora of oceanic islands, namely, that 
the inhabitants, when not identically the same, yet are 
plainly related to the inhabitants of that region whence 
colonists could most readily have been derived,—the 
colonists having been subsequently modified and better 
fitted to their new homes,—is of the widest applica¬ 
tion throughout nature. We see this on every moun¬ 
tain, in every lake and marsh. For Alpine species, 
excepting in so far as the same forms, chiefly of plants, 
have spread widely throughout the world during the 
recent Glacial epoch, are related to those of the sur¬ 
rounding lowlands;—^thus we have in South America, 
Alpine humming-birds, Alpine rodents, Alpine plants. 
