208 ZOOLOGICAL GEOGRAPHY. [PART II. 
tion all tends in one direction—to produce a more sombre 
plumage, a greater strength of feet and legs, and a more robust 
bill. We further find, that four of the land-birds, including the 
oriole, snow-bunting, and hoopoe, are not resident birds, but 
straggle accidentally to the islands by stress of weather; and 
we are told that every year some fresh birds are seen after 
violent storms. Add to this the fact, that the number of 
species diminishes in the group as we go from east to west, and 
that the islands are subject to fierce and frequent storms 
blowing from every point of the compass,—and we have all the 
facts requisite to enable us to understand how this remote 
archipelago has become stocked with animal life without ever 
probably being much nearer to Europe than it is now. For 
the islands are all volcanic, the only stratified rock that occurs 
being believed to be of Miocene date. 
Madeira and the Canaries.—Coming next to Madeira, we find 
the number of genera of land birds has increased to twenty-eight, 
of which seventeen are identical with those of the Azores. Some 
of the commonest European birds—swallows, larks, sparrows, 
linnets, goldfinches, ravens, and partridges, are among the addi- 
tions. A gold-crested warbler, Regulus Maderensis, and a pigeon, 
Columba Trocaz, are peculiar to Madeira. 
In the Canaries we find that the birds have again very much 
increased, there being more than fifty genera of land birds; but 
the additions are wholly European in character, and almost all 
common European species. We find a few more peculiar spe- 
cies (five), while some others, including the wild canary, are 
common to all the Atlantic Islands or to the Canaries and 
Madeira. Here, too, the only indigenous mammalia are two 
European species of bats. 
Land Shells.—The land shells of Madeira offer us an instruc- 
tive contrast to the birds of the Atlantic Islands. About fifty- 
six species have been found in Madeira, and forty-two in the small 
adjacent island of Porto Santo, but only twelve are common to 
both, and all or almost all are distinct from their nearest allies 
in Europe and North Africa, Great numbers of fossil shells 
are also found in deposits of the Newer Pliocene period; and 
