Seite - 434 - in The Origin of Species

434 ORIGIN OF SPECIES birds are blown to Madeira; this island is inhabited by gcf kinds, of which one alone is peculiar, though very closely related to a European form ; and three or four other species are confined to this island and to the Canaries. So that the Islands of Bermuda and Madeira have been stocked from the neighbouring continents with birds, which for long ages have there struggled together, and have become mutually co-adapted. Hence when settled in their new homes, each kind will have been kept by the others to its proper place and habits, and will consequently have been but little liable to modification. Any tendency to modification will also have been checked by intercrossing with the unmodified immi- grants, often arriving from the mother-country. Madeira again is inhabited by a wonderful number of peculiar land- shells, whereas not one species of sea-shell is peculiar to its shores; now, though we do not know how sea-shells are dis- persed, yet we can see that their eggs or larvae, perhaps at- tached to seaweed or floating timber, or to the feet of wading- birds, might be transported across three or four hundred miles of open sea far more easily than land-shells. The dif- ferent orders of insects inhabiting Madeira present nearly parallel cases. Oceanic islands are sometimes deficient in animals of cer- tain whole classes, and their places are occupied by other classes; thus in the Galapagos Islands reptiles, and in New Zealand gigantic wingless birds, take, or recently took, the place of mammals. Although New Zealand is here spoken of as an oceanic island, it is in some degree doubtful whether it should be so ranked; it is of large size, and is not sep- arated from Australia by a profoundly deep sea; from its geological character and the direction of its mountain-ranges, the Rev. W. B. Clarke has lately maintained that this island, as well as New Caledonia, should be considered as appur- tenances of Australia. Turning to plants. Dr. Hooker has shown that in the Galapagos Islands the proportional num- bers of the dififerent orders are very different from what they are elsewhere. All such differences in number, and the ab- sence of certain whole groups of animals and plants, are gen- erally accounted for by supposed differences in the physical conditions of the islands; but this explanation is not a little