Page:Popular Science Monthly Volume 37.djvu/332

318 off-shore islands, like the Bermudas, the remainder drowned in the sea.

Of the immigrants to the Galapagos the majority doubtless die and leave no sign. A few will remain, multiply, and take possession, and their descendants are thus native to the islands. But, isolated from the great mass of their species and bred under new surroundings, these island birds come to differ from their parents and still more from the great mass of the land species of which their ancestors were members. Separated from these, their individuality would assert itself. They would assume with new environment new friends, new foes, new conditions. They would develop qualities peculiar to themselves—qualities intensified by isolation. "Migration" says Dr. Coues, "holds species true; localization lets them slip." This would be more exactly the truth should we say that localization holds peculiarities true; migration lets them slip. Local peculiarities disappear by wide association and are intensified when individuals of similar peculiarities are kept together. Should later migrations of the original land species come to the islands, the individuals surviving would in time form distinct species, or more likely, mixing with the mass of those already arrived, their special characters would be lost in those of the majority.

The Galapagos, first studied by Mr. Darwin, serve to us only as an illustration. The same problems come up in one guise or another in all questions of geographical distribution, whether of continent or island.

The relations of the fauna of different regions are intimate in direct relation to the ease by which barriers may be crossed. Distinctness is in direct proportion to isolation. What is true in this regard of the fauna of any region as a whole is likewise true of any of its individual species. The degree of resemblance among individuals is in direct proportion to the freedom of their movement, and variation within what we call specific limits is again proportionate to the barriers which prevent equal and perfect diffusion.

The various divisions or realms into which the surface of the earth may be divided on the basis of the differences in animal life each has its boundary in the obstacles offered to the spread of the average animal. Each species broadens its range as far as it can. It struggles knowingly or not to overcome the barriers of ocean or river, of mountain or plain, of woodland or desert, of moisture or drought, of cold or heat, of lack of food or abundance of enemies, whatever these barriers may be. Were it not for these barriers, every species would become what only man now is, practically cosmopolitan. Man is pre-eminently the barrier-crossing animal. The degree of hindrance offered by any barrier to the