If we keep in view these facts--that the minor features of the earth's surface are everywhere slowly changing; that the forms, and structure, and habits of all living things are also slowly changing; while the great features of the earth, the continents, and oceans, and loftiest mountain ranges, only change after very long intervals and with extreme slowness; we must see that the present distribution of animals upon the several parts of the earth's surface is the final product of all these wonderful revolutions in organic and inorganic nature. The greatest and most radical differences in the productions of any part of the globe must be dependent on isolation by the most effectual and most permanent barriers. That ocean which has remained broadest and deepest from the most remote geological epoch {8}will separate countries the productions of which most widely and radically differ; while the most recently-depressed seas, or the last-formed mountain ranges, will separate countries the productions of which are almost or quite identical. It will be evident, therefore, that the study of the distribution of animals and plants may add greatly to our knowledge of the past history of our globe. It may reveal to us, in a manner which no other evidence can, which are the oldest and most permanent features of the earth's surface, and which the newest. It may indicate the existence of islands or continents now sunk beneath the ocean, and which have left no record of their existence save the animal and vegetable productions which have migrated to adjacent lands. It thus becomes an important adjunct to geology, which can rarely do more than determine what lands have been raised above the waters, under what conditions and at what period; but can seldom ascertain anything of the position or extent of those which have sunk beneath it. Our present study may often enable us, not only to say where lands must have recently disappeared, but also to form some judgment as to their extent, and the time that has elapsed since their submersion. Having thus briefly sketched the nature and objects of the subject we have to study, it will be necessary--before entering on a detailed examination of the zoological features of the different parts of the earth, and of the distribution of the orders, families, and genera of animals--to examine certain preliminary facts and principles essential for our guidance. We must first inquire what are the powers of multiplication and dispersal of the various groups of animals, and the nature of the barriers that most effectually limit their range. We have then to consider the effects of changes in physical geography and in climate; to examine the nature and extent of such changes as have been known to occur; to determine what others are possible or probable; and to ascertain the various modes in which such changes affect the structure, the distribution, or the very existence of animals.