And so is it also with the animated world of past ages; in similar deposits of sand, mud, or lime, in adjoining regions of the same geological age, identical remains of animals and plants may be found, while at greater distances, but under similar circumstances, representative species may occur.  In very remote regions, however, whether the circumstances be similar or dissimilar, the general aspect of the organic world differs greatly, remoteness in space being thus in some measure an indication of the degree of affinity between different faunae.  In deposits of different geological periods immediately following each other we sometimes find remains of animals and plants so closely allied to those of earlier or later periods that at first sight the specific differences are hardly discernible.  The difficulty of solving these questions, and of appreciating correctly the differences and similarities between such closely allied organisms, explains the antagonistic views of many naturalists respecting the range of existence of animals, during longer or shorter geological periods; and the superficial way in which discussions concerning the transition of species are carried on is mainly owing to an ignorance of the conditions above alluded to.  My own personal observation and experience in these matters have led me to the conviction that every geological period has had its own representatives, and that no single species has been repeated in successive ages.

The laws regulating the geographical distribution of animals and their combination into distinct or zoological provinces called faunae with definite limits are very imperfectly understood as yet; but so closely are all things linked together from the beginning till to-day that I am convinced we shall never find the clue to their meaning till we carry on our investigations in the past and the present simultaneously.  The same principle according to which animal and vegetable life is distributed over the surface of the earth now prevailed in the earliest geological periods.  The geological deposits of all times have had their characteristic faunae under various zones, their zoological provinces presenting special combinations of animal and vegetable life over certain regions, and their representative types reproducing in different countries, but under similar latitudes, the same groups with specific differences.

Of course, the nearer we approach the beginning of organic life, the less marked do we find the differences to be, and for a very obvious reason.  The inequalities of the earth’s surface, her mountain-barriers protecting whole continents from the Arctic winds, her open plains exposing others to the full force of the polar blasts, her snug valleys and her lofty heights, her table-lands and rolling prairies, her river-systems and her dry deserts, her cold ocean-currents pouring down from the high North on some of her shores, while warm ones from tropical seas carry their softer influence to others,—­in short, all the contrasts in the external configuration of the globe, with the physical conditions attendant upon them, are naturally accompanied by a corresponding variety in animal and vegetable life.