Page:Popular Science Monthly Volume 37.djvu/335

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EVOLUTION AND DISTRIBUTION OF ANIMALS.

have been long shut off from communication with the great land masses." The rapid multiplication which certain holarctic animals and plants have shown when transported to the Australian realm, demonstrates what might have taken place if impassable barriers had not previously shut them out.

Each of these great realms may be indefinitely subdivided into provinces and sections, for there is no end to the possibility of analysis. No township or school district has exactly the same animals or plants as any other; and, finally, in ultimate analysis no two animals or plants are alike. Modification comes with the growth of each new individual, and steadily increases with the individual's separation in time or space from the parent stock. Moreover, we observe apparent anomalies of distribution in every realm: here appears an animal, there a plant, which seems to have a character or a place which it ought not to hold. To the result of unexpected or chance crossing of barriers these apparent anomalies in geographical distribution are due. Anomalies in distribution, like anomalies in evolution, would cease to be such if we knew all the facts and circumstances of their previous history. The present range of the tapir in Farther India and in the northern part of South America, two widely separated regions, is at first sight an anomaly of distribution. This anomaly disappears when we know that formerly the tapir ranged over the holarctic realm and became gradually extinct with the changing climate. The bones of a tapir, much like one of the South American species, are found in recent clays in Indiana (Ellettsville), and similar remains exist in France, in China, and in Burmah. The isolated, unexterminated colonies are now left at the extremes of the animal's former range, and these colonies at present constitute what we call distinct species.

The more extended are our studies the fewer are the anomalies which arrest our attention, and the fewer are the distinctive or characteristic forms. There is little foundation for the current belief that each species of animal has originated in the area it now occupies, for in many cases our knowledge of paleontology shows the reverse of this to be true. Even more incorrect is the belief that each species occupies the district or the surroundings best fitted for its habitation. This is manifest in the fact of the extraordinary fertility and persistence shown by many kinds of animals and plants in taking possession of new lands, which have become, through the voluntary or involuntary interference of man, open to their invasion. Facts of this sort are the "enormous increase of rabbits and pigs in Australia and New Zealand, of horses and cattle in South America, and of the sparrow in North America, though in none of these cases are the animals natives of the countries in which they thrive so well" (Wallace).