Читать книгу Wild Animals of North America - Edward William Nelson - Страница 10

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The differences in the geographic distribution of mammal life between North and South America and the relationships between our fauna and that of the Old World are parts of the latest chapter of a wonderful story running back through geologic ages. The former chapters are recorded in the fossil beds of all the continents. While only a good beginning has been made in deciphering these records, enough has been done by the fascinating researches of Marsh, Cope, Osborn, Scott, and others to prove that in all parts of the earth one fauna has succeeded another in marvelous procession.

It has been shown also that these changes in animal life, accompanied by equal changes in plant life, have been largely brought about by variations in climate and by the uplifting and depressing of continental land-masses above or below the sea. The potency of climatic influence on animal life is so great that even a fauna of large mammals will be practically destroyed over a great area by a long-continued change of a comparatively few degrees (probably less than ten degrees Fahrenheit) in the mean daily temperatures.

The distribution of both recent and fossil mammals shows conclusively that numberless species have spread from their original homes across land bridges to remote unoccupied regions, where they have become isolated as the bridges disappeared beneath the waves of the sea.

Photograph by Gus A. Swanson

THEIR LIVING LIES BENEATH THE SNOW

All nature loves kindness and trusts the gentle hand. Contrast these sheep, ready to fly at the slightest noise, with those in the picture on page 396, peacefully feeding in close proximity to a standing express train. Every one appreciates a good picture of a living animal more than the trophy of a dead one!