Page:Popular Science Monthly Volume 10.djvu/19

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9
AMERICAN ZOÖLOGISTS AND EVOLUTION.

his work ranks among the most important contributions to this science. Mr. Allen finds that there are marked geographical variations in mammals and. birds. He shows that northern mammals of the same species are more thickly and softly furred, and that toward the south the peripheral parts, such as the ears and feet, are more developed. The same law holds good in birds, a diminution in size being observed toward the south, and the individuals being darker in color.

As one goes south he meets with the same species of birds, whose bodies are shorter, but whose beak, tail, and claws, are longer. On the Plains, also, he found the birds with plainer tints, while southward the colors became more intense. On drawing up a table indicating the regions of lighter varieties, and comparing it with a chart of mean annual rainfall, Mr. Allen found the lighter forms occurred in dry regions, and the dark forms in relatively humid regions. To sum up: Mr. Allen finds in latitudinal variation climatic influences affecting color as well as altering the size of bill, claw, and tail, while longitudinal variation usually affects color alone.

He states that these laws are now so well known that a species may be predicted to assume a given color if under certain specific climatic conditions.

Mr. Robert Ridgway[1] has in a similar way called attention to the relation between color and geographical distribution in birds as exhibited in melanism and hyperchromatism, and has shown that red areas "spread" or enlarge their field in proportion as we trace certain species to the Pacific coast, and that in the same proportion yellow often intensifies in tint.

The results of these investigations can be easily understood. Nearly if not quite one hundred and fifty species of birds, which were recognized as distinct, are at once reduced to varieties, though less than twelve years ago they were looked upon as good species, with which no external influence had anything to do. Nearly if not quite a fifth of the number of species of birds have been reduced by the investigations of Baird, Allen, Coues, and Ridgway.

The mammals, through the same study of geographical variation, will have been reduced at least one-fourth. Already Mr. Allen[2] has studied the geographical variation of the squirrels, and the result is that a reduction has been made of one-half the number of species before recognized. Prof. Baird, in his monograph of North American squirrels, reduced the number from twenty-four, as acknowledged by Audubon and Bachman, to ten well-established species and two doubtful varieties. Allen, with still greater advantage in the shape of a mass of material from the Western surveys, l-educed the ten species to five species, with seven geographical varieties.

  1. American Journal of Science and Arts, vol, iv., December, 1872, p. 454, and vol. v., p. 39.
  2. "Proceedings of the Boston Society of Natural History," vol. xiv., p. 276.