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Page:Popular Science Monthly Volume 82.djvu/392

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388
THE POPULAR SCIENCE MONTHLY

NATURAL SELECTION
By Professor T. D. A. COCKERELL

UNIVERSITY OF COLORADO

THE lowest forms of life show the greatest stability in their specific characters. Any one who will examine a little pond water under the microscope will see numerous minute Protozoa belonging to different families, genera and species. Close study brings out the fact that although we regard these as very low types, they are complicated little animals, with remarkable characters. When we look at lists of these creatures, based on collections from different parts of the world, we are astonished to find that most of the species are the same, no matter how remote the localities. When identifications were based merely on comparisons of descriptions and figures, we suspected that the alleged wide distribution of some of these fresh-water Protozoa might be due to mistakes. In recent years, however, protozoologists have frequently traveled, and Dr. Penard, the greatest authority on rhizopods, has been able to determine by study on the spot the identity of Rocky Mountain forms with those of Switzerland. Even more remarkable are the results of Dr. Edmondson, who has visited Tahiti, high altitudes in the Rocky Mountains, and various places in the western central states, with the result of entirely confirming the opinion that most species of fresh-water Protozoa are spread over the world, almost without regard to climate or altitude.[1]

These studies and reports, however, have been based on the species as generally understood. These species are not entirely uniform, but consist of groups of minor races, which also appear to have constant characters and to be of general distribution. Dr. Penard told me that he could greatly increase the list of "species" of rhizopods were he to describe as distinct all the apparently constant forms which he had learned to recognize, and which, so far as he knew, did not conjugate one with the other. He did not describe and name them because their separation required such critical comparisons and familiarity with the subject that very few naturalists would be willing to consider them. Professor Jennings, in his studies of Paramæcium, has isolated a number of races or varieties which possess constant characters by which they can be recognized, and which are believed to be common throughout the country.

  1. University of Colorado Studies, IX., pp. 65-74; Science, September 9, 1910.