Page:Popular Science Monthly Volume 78.djvu/531

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THE

POPULAR SCIENCE

MONTHLY

 

JUNE, 1911




THE MEASUREMENT OF NATURAL SELECTION
By Dr. J. ARTHUR HARRIS

STATION FOR EXPERIMENTAL EVOLUTION, COLD SPRING HARBOR, N. Y.

I. The Status of Darwinism

BY organic evolution the broad-minded biologist of to-day understands merely the natural as opposed to the supernatural processes by which the hundreds of thousands of kinds of organisms which now inhabit or have inhabited the surface of the earth have come to possess the morphological and physiological peculiarities which distinguish them from each other. He believes that these differentiable types have been derived by a natural and relatively gradual process from earlier and, in the main, simpler forms.

This belief he shares with all his associates; the evidence in favor of it is considered by scientific men to be so strong that it has become scientific faith and scientific dogma. To-day, only the nature of the processes by which this evolution has proceeded interests biologists. Darwin's theory, and in large measure Darwin's evidence, have accomplished this. At one time Darwinism and organic evolution were synonyms, but now the suffixes "ian" and "ism" and the prefixes "neo" and "ultra" and "post" are combined with half a dozen different names and discussed with a glibness which is bewildering to some of those who are more interested in measuring the intensity of the factors which may have been active in organic evolution than in formulating theories concerning it.

Darwin's theory, viewed from such a distance that trivial details blend into large outlines, involves three propositions:

First, that variations from the typical condition of an existing species do occur.

Second, that these deviations may be inherited.

Third, that in the competition for existence which must result from