Page:Popular Science Monthly Volume 78.djvu/533

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MEASUREMENT OF NATURAL SELECTION

For the evolutionist a question of fundamental importance is this: Do the offspring of selected individuals differ from unselected individuals of the same population? This is, however, really a problem of variation and heredity and not of selection at all. Given variations which are heritable, stringent selection will change the type of the population. If after this change of type no more heritable variations occur, selection can effect no further change.

The history of cultivated varieties shows us that much can be accomplished by selection, but neither the history of animals and plants under domestication nor any amount of experimental evidence would be sufficient to demonstrate the correctness of the third Darwinian proposition.

This has often been recognized. "The real difficulty of Darwin's theory is the transition from artificial to natural selection," said Paul Janet. Darwin himself frankly tells us, "I soon perceived that selection was the keystone of man's success in making useful races of animals and plants. But how selection could be applied to organisms living in a state of nature remained for some time a mystery to me."

Experimental breeding and statistical studies in variation and heredity can teach us much, but evolution, for the most part, has occurred outside the breeding pen. Some species have originated in the greenhouse and some in a hanging drop culture, but most species have come into existence and biological dynasties risen to dominance and sunk into decadence in the fields and swamps and mountains where organisms live in competition and cooperation, as host and parasite, as destroyer and destroyed. From the standpoint of evolution the vital question concerning selection is: Does selection (natural, sexual or genetic) occur in nature?

Is a selective death rate such an important factor that equipped with proper instruments the biologist can go out into free nature and measure its intensity? If he can, then the Darwinian theory of evolution must detain us longer; if he can not, we must lay Darwinism on one side, and maintain towards it, as towards all other theories for which critical evidence is wanting, an attitude of agnosticism.

III. The Measurement of Selective Elimination'

The hypothesis of the existence of the evolutionary factor known as natural selection is dependent upon the assumption that individuals vary in their capacity to withstand the pressure of their environment, and that the differences in resistance to untoward external conditions are associated with and due to differences in the physical, physiological or psychical characteristics of the organism.

It does not assume that every death is selective. Many are due to factors which eliminate irrespective of any particular character; many survivals are due to a fortuitous combination of favorable environmental