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Page:Popular Science Monthly Volume 34.djvu/501

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ON THE CAUSES OF VARIATION.

than the indigenes or endemic forms. This is readily comprehended on two grounds: First, that species which liave, in the course of time, experienced a greater struggle among themselves in large areas, have an advantage over those in more limited areas in which the struggle has been less intense; secondly, that species which have accommodated themselves to the changes in life conditions which civilized man induces, have a great advantage when, following man's migrations, they are brought into competition with species which have not yet been subjected to such conditions. Again, no valid reason can be urged why, within a given area, one species predominates over another in so far as mere adaptation is concerned. The influences of environment alone would tend to unify the fauna and flora of a given region. Theoretically, so far as climate and physical conditions are concerned, there is no reason, through regions where these are uniform, why a single animal should not prevail to the exclusion of all others, providing it was vegetarian, or that the particular plant which furnished food to such an animal should not prevail to the exclusion of all others. The hickory and the blade of grass must be considered equally adapted to the environment with the oak, and so on all through the multifarious forms of both vegetal and animal life: so that this diversity of form can best be explained by some principle like natural selection, and by the interrelation and interaction of organisms and the struggle between them for existence. This is illustrated in many directions. To take a striking example: no one doubts that if the larger carnivora of Europe and Asia were introduced into Australia, the marsupials would soon have to give way, and could survive only by the acquisition of special functional modifications and larger intelligence such as we find in our opossum. Yet it would be folly to conclude that the marsupials are less well fitted to the physical conditions which obtain in Australia than their introduced exterminators.

From what has preceded, we are, I think, justified in rejecting the interpretations of both extremists as to the scope and meaning of natural selection. It can not be debased to the mere expression of the universally observed fact of variability; yet it must be restricted, because it not only implies something to be selected, but its promulgator limits its scope to the selection of something that is useful. As a philosophy it considers only processes, and leaves remote origin and cause untouched. The following limitations are probably justified to-day, and will help to more exact use of the term:

1. It deals only with individual variation from whatever cause, and should not be applied to simultaneous variation in masses.