Page:Popular Science Monthly Volume 37.djvu/524

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are birches even where snow falls during every month of the year, and the distant sun gives only a glimpse of summer in July. Competition with other plants is, of course, not severe in such regions, but the birches must struggle against the weather. They can live and multiply, if only they can adjust themselves to the conditions of life. They must keep down their size, they must carry as little foliage as possible, and their stems must be tough enough to resist snow, and hardy enough to withstand almost perpetual frost. Their year's growth must be finished in a very short time, and leaves, flowers, and seeds must follow in the most rapid succession. In short, there is room for birch-trees here, if only the trees can be reduced to their lowest terms. And so birch-trees have crept up the mountain-sides even to the very edges of the perpetual snow. But such trees! All trees requiring sunshine, or long time for their summer's growth, are rigidly kept away by "natural selection." The cold climate dwarfs the individual, and the hard conditions exclude every individual not dwarfed. I have before me three birch-trees from a Norwegian mountain called the Suletind—the little trees known to the Norwegian peasants as "Hundsöire," or "dogs'-ears." The trunk of each tree is barely an inch in height. There are no branches, and but three leaves. Half inclosed by the uppermost leaf is the single little catkin of flowers. Leaves in June, blossoms in July, fruit in August, and then the little tree is ready for its nine months' sleep. These little trees are the Lapps of forest vegetation.

All natural history is full of similar cases of modifications. Everywhere there is the most perfect adaptation of life to its conditions. But this adaptation must come about through the survival of those organisms fittest to live under the conditions, while the unfit die out and leave no progeny. But fitness is a relative term; for in many cases, as with the Norwegian dwarf birches, the deformed or stunted may be the only ones fitted to survive. An advantage ever so slight must in the long run conquer. The gambler recognizes that final victory must always go with the percentage of the dealer.

The restlessness of individuals is the key to all these problems. Each species of animal or plant is first the product of heredity, and then of the various influences, reactions, and extinctions to which we give the name of natural selection. Each species may be conceived as making every year inroads on territory occupied by other species. If these colonies are able to hold their own in the struggle for possession, they will multiply in the new conditions, and the range of the species becomes widened. If the surroundings are different, new species or varieties may be formed with time; and these new forms may invade the territory