The Last Harvest by John Burroughs

Natural Selection turns out to be of only secondary importance. It is not creative, but only confirmative. It is a weeding-out process; it is Nature's way of improving the stock. Its tendency is to make species more and more hardy and virile. The weak and insufficiently endowed among all forms tend to drop out. Life to all creatures is more or less a struggle, a struggle with the environment, with the inorganic forces, -- storm, heat, cold, sterile land, and engulfing floods, -- and it is a struggle with competing forms for food and shelter and a place in the sun. The strongest, the most amply endowed with what we call vitality or power to live, win. Species have come to be what they are through this process. Immunity from disease comes through this fight for life; and adaptability -- through trial and struggle species adapt themselves, as do our own bodies, to new and severe conditions. The naturally weak fall by the wayside as in an army on a forced march.

Every creature becomes the stronger by the opposition it overcomes. Natural Selection gives speed, where speed is the condition of safety, strength where strength is the condition, keenness and quickness of sense-perception where these are demanded. Natural Selection works upon these attributes and tends to perfect them. Any group of men or beasts or birds brought under any unusual strain from cold, hunger, labor, effort, will undergo a weeding-out process. Populate the land with more animal life than it can support, or with more vegetable forms than it can sustain, and a weeding-out process will begin. A fuller measure of vitality, or a certain hardiness and toughness, will enable some species to hold on longer than others, and, maybe, keep up the fight till the struggle lessens and victory is won.