1. The number of individual varieties constantly being formed is almost infinite, but the number of places in nature is very limited. Now, among the infinite number of slight individual varieties formed with every generation, the competitive struggle will be severest between those most nearly alike, because they are competitors for the _same_ place. Only one kind succeeds, viz., the fittest. Intermediate forms are, therefore, exactly those which are eliminated in the most wholesale way. 2. Add to this the fact that, as soon as divergence, from whatsoever cause, reaches a certain point, sexual repugnance or cross-sterility, or both, come in to perpetuate and increase the separation already commenced. 3. Add to this, again, that migrations in higher animals, and involuntary dispersals in lower animals and in plants, and the mingling together of different faunas and floras, produces a still fiercer struggle for life, especially between natives and invaders, and thus great numbers of forms are destroyed; all but the fittest are weeded out, and therefore the distinctness of the remainder is greatly increased. Periods of great changes of physical geography and of climate, and therefore of wide and general migrations, are also periods of great weedings-out of unfit forms. Thus it happens that existing faunas and floras are little else than isolated _remnants_. To illustrate, again, by a growing tree: If all the buds of a tree lived and grew, they would soon become so numerous that they would together form a solid hemispherical mass, like a coral-head, with no room between for leaf or light or air. But ninety-nine one-hundredths of buds die in the struggle for light and air, and therefore the survivors are distinct growing points, widely separated from each other. Species are such extreme, but separated, twiglets of the tree of life.