It is only necessary to add a few experimental considerations to complete a rapid sketch of the laws of variability. First, correlation of growth: Homologous organs tend to vary in the same direction, and together. Are the fingers joined or divided? The hand follows similar variations. Then there is a compensation of growth which prevents the excess of the preceding rule; when one organ is developed, another is atrophied. Also vital competition. Every organized being must be in harmony with the conditions of its existence or it will not subsist; the monster may appear, but will not live. Lastly, by virtue of natural selection, the individual must likewise possess the means of perpetuating its species. Otherwise, a series of transformations will come to pass in the course of successive generations, improving the organism and adapting it more and more to the exigencies of its habitat. The least prolific species of to-day fulfill these conditions so well that they of themselves alone would cover the surface of the earth if their multiplication was not checked by that of other species. But as only a limited quantity of life is possible on our planet, the less well-adapted organisms perish. The struggle therefore produces a selection. It is hence presumed that in the same species only varieties manifesting tendencies in most complete harmony with the method of their existence will be preserved, all the intermediate varieties being destroyed. Consequently, if we push the doctrine of Darwin to its extreme limits, we arrive at the idea, now rejected, that in the beginning only a single germ arose at one point on the globe. All the analogies, on the other hand, lead us to suppose that the earth was fruitful over its entire surface.