“His second point is what he terms the universal and necessary struggle for existence. This follows from the high geometrical ratio of increase common to all beings. If there were no catastrophes, any one of the existing species would be sufficiently numerous in a few thousand years to cover the whole earth, to the exclusion of everything else.
“His third point is to prove that this struggle is directed by the law of natural selection. Even the races of domestic animals may be constantly improved and modified by choosing the best individuals for propagation. Nature brings the same discipline to bear upon the whole domain of animal and vegetable life. She seizes at once upon any slight variation that is favorable, and perpetuates it; in the universal pressure, any variation that is injurious is immediately extinguished.”