The evidence upon which the doctrine of evolution rests is too extensive and too technical to be even summarized here but it is regarded as trustworthy by most scientists.[3] For fifty years it has been studied, discussed, and tested by scholars with the result that educated men are now disposed to accept the doctrine so far as its main principles are concerned although they differ about various details. It is astonishing how little we know, after all, about the beginnings of things. We do not know when or how life began upon the earth. We do not know the exact origin of man. But we do know that all forms of life and institutions have grown; they were not created in the shape we now have them. All the general laws of life which apply to plants and animals apply also to man. Alike they are born, they are nourished, they mature, and they produce descendants like themselves. [Sidenote: The principle of “natural selection” and the struggle for existence.] The method of evolution, according to Darwin’s theory is based upon the principle of _natural selection_. It is a well-known law of nature that “like begets like”, in other words that offspring resemble the parent-stock although there may be some individual differences. If it were not so, a definite species would never be perpetuated. All forms of life, moreover, reproduce themselves more or less abundantly. It is said that the progeny of a single starfish exceeds half a million per year. Even the elephants, which are the slowest breeding of all animals, produce a sufficiently numerous offspring to over-run the whole of Africa if every young elephant grew to maturity. But nowhere does the entire progeny of any organisms, whether plants, animals, or human beings, survive to full growth. If every acorn became an oak tree, there would in time be no room for anything else on the surface of the earth. If every tadpole grew to be a frog, there would be no room for anything else in the waters of the earth. All life, however, is a struggle for existence, a relentless competition for air, sunshine, moisture, and soil on the part of plants, and for food and shelter on the part of animals. The further down we go in the scale of life the more bitter this struggle for existence becomes; small animals eat up the plants; large animals feed on the smaller ones. Higher in the scale, the struggle is not so keen, and among mankind it is the least strenuous of all.