THE FACTORS OF EVOLUTION
CHAPTER I
All the world, or at any rate all that portion of the world which has a sufficient knowledge of the facts, is practically agreed that the plant and animal kingdoms have arisen by evolution; but all the world is not agreed as to what have been the factors in that evolution. It is not now disputed that natural selection, by perpetuating and accumulating inborn favourable variations, has been a cause of evolution, but it is contended by many biologists, some of whom are of the greatest eminence, that the accumulation of inborn variations has not been the sole nor even the principal cause of evolution; but that the accumulation of acquired variations has been an additional and even the principal cause. To take an example; hares run swiftly, their rapid pace being due to structural evolution from a slower ancestry. By the one school of biologists it is held that the great speed of hares is due to their having lived under conditions which permitted only the naturally swift to survive, so that while those animals that were naturally slow perished, the others continued the race; the other school, while admitting that the evolution of speed in hares may have been partly so caused, contend that it
54
