Even in the case of the anthropoid apes the transition to the instrument and to a product of labour entirely artificial and perfectly independent of the animal's own body, is only very slowly completed. This is clear from a consideration of the slow process by which man has progressed in perfecting the implements which he has invented. From the action of the bird which beats open a nut with its beak, or the squirrel which cracks it with its teeth, up to that of man who, in order to open the nut, makes use of a stone lying near him, is only a step, and yet by that step the destiny of the _genus homo_ is settled. The application of natural objects, such as sticks and stones, to the purposes of daily life, to defence against animals and men, to hunting, to cutting down fruits, and so on, does not certainly become a habit all at once. Indeed, a very long time elapsed before this adaptation became a general and even a conscious one, and it was only possible when the advantages of such objects had been perceived through many experiences. It needed a still longer time before man learned to choose between the various objects offered to him by nature, and understood how to distinguish a more pointed and sharper or a harder stone from one of those less useful for his purpose. Perhaps it required the experience and disappointments of uncounted ages to bring the consciousness of purpose even up to this point. But when this was once done, when man could judge as to the usefulness of the implement which nature offered him, then a further step of progress, and certainly the most important in this series of developments, was taken. To natural selection follows immediately artificial. The need for suitable and useful implements became more general and greater, and at the same time it became more difficult to satisfy, since nature is not so generous with objects of this kind, and (as was soon seen) only very few substances united all these qualities which hitherto had been recognised as necessary or useful. But by this time individuals who were already better provided for had made other discoveries; they had, for example, in cracking a nut, broken a stone with which they cracked it, and noticed that the broken pieces had greater sharpness and pointedness on their edges than those which nature afforded; or they had found the pieces of some tree split by lightning, and discovered their greater hardness and capacity for resistance. What was more natural under the pressure of the necessity, than to produce intentionally those processes by which the objects afforded by nature became more usable--to break the stone in pieces or to burn the wood?