To turn to a later but very ancient period, what can be more astonishing than those first air-breathing vertebrates of the Coal formation referred to in a previous chapter, with all their special arrangements for an aërial habitat? I have mentioned their footprints, and when we see the quarrymen split open a slab of sandstone and expose a series of great plantigrade tracks, not unlike those of a human foot, with the five toes well-developed, we are almost as much astonished as Crusoe was when he saw the footprints on the sand. Crusoe inferred the presence of another man in his island; we infer the earliest appearance of an air-breathing vertebrate and the pre-human determination of the form and number of parts of the human foot and hand, to appear in the world long ages afterward. We see also that already that decimal system of notation which we have founded on the counting of our ten fingers was settled in the framework of most unmathematical Batrachians. It has approved itself ever since as the typical and most perfect number of parts for such organs. If sceptically inclined, we may ask, Why five rather than four or six? In the case of man we see that individuals who have lost one finger have the use of the hand impaired, while the few who happen to have six do not seem to be the better. How it was with the old Batrachians we do not know; but it is certain that if we could have amputated the claw-bearing little toe of _Sauropus unguifer_, or the reflexed little toe of _Cheirotherium_, we should have much injured their locomotive power. The vegetable kingdom is full of similar examples of the early settlement of great questions. Perhaps nothing is more marvellous than the power of the green cells of the leaf as workers of those complex and inimitable chemical changes whereby out of the water, carbon dioxide and ammonia of the soil and the atmosphere, the living vegetable cell, with the aid of solar energy, elaborates all the varied organic compounds produced by the vegetable kingdom. Yet this seems all to have been settled and perfected in the old Silurian period, long before any kind of plant now living was on the earth. Perhaps in some form it existed even in the Laurentian age, and was instrumental in laying up its great beds of carbon. So all that is essential in plant reproduction, whether in that simpler form in which a one-celled spore is the reproductive organ, or in that more complex form in which an embryo plant is formed in the seed, with a store of nourishment laid up for its sustenance.