THE LINEAGE OF MAN
Each little red muscle fibre is a tiny “gas engine,” consuming the oxygen of the blood stream; it is touched off, so to speak, by the nerve current which is conveyed through a nerve fibril from the larger nerves that pass down from the spinal nerve cord to the muscle.
All the complicated locomotor apparatus of the vertebrates, including man, has evolved according to clearly discernible stages out of this relatively simple ground plan. In support of this statement Nature supplies us with hundreds of different variations of this simple theme. The ultimate causes of evolution may be as mysterious as you like, the origin of the vertebrates from invertebrates may be obscure if you will, but the main stages in the evolution of the locomotor apparatus, from fish to man, are now a matter of record, and the same is true as to the evolution of the human skull, brain, and spinal cord.
The following story of evolution has not been built up like a system of metaphysics or philosophy out of abstruse untested reasoning; it is the plain result of many more or less independent lines of research and discovery pursued by geologists, palaeontologists, zoologists, embryologists, and other scientists for more than a century. Naturally, within the limits of the space available in this book I cannot review the evidences that have led to this general picture of vertebrate evolution.
Early Evolution of the Fishes
Whatever may have been the origin of vertebrates, by the time the Devonian period was reached a very great advance had been made toward the higher forms, for at that very distant time, probably half a billion years ago, there were already in existence shark-like fishes that resembled man in possessing the following important structural characters:
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