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 verte- 

 brates, including man, has evolved according to clearly dis- 

 cernible 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 loco- 

 motor apparatus, from fish to man, are now a matter of rec- 

 ord, 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 verte- 

 brate 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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