MATERIALIST MONISM 



on the chorda of its worm ancestors and whose 

 tiny string of spinal nerves above the chorda 

 indicates a new feature which from now on be- 

 comes the characteristic mark of all vertebrates. 



During the last stages of the transition toward 

 the Silurian period, worm-like fishes appear with 

 a better developed chorda, a more highly organ- 

 ized nerve system, and the rudiments of a skull 

 with brains. The spinal chord above the chorda 

 is becoming the halfway house between a brain 

 and a complicated nerve system. 



The selachian and ganoid fishes of the Silurian 

 period have developed the chorda into a cartilage 

 backbone and skeleton, their brain is much bet- 

 ter organized, and they have acquired the definite 

 beginnings of that system of five fingers or toes 

 on four limbs, which we see later as a typical 

 mark of the highest vertebrates. 



Ages pass and accumulated changes in the 

 earth's crust and surface bring on the Devonian 

 period. Dry land has been lifted out of the 

 oceans, and some fishes have been compelled to 

 live for months with little or no water. They 

 have developed their airbladder into a lung and 

 their forefins into strong supports for walking, 

 climbing and digging. Their cartilage skeleton 

 has become bony. The decisive step has been 



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