THE ORIGIN OF MAN 41 



The earliest known fishes are the ostracoderms 

 found in the Middle Ordovician. It is thought 

 that freshwater fishes existed in the Ordovician 

 and which evolved into the lung-fishes, animals 

 that were then more or less adapted to double 

 breathing, that is, had not only functional gills 

 as have all fishes, but in addition lungs, of the 

 Silurian and Devonian. Therefore it appears 

 that the freshwater fishes of the Devonian con- 

 sisted of spinous sharks, ostracoderms, lung- 

 fishes and ganoids. Hence the most fundamental 

 evolution among the fishes had taken place pre- 

 vious to the Devonian, and the vista in the Ordo- 

 vician shows that the greater part of it had oc- 

 curred before the middle of that period. When 

 the seas spread over the land, the primitive fishes 

 occupied their widened habitat, and when the 

 continent emerged, some of them were probably 

 retained in the relic seas. This possibly explains 

 to some extent how fishes came to be found in 

 inland lakes and rivers. Thus caught in the relic 



