In Silurian and Devonian Epochs 113 



lived, rapid destructive transfer of sand, gravel, and even 

 small stones, all must have combined to produce a biolog- 

 ical wear and tear that would cause extensive migrations, 

 exposure to new environal surroundings and factors, rapid 

 specific variations, and evolution of increasingly divergent 

 types of organisms, often also sudden as well as widespread 

 death. 



But in line with such results the question may well be 

 asked : Why did not rapid and wide migrations occur 

 from freshwater to littoral or suboceanic habitats? The 

 answer is evidently got when we remember that these 

 fishes were wholly or largely devoid of hard teeth, that they 

 were mainly of heavy flattened build, and were suited for 

 shuffling or grubbing after their food over muddy or sandy 

 surfaces. Above all, there is strong likelihood that the 

 seas and littoral regions were abundantly stocked with me- 

 dusoid and cephalopod organisms. For though we are 

 still unacquainted with the offensive or defensive parts of 

 these, it seems fairly probable that some of these were 

 formidable antagonists, which could drive back into fresh- 

 water such of the clumsy fishes as ventured into the sea. 

 So while related types to some of these primitive fishes 

 survived into the Lower and even into the Upper Devonian, 

 their place was gradually taken by more lithe, flexible and 

 resisting descendants. 



We are therefore compelled to conclude that the most 

 primitive fishes known to us, originated wholly in fresh- 

 water areas, and are there known to us in countless myriads 

 as fossilized forms. The associated animals and plants con- 

 stitute with these a biological assemblage that are totally 

 different from marine organisms of the same period. 



B. The Physical and biological environment of Devon- 

 ian fishes. 



It is now generally conceded that at least throughout 

 Europe and North America a complete or almost complete 

 continuity in succession of the rocks can be traced from 

 Upper Silurian beds such as the Temeside, to others whose 

 organisms indicate transition to Devonian or Old Red 

 times. But the rocks of Central Russia show marked 



