RIVER OF LIFE 49 



but there must have been a long period of evolution before 

 this became cased in bony vertebrae. 



Before the vertebrate evolution could follow its full course 

 it was necessary for the plant life of the ocean to set the stage 

 by blazing a new trail away from the waters of the ocean. 

 Although the primeval seaweeds, being soft and usually with- 

 out skeletons, have only rarely left their story in the sediments, 

 we can imagine that they gradually became accustomed to the 

 intermittent drying out of the intertidal zone or of the fresh 

 waters into which they may have spread. Some of the more 

 vigorous, when their spores were washed up above the water 

 line, became adapted to life completely out of the water. This 

 was a momentous step forward. Without land plants there 

 could be no land animals. With gradual increase in complexity 

 and abundance they provided the necessary food, so that in- 

 sects, and finally vertebrates, were able to follow them in a 

 slow progress to the land. 



The long-drawn-out migration of the vertebrates started 

 when the worm-like prototype adopted swimming habits and 

 became a primitive fish, with a bony shield covering its head 

 but without jaws, and in some ways much more like its de- 

 generate descendants, the lampreys, than any other fish today. 

 These early jawless fish, the ostracoderms, began life as far back 

 as the Ordovician period. It is believed by some that their 

 start was made in fresh water or in the brackish lagoons of a 

 low swampy coast, so that the chemical control system that 

 made possible the move to land was already there, stabilizing 

 their bloodstreams in the chemical image of the sea from 

 which the wormlike ancestor had originally started. 



Whether the start was in the seas or in fresh water streams 

 we cannot be sure; but for a while evolution returned to salt 

 water, where it produced the primeval sharks, once the domi- 

 nant fishes of the ocean. These were followed by three im- 

 portant groups of fishes: the armoured fishes, represented today 



