228 THE FOUNDATIONS OF ZOOLOGY 



Marine life is older than terrestrial life, and as all marine life 

 has shaped itself in relation to the pelagic food supply, this itself 

 is the only form of life which is independent, and it must there- 

 fore be the oldest. There must have been a long period in prime- 

 val times when there was a pelagic fauna and flora rich beyond 

 limit in individuals, but made up of only a few simple types. 

 During this time the pelagic ancestors of all the great groups of 

 animals were slowly evolved, as well as other forms which have 

 left no descendants. So long as life was restricted to the surface 

 no great or rapid advancement, through the influences which now 

 modify species, was possible, and we know of no other influences 

 which might have replaced them. We are therefore forced to be- 

 lieve that the differentiation and improvement of the primitive 

 flora and fauna were slow, and that, for a vast period of time, 

 life consisted of an innumerable multitude of minute and simple 

 pelagic organisms. During the time which it took to form the 

 thick beds of older sedimentary rocks the physical conditions of 

 the ocean gradually took their present form, and during a part, 

 at least, of this period the total amount of life in the ocean may 

 have been very nearly as great as it is now without leaving any 

 permanent record of its existence, for no . rapid advance took 

 place until the advantages of life on the bottom were discovered. 



We must not think of the populating of the bottom as a 

 physical problem, but as discovery and colonization, very much 

 like the colonization of islands. Physical conditions for a long 

 time made it impossible, but its initiation was the result of bio- 

 logical influences, and there is no reason why its starting-point 

 should necessarily be the point where the physical obstacles first 

 disappeared. It is useless to speculate upon the nature of the 

 physical obstacles; there is reason to think one of them, probably 

 an important one, was the deficiency of oxygen in deep water. 



Whatever their character may have been, they were all, no 

 doubt, of such a nature that they first disappeared in the shallow 

 water around the coast, but it is not probable that bottom life 

 was first established in shallow water, or before the physical con- 

 ditions had become favorable at considerable depths. 



The sediment near the shore is destructive to most surface 

 animals, and recent explorations have shown that a stratum of 



