282 THE ORIGIN OF THE OLDEST FOSSILS, ETC. 



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 believe 

 that the differentiation and improvement of the primitive flora and 

 fauna was slow, and that, for a vast period of time, life consisted 

 of an innumerable multitude of minute and simple pelagic organ- 

 isms. 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 phy- 

 sical problem, but as discovery and colonisation, very much like 

 the colonisation of islands. Physical conditions for a long time 

 made it impossible, but its initiation was the result of biological 

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

 necessarily be the point where the physical obstacles first disap- 

 peared. It is useless to speculate upon the nature of the physical 

 obstacles ; there is reason to think one of them, probably an im- 

 portant 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 conditions had 

 become favourable at considerable depths. 



1'he sediment near the shore is destructive to most surface 

 animals, and recent explorations have shown that a stratum of 

 water of very great thickness is necessary for the complete devel- 

 opment of the floating microscopic fauna and flora, and it is a 

 mistake to picture them as confined to a thin surface stratum. 

 Pelagic plants probably flourish as far down as light penetrates, 

 and pelagic animals are abundant at very great depths. As the 

 earliest bottom animals must have depended directly upon the 

 floating organisms for food, it is not probable that they first 

 established themselves in shallow water, where the food supply 



