470 THE JOURNAL OF GEOLOGY. 



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 

 water of very great thickness is necessary for the complete 

 development 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 flourished as far down as light pene- 

 trates, 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 

 is both scanty and mixed with sediment ; nor is it probable that 

 their establishment was delayed until the great depths had 

 become favorable to life. 



The belts around elevated areas far enough from shore to be 

 free from sediment, and deep enough to permit the pelagic fauna 

 to reach its full development above them, are the most favorable 

 spots, and palaeontological evidence shows that they were seized 

 upon very early in the history of life on the bottom. 



It is probable that colony after colony was established on the 

 bottom and afterwards swept away by geological change like a 

 cloud before the wind, and that the bottom fauna, which we know 

 was not the first. Colonies which started in shallow water, were 

 exposed to accidents from which those in great depths were free ; 

 and in view of our knowledge of the permanency of the sea-floor 

 and of the broad outlines of the continents, it is not impossible 

 that the first fauna which became established in the deep zone 



