160 W. K. BROOKS. 



The Discovery of the Bottom, and its 

 Effect on Evolution. 



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

 cal problem, but as colonization, very much like the colonization 

 of oceanic islands. Physical conditions for a lono; time made it 

 impossible, but its initiation was the result of biological influences, 

 and there is no reason why the starting-point should be the point 

 whfere the physical obstacles were first removed. It is useless to 

 speculate upon the character of the physical obstacles ; there is 

 reason to believe that one of them, probably a very 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 most 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 favorable at considerable depths. 



The sediment near the shore is destructive to most pelagic ani- 

 mals, and recent explorations have shown that a stratum of water 

 of very great thickness is necessary for the complete development 

 of the pelagic flora and fauna. It is a mistake to picture pelagic 

 life as confined to a thin surface stratum. Pelagic plants probably 

 flourish as far down as the 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 is not only scanty in amount but 

 also mixed with sediaient; nor is it probably that their estab- 

 lishment on the bottom was delayed until the great depths had 

 become favorable to life. 



The belts around elevated areas which are far enough from shore 

 to be free from sediment and to have above them a sufficient depth 

 of water to permit the pelagic fauna to reach its full development, 

 are the most favorable spots, and I shall soon show that there is 

 palseontological evidence which indicates that they were seized 

 upon very early in the history of bottom life. It is very probable 

 that colony after colony was established on the bottom, and after- 



