168 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



advancement took place until the advantages of a life on the bottom 

 were discovered. 



The Discovery of the Bottom, and its Effect on Evolution. 



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

 problem, but as colonization, very much like the colonization of oceanic 

 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 the starting-point should be the point where 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 estab- 

 lished in shallow water, or before the physical conditions had become 

 favorable at considerable depths. 



The sediment near the shore is destructive to most pelagic animals, 

 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 sediment ; nor is it probable 

 that their establishment on the bottom wag 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 palaeontological 

 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 afterwards swept away, like clouds before 

 the wind, by geological changes, and that the bottom fauna which we 

 know was not the first. 



