iVATURAL selection; AND THE ANTIQUITY OF LIFE 229 



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

 velopment 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 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 estab- 

 lished themselves in shallow water, where the food supply is both 

 scanty and mixed with sediment ; nor is it probable that their es- 

 tablishment was delayed until the great depths had become favor- 

 able 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 paleontological 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 

 around the continents may have persisted and given rise to modern 

 animals. However this may be, we must regard this deep zone 

 as the birthplace of the fauna which has survived, as the ancestral 

 home of all the improved metazoa. 



The effect of life upon the bottom is more interesting than 

 the place where it began, and we are now to consider its influence 

 upon animals, all whose ancestors and competitors and enemies 

 had previously been pelagic. The cold, dark, silent, quiet depths 

 of the sea are monotonous compared with the land, but they intro- 

 duced many new factors into the course of organic evolution. 



It is doubtful whether the animals which first settled on the 

 bottom secured any more food than the floating ones, but they 

 undoubtedly obtained it with less effort, and were able to devote 

 their superfluous energy to growth and to multiplication, and thus 



