THE ORIGIN OF THE O ID EST FOSSILS, ETC. 46 7 



Embryology also gives us good ground for believing that all 

 these animals are still more remotely descended from minute 

 and simple pelagic ancestors, and that the history of all the 

 highly organized inhabitants of the water has followed a round- 

 about path from the surface to the bottom and then back into 

 the water. 



When this fact is seen in all its bearings and its full signifi- 

 cance is grasped, it is certainly one of the most notable and 

 instructive features of evolution. 



The food-supply of marine animals consists of a few species 

 of microscopic organisms which are inexhaustible and the only 

 source of food for all the inhabitants of the ocean. The supply 

 is primeval as well as inexhaustible, and all the life of the ocean 

 has gradually taken shape in direct dependence upon it. 



In view of these facts we cannot but be profoundly impressed 

 by the thought that all the highly organized marine animals are 

 products of the bottom or the shore or the land, and that while 

 the largest animals on earth are pelagic the few which are primi- 

 tively pelagic are small and simple. 



The reason is obvious. The conditions of life at the surface 

 are so easv that there is little fierce competition, and the 

 inorganic environment is so simple that there is little chance for 

 diversity of habits. 



The growth of terrestrial plants is limited by the scarcity of 

 food, but there is no such limit to the growth of pelagic plants 

 or the animals which feed on them, and while the balance of life 

 is no doubt adjusted by competition for food this is never very 

 fierce, even at the present dav, when the ocean swarms with 

 highly organized wanderers from the bottom and the shore. 

 Even now the destruction or escape of a microscopic pelagic 

 organism depends upon the accidental proximity or remoteness 

 of an enemy rather than upon defense or protection, and survival 

 is determined by space relations rather than a struggle for 

 existence. 



The abundance of food is shown by the ease with which 

 wanderers from the land, like sea birds, find places for them- 



