CRITICAL EPISODE IN EVOLUTION, 
127 
the largest animals on earth are pelagic the few which are prim¬ 
itively pelagic are small and simple. The reason is obvious. The 
conditions of life at the surface are so easy that there is little 
fierce competition, and the inorganic environment is so simple 
that there is little chance for diversity of habits. 
Growth of terrestrial plants is limited by the scarcity of food, 
but there is no such limit to the growth of pelagic plants or ani¬ 
mals which feed upon them; and while the balance of life is no 
doubt adjusted by competition for food this is never very fierce, 
even at the present day, when the ocean swarms with highly or¬ 
ganized 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 the defense or protection, and survival is determined 
by space relations rather than a struggle for existence. 
Easy character of pelagic life is shown by the fact that the 
larvae of innumerable animals from the bottom and the shore 
have retained the pelagic habit; and I shall soon give reasons 
for believing that the larva of a shore animal is safer at sea than 
near the land. 
It is not probable that bottom life was first established in shal¬ 
low water, or before the physical conditions had become favorable 
at considerable depths. The sediment near the shore is destruc¬ 
tive 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 layer. 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 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. 
Belts around elevated areas far enough from the shore to 
be free from sediment, and deep enough to permit the pelagic 
fauna to reach its full development above them, are most fav¬ 
orable spots, and paleontological evidence shows that they were 
seized upon very early in the history of the life on the bottom. 
