126 
CRITICAL EPISODE IN EVOLUTION 
is bathed on all sides by nutritive fluid, it is advantageous for 
the new plant-cells, which are formed by cell-multiplication, to 
separate from one another as soon as possible, in order to expose 
the whole of their surface to the water. Cell-aggregation, the 
first step toward higher organization, is therefore disadvantageous 
to the pelagic plants, and as the environment at the surface of the 
ocean is so monotonous, there is little opportunity for an aggre¬ 
gation of cells to gain any compensating advantage by seizing upon 
a more favorable habitat. The pelagic plants have retained their 
primitive simplicity; and the most distinctive peculiarity of the 
microscopic food-supply of the ocean is the very small number of 
forms which make up the enormous mass of individuals. 
All the animals of the ocean are dependent upon this supply 
of microscopic food, and many of them are adapted for preying 
upon it directly, but a review of the animal kingdom will show 
that no highly organized animal has ever been evolved at the 
surface of the ocean although all depend upon the food-supply 
of the surface. 
Animals which now find their home in the open waters of the 
ocean are, almost without exception, descendants of forms which 
once lived upon or near the bottom, or along the sea-shore, or 
upon the land; and all the exceptions are simple animals of 
minute size. A review of the whole animal kingdom would take 
more space than we could spare, but it would show that the 
evidence from embryology, from comparative anatomy, and from 
paleontology, all bears in the same direction and proves that 
every large and highly organized animal in the open ocean is 
descended from ancestors whose home was not open water but 
solid ground, either on the bottom or on the shore. 
Embryology also gives us grounds 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 roundabout path from 
the surface to the bottom and then back into the water. When 
this fact is seen in all of its bearings and its full significance is 
grasped, it is certainly one of the most notable and instructive 
features of evolution. 
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 
