CRITICAL EPISODE IN EVOLUTION 
125 
when it was represented by minute and simple floating ancestors. 
Broadly stated, the history of each great line has been like that 
of the Echinoderms and the Brachiopods. The oldest Pteropod 
or Lamellibranch or Echinoderm or Crustacean or Vertebrate 
which we know from fossils exhibits its own type struc¬ 
ture with perfect distinctness, and later influences have done no 
more than to expand and diversify the type, while anatomy fails 
to guide us back to the point where these various lines met one 
another in a common source, although it forces us to believe that 
the common source once had an individual existence. Embryology 
teaches that each line once had its own representative at the sur¬ 
face of the ocean, and that the early stages in its evolution have 
passed away and left no record in the rocks. 
Modern microscopic research has shown that the simple uni¬ 
cellular plants, and the Globigerinae and Radiolarians which feed 
upon them, are so abundant and prolific that they meet all demands 
and supply the food of all the animals of the ocean. This is the 
fundamental conception of marine biology. The basis of all life 
in the modern ocean is found in the micro-organisms of the sur¬ 
face. This is not all. The simplicity and abundance of the 
microscopic forms and their importance in the economy of nature 
show that the organic world has gradually taken shape around 
them as a center, or starting point, and has been controlled by 
them. They are not only the fundamental food-supply, but the 
primeval supply, which has determined the whole course of the 
evolution of marine life. Pelagic plant-life of the ocean has 
retained its primitive simplicity on account of the very favorable 
character of its environment, and the higher rank of littoral veg¬ 
etation and that of the land is the result of hardship. 
On land the mineral elements of plant-food are slowly supplied, 
as the rains dissolve them; limited space brings crowding and 
competition for this scanty supply; growth is arrested for a 
great part of each year by drought or cold; the diversity of the 
earth’^ surface demands diversity of structure and habit, and 
the great size and complicated structure of territorial plants are 
adaptations to these conditions of hardship. 
At the surface of the ocean the abundance and uniform distri¬ 
bution of mineral food in solution; the area which is available 
for plants; the volume of sunlight and the uniformity of temper¬ 
ature are all favorable to the growth of plants; and as each plant 
