92 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



On the 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's surface demands diversity of structure and habit, and the great size and 

 complicated structure of terrestrial plants are adaptations to these conditions of 

 hardship. 



The conditions of the surface of the ocean; the abundance and uniform distribu- 

 tion of mineral food in solution; the area which is available for plants; the volume of 

 sunlight and the uniformity of the temperature, are all favorable to the growth of 

 plants, and as each plant is bathed on all sides by a nutritive fluid, it is advantageous 

 for the new plant cells, which are formed by cell multiplication, to separate from each 

 other as soon as possible in order to expose the whole of their surface to the water. 

 Cell aggregation, the first step towards higher evolution, is therefore disadvantageous 

 to the pelagic plants, and as the environment is so homogeneous at the surface of the 

 ocean that there is little opportunity for an aggregation of cells to gain a compensating 

 advantage by seizing upon a more favorable habitat, the pelagic plants have retained 

 their primitive simplicity. 



The list of pelagic microorganisms is a long one, but a few forms are so predomi- 

 nant that the others have little significance at the present day in comparison, and we 

 may regard the great primary food-supply as made up of two simple protozoa, globig- 

 erina and the radiolarians, and some five or six unicellular plants. 



Of these only two, the radiolarians and the diatoms, show any great diversity of 

 species; and while the radiolarians are so diversified that the Challenger collection 

 alone furnished more than 4,000 species, this variety does not obscure the primitive 

 simplicity of the type, and the most distinctive peculiarity of the microscopic food 

 supply of the ocean is the very small number of the forms which go to make up the 

 enormous mass of individuals. 



