118 THE MICROSCOPE. 



val supply, which has determined the whole course of the evo- 

 lution of marine life. 



The pelagic plant life of the ocean has retained its primitive 

 simplicity on account of the very favorable character of its en- 

 vironment, and the higher rank of the littoral vegetation and that 

 of the land is the result of hardship. 



On the land the mineral elements of plant food are slowly sup- 

 plied 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 distribution of mineral food in solution; the area which 

 is available for plants; the volume of sunlight and the uniform- 

 ity 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 ag- 

 gregation, the first step towards higher evolution, is therefore dis- 

 advantageous to the pelagic plants and as the environment is so 

 homgeoneous at the surface of the ocean that there is little op- 

 portunity for an aggregation of cells to gain a compensating ad 

 vantage by seizing upon a more favorable habitat, the pelagic 

 plants have retained their primitive simplicity- 



The list of pelagic micro-organisms is a long one, but a few 

 forms are so predominant 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 about five unicellular plants. 



Of these only two, the radiolarians and the diatoms, show any 

 great diversity of species, and while the radiolarians are so di- 

 versified that the Challenger collection alone furnished more 

 than four thousand species, this variety does not obscure the 

 primitive simplicity of the type, and the most distinctive pecu- 

 liarity 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. 



