148 JOHNS HOPKINS UNIVEESITY MORPHOLOGICAL MONOGRAPHS. 



forms and their importance in the economy of nature show that the 

 organic world has gradually shaped itself around 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. 



The pelagic plant-life of the ocean has retained its primitive sim- 

 plicity on account of the very favorable character of its environment, 

 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 supplied 

 as the rains dissolve them ; limited space brings crowding and com- 

 petition 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 compli- 

 cated 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 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 oppor- 

 tunity 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 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, Globigerina and the Radiolarians, and 

 some five or six unicellular plants. 



Of these only two, the Radiolarians and the Diatomes, show any 

 great diversity of species, and while the Radiolarians are so diversified 

 that the Challenger collection alone furnished more than four thousand 

 species, this variety does not obscure the primitive simplicity of the type, 



