466 THE JOURNAL OF GEOLOGY. 



the great size and complicated structure of terrestrial plants are 

 adaptations to these conditions of hardship. 



At the surface of the ocean the abundance and uniform dis- 

 tribution of mineral food in solution ; the area which is avail- 

 able 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 advan- 

 tageous for the new plant-cells which are formed by cell-multipli- 

 cation, to separate from each other as soon as possible, in order 

 to expose the whole of their surface to the water. Cell-aggre- 

 gation, the first step towards 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 aggregation of cells to gain any compensa- 

 ting 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. 



The animals which now find their home in the open waters 

 of the ocean are, almost without exception, descendants of forms 

 which 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 can spare, but it would show that the evi- 

 dence from embryology, from comparative anatomy and from 

 palaeontology, 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. 



