NATURAL SELECTION, AND THE ANTIQUITY OF LIFE 225 



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 sur- 

 face demands diversity of structure and habit, and 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 distri- 

 bution of mineral food in solution, the area which is available for 

 plants, the volume of sunlight and the uniformity of the tempera- 

 ture 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 

 toward 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 compensating advantage by seizing upon a more 

 favorable habitat. The pelagic plants have retained their primi- 

 tive simplicity, and the most distinctive peculiarity of the micro- 

 scopic 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 sur- 

 face 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 seashore, 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 evidence from 

 embryology, from comparative anatomy, and from paleontology 

 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 

 Q 



