THE ORIGIN OF THE OLDEST FOSSILS, ETC. 279 



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 organisation, 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 

 favourable habitat. The pelagic plants have retained their prim- 

 itive 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 organised 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 evidence from 

 embryology, from comparative anatomy and from palaeontology, 

 all bears in the same direction, and proves that every large and 

 highly organised 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. 



Embryology also gives us good ground for believing that all 

 these animals are still more remotely descended from minute and 

 simple pelagic ancestors, and that the history of all the highly 

 organised inhabitants of the water has followed a roundabout path 

 from the surface to the bottom, and then back into the water. 

 When this fact is seen in all its bearings, and its full significance 

 is grasped, it is certainly one of the most notable and instructive 

 features of evolution. 



The food-supply of marine animals consists of a few species 

 of microscopic organisms which are inexhaustible, and the only 



