THE ORIGIN OF THE OLDEST FOSSILS. 367 



by a nutritive fluid, it is advantageous for the new plant-cells which 

 are ibrmed 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 i)elagic i^lants, 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 

 ]»rimitive 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 uj)on this supi)ly of 

 jnicroscopic 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 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 j)aleontology 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. 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 liistory of all the highly organized 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 micro- 

 scopic organisms which are inexhaustible and the only source of food 

 for all the inhabitants of the ocean. The supply is primeval as well as 

 inexhaustible, and all the life of the ocean has gradually taken shape 

 in direct dependence upon it. In view of these t^icts we can not but 

 be profoundly impressed by the thought that all the highly organized 

 marine animals are products of the bottom or the shore or the land, 

 and that while the largest animals on earth are pelagic the few which 

 are primitively pelagic are small and simple. The reason is obvious. 

 The conditions of life at the surface are so easy that there is little 

 fierce competition, and the inorganic environment is so simple that 

 there is little chance for diversity of habits. 



The growth of terrestrial plants is limited by the scarcity of food, 

 but there is no such limit to the growth of pelagic plants or the animals 



