154 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



In its descent from an inhabitant of the bottom and in its secondary 

 adaptation to a pelagic life, its history resembles that of all the highly 

 organized pelagic animals. 



Embryology also gives us good ground for believing that Salpa 

 follows the analogy of all the metazoa in its still more remote descent 

 from a small and simple pelagic ancestor, and there is good ground for 

 believing that the earliest metazoa were all pelagic, and that they were 

 represented at a very early period in the history of life by floating or 

 swimming animals of minute size and simple structure. We may see 

 in the free larval forms of many marine metazoa, such as the tornaria 

 of balanoglossus, the swimming echinoderm larva, the ascidian tadpole, 

 the floating ciliated larvae of annelids, brachiopods and molluscs, in the 

 coelenterate planula, and, as I believe, in the crustacean nauplius, traces 

 of this primitive mode of life ; often obscured or complicated by more 

 recent adaptation and sometimes almost obliterated by secondary 

 changes. 



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

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

 features of the history of evolution. 



The food-supply of the- ocean consists of a few species of unicellular 

 microscopic plants, and of a few simple protozoa which feed upon them. 

 This supply is inexhaustible, and it is the only source of food for all the 

 inhabitants of the ocean, except a few which live upon floating sargas- 

 sum and the -littoral algae, and the drainage from the land. 



Many marine animals are adapted for direct subsistence upon these 

 organisms, and some' of them, like Salpa, are universally distributed and 

 are found in enormous numbers in all parts of the ocean. 



The food-supply is not only inexhaustible, it is also primeval, and all 

 the life of the ocean has gradually taken shape in direct dependence 

 upon it during the history of its evolution. 



In view of all these facts we cannot but be profoundly impressed by 



the thought that all the highly organized marine animals are products 



'of the bottom, or of the shore, or of the land, and that while the largest 



animals on earth are pelagic, the few which are primitively pelagic are 



very small and very simple. 



The reason is obvious. The conditions of pelagic life are so easy 

 that there is no 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, 



