166 W. K. BROOKS. 



tion in later times gives us no index as to the time which was 

 required to evolve, from pelagic ancestors, the oldest animals 

 which were likely to be preserved as fossils. 



Life on the bottom also introduced another most important 

 influence in evolution — competition between blood relations. In 

 the animals which we know most intimately, divergent modifica- 

 tion, with the extinction of connecting forms, results from the fact 

 that the fiercest competitors of each animal are its closest allies, 

 which, having the same habits, living upon the same food, and 

 avoiding enemies in the same way, are constantly striving to hold 

 exclusive possession of all the essentials to their life. When a 

 stock gives rise to two divergent branches, each of them escapes 

 competition with the other, so far as they differ in structure and 

 habits, while the parent stock, competing with both at a disadvan- 

 tage, is exterminated. 



Among the animals which we knoAV best, evolution leads to a 

 branching tree-like phylogeny with the topmost twigs represented 

 by living animals, while the rest of the tree is buried in the dead 

 past. The connecting form between two species must, therefore, 

 be constructed in imagination or sought in the records of the past. 



Even at the present day things are somewhat diiferent in the 

 open ocean, and they must have been very diiferent in the primi- 

 tive ocean, for a pelagic animal has no fixed home, one locality is 

 like another, and the competitors and enemies of each individual 

 are determined, in great part, by accidents. We accordingly 

 find, even now, that the evolution of pelagic animals is often 

 linear instead of divergent, and the early steps in the series often 

 live on side by side with the later and more evolved forms. The 

 radiolarians and the medusse and the siphonophores furnish many 

 well-known illustrations of this feature of pelagic life. 



No one is much surprised to find in the South Pacific or in the 

 Indian Ocean a salpa, or a pelagic crustacean, or a surface fish 

 which has previously been known only in the North Atlantic, and 

 the list of species of marine animals which are found in all seas is 

 a very long one. The fact that pelagic animals are so independent 

 of those laws of geographical distribution which limit land animals 

 is additional evidence of the easy character of the conditions of 

 pelagic life. We have seen that one of the first results of life 



