234 THE FOUNDATIONS OF ZOOLOGY 



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 that is essential to their welfare. 



When a stock gives rise to two divergent branches, each es- 

 capes competition with the other so far as they differ in struct- 

 ure or habits, while the parent stock competing with both at a 

 disadvantage is exterminated. 



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

 branching tree-like genealogy, 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 sought in the records of the past or reconstructed by compari- 

 son. Even at the present day things are somewhat different in 

 the open ocean, and they must have been very different in the 

 primitive ocean, for a pelagic animal has no fixed home, one local- 

 ity is like another, and the competitors and enemies of each indi- 

 vidual 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 ancient forms, such as the sharks, 

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

 The radiolarians and medusae and siphonophores furnish many 

 well-known illustrations of this feature of pelagic life. 



No naturalist is surprised to find in the South Pacific or in the 

 Indian Ocean a Salpa or a pelagic crustacean or a surface fish or 

 a whale which was previously known only from the North At- 

 lantic, 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. 



One of the first results of life on the bottom was to increase 

 asexual multiplication and to lengthen the time during which 

 buds remain united to and nourished by their parents, and to 

 crowd individuals of the same species together and to cause com- 

 petition between relations. We have in this and other obvious 

 peculiarities of life on the bottom a sufficient explanation of the 

 fact that since the first establishment of the bottom fauna, evolu- 

 tion has resulted in the elaboration and divergent specialization of 



