W. K. BROOKS ON THE GENUS SALPA. 173 



emptiness of the water in coral sounds. The only larvae which have 

 much chance of establishing themselves for life are those which are so 

 fortunate as to be swept out into the open ocean, where they can com- 

 plete their larval life under the milder competition of the pelagic fauna, 

 and while it is usually stated that the pelagic habit has been retained by 

 the larvae of bottom animals for the purpose of distributing the species, 

 it is more probable that it has been retained on account of its com- 

 parative safety. 



There can be no doubt, in view of these facts, that competition came 

 swiftly after the establishment of the first bottom fauna, and that it soon 

 became very rigorous and led to rapid evolution ; and we must also 

 remember that life on the bottom introduced many new opportunities 

 for divergent modification and for the perfecting of animals. 



The increase in size, which came with the economy of energy, 

 increased the possibilities of variation, and led to the natural selection of 

 those peculiarities which improved the efficiency of various parts of the 

 body in their functions of relations to each other, and this has certainly 

 been an important factor in the evolution of complicated organisms. 



The new mode of life also permitted the acquisition of protective 

 shells, hard supporting skeletons, and other imperishable structures, and 

 it is therefore probable that the history of evolution 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 modification, with the extinc- 

 tion of connecting forms, results from the fact that the fiercest com- 

 petitors 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 essen- 

 tials 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 

 disadvantage, is exterminated. 



Among the animals which we know 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. 



