THE ORIGIN OF THE OLDEST FOSSILS, ETC. 287 



pelagic fauna, and while it is usually stated that the larvae of bottom 

 animals have retained the pelagic habit for the purpose of distri- 

 buting the species, it is more probable that it has been retained on 

 account of its comparative safety. 



These facts show that competition must have come quickly 

 after the establishment of the first fauna on the bottom, and that 

 it soon became very rigorous and led to severe selection and rapid 

 modification ; and we must also remember that life on the bottom 

 brought with it many new opportunities for divergent specialisation 

 and improvement. The increase in size which came with the 

 economy of energy increased the possibilities of variation, and led 

 to the natural selection of peculiarities which improved the efficacy 

 of the various parts of the body in their functions of relation to 

 each other, and this has been an important factor in the evolution 

 of complicated organisms. 



The new mode of life also permitted the acquisition of pro- 

 tective shells, hard-supporting skeletons, and other imperishable 

 parts, and it is therefore probable that the history of evolution in 

 later times gives no index as to the period which was required to 

 evolve from small, simple pelagic ancestors the oldest animals 

 which were likely to be preserved as fossils. 



Life on the bottom also introduced another important evolu- 

 tionary influence : competition between blood-relations. In those 

 animals which we know most intimately, divergent modification, 

 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 that is essential to their welfare. 



When a stock gives rise to two divergent branches, each 

 escapes competition with the other so far as they differ in structure 

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

 vantage, 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 comparison. 



