THE ORIGIN OF THE OLDEST FOSSIIS, ETC. 475 



of distributing 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 

 specialization and improvement. The increase in size which 

 came with 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 import- 

 ant 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 struc- 

 ture 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 



