CRITICAL EPISODE IN EVOLUTION 
129 
to force themselves in among the old ones and colonization soon 
came to an end. 
Nothing could illustrate the fierceness of the struggle for food 
among the animals on a crowded sea-bottom more vividly than 
the emptiness of the water in coral sounds where the bottom is 
, practically one enormous mouth. The only larvae which have 
much chance to establish themselves for life are those which are 
so fortunate as to be swept out into the open ocean where they 
can complete their larval life under the milder competition of 
the pelagic fauna, and while it is usually stated that the larvae 
of bottom animals have retained the pelagic habit for the purpose 
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 also remember that life on the bottom 
brought with it many new opportunities for divergent specilization 
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 proved the efficacy of the 
various parts of the body in their functions of relation to one 
another, 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 evolutionary influence — com¬ 
petition 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 compet¬ 
itors of each animal are its closest allies, which, having the same 
habits, living upon the same food, and avoiding the 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 
