But to the vertebrate these lowest depths of stagnation
and degeneration are, as a rule, impossible. From true fish upward
parasitism and sessile life are practically impossible. Here
stagnation and degeneration mean, as a rule, extinction. Of all the
relatives of vertebrates back to worms only the very aberrant lines
of amphioxus and of the tunicata remain. Of the rest not a single
survivor has yet been discovered. And yet what hosts of species must
have peopled the sea. The primitive round-mouthed fishes have
practically disappeared. The ganoids survive in a few species out of
thousands. The amphibia of the carboniferous and the next period and
the reptiles of the mesozoic have disappeared; only a few feeble
degenerate remnants persist. And this was necessarily so. Each
advancing form crowded hardest on those which occupied the same
place and sought the same food, that is, the members of the same
species. And the first to suffer from its competition were its own
brethren. Death, rarely commuted into life imprisonment, is the
verdict pronounced on all forms which will not advance. And does not
the same law of advance or extinction apply to man? What is the
record of successive civilizations but its verification?
Notice once more that as we ascend in the scale of development
natural selection selects more unsparingly and the path to life
narrows.
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