The internal skeleton had to become far more highly
developed before its great advantages, and freedom from
disadvantages, became apparent. The mollusk and crab were working a
mine rich in surface deposits although soon exhausted. The
vertebrate lead was poor at the surface, and only later showed its
inexhaustible richness. It looked as if the vertebrate were making a
very poor speculation.
Whether this explanation be true or not, a glance at a chart,
showing the geological succession of occurrence of the different
kingdoms, proves that in the oldest palaeozoic periods there were
well-developed cuttlefish and crabs before there were any
vertebrates worthy of the name. If any were present, their skeleton
was purely cartilaginous and not preserved.
I think we may go farther, although in this latter consideration we
may very possibly be mistaken. We have already seen that the
progress made by any animal may be measured more or less accurately
by the length of time during which its ancestors maintained a
swimming life. The ancestors of the coelenterates settled to the
bottom first. Then successively those of flatworms, mollusks,
annelids, and crabs. All this time the ancestors of vertebrates were
swimming in the water above. Food was probably more abundant,
certainly more easily and economically obtained by a creeping life,
on the bottom.
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