They transformed the annelid parapodia into legs and
developed wings. They attained life in the air. They devoted the
muscles of the body largely to the extremities and gained swift
locomotion. They have a fair circulatory and an excellent
respiratory system. Best of all, they developed a head and a brain
by fusing the three anterior ganglia of the body. The insect could
and does think. Such a structure ought to lead to great and high
results. But actually their possibilities were very limited. They
have not progressed markedly during the last geological period.
Their external skeleton was easily attained and brought speedy
advantages, which for a time placed them far above all competitors.
But it limited their size and length of life and opportunities, and
finally their intelligence. They remained largely the slaves of
instinct. They followed an attractive and exceedingly promising
path, but it led to the bottom of a cliff, not to the summit.
The mollusks, clams, and snails took an easier, down-hill road. They
formed a shell, and it developed large enough to cover them. It
hampered and almost destroyed locomotion and reduced nerve to a
minimum. But nerves are nothing but a nuisance anyhow. And why
should they move? Food was plenty down in the mud, and if danger
threatened, they withdrew into the shell.
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