But the plan or fundamental scheme of these eyes is
correct and capable of indefinitely greater development than the
organs of touch or smell, perhaps greater even than the otolith
vesicle.
And the reflex influence of the eye on the brain was the greatest
advantage of all. Hitherto with feeble muscles and sense-organs it
has hardly paid the animal to devote more material to building a
larger brain. It was better to build more muscle. But now with
stronger muscles at its command, and better sense-organs to report
to it, every grain of added brain material is beginning to be worth
ten devoted to muscle. The muscular system will still continue to
develop, but the brain has begun an almost endless march of
progress. The eye becomes of continually increasing advantage and
importance because it has a capable brain to use it; and brain is a
more and more profitable investment, because it is served by an
ever-improving eye.
[Illustration: 9. MYRMELEO FORMICARIUS. ANT-LION. HERTWIG, FROM
SCHMARDA.
1, adult; 2, larva; 3, cocoon.]
The annelid had hit upon a most advantageous line of development,
which led ultimately to the insect. The study of the insect will
show us clearly the advantages and defects of the annelid plan.
First of all, the insect, like the mollusk, has an external
skeleton.
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