We
find a few stray wings in the Silurian, and a large number of
wings and fragments in the Devonian, but it is in the Coal-forest
that we find the first great expansion of insect life, with a
considerable development of myriapods, spiders, and scorpions.
Food was enormously abundant, and the insect at least had no
rival in the air, for neither bird nor flying reptile had yet
appeared. Hence we find the same generous growth as amongst the
Amphibia. Large primitive "may-flies" had wings four or five
inches long; great locust-like creatures had fat bodies sometimes
twenty inches in length, and soared on wings of remarkable
breadth, or crawled on their six long, sprawling legs. More than
a thousand species of insects, and nearly a hundred species of
spiders and fifty of myriapods, are found in the remains of the
Coal-forests.
From the evolutionary point of view these new classes are as
obscure in their origin, yet as manifestly undergoing evolution
when they do fully appear, as the earlier classes we have
considered. All are of a primitive and generalised character;
that is to say, characters which are to-day distributed among
widely different groups were then concentrated and mingled in one
common ancestor, out of which the later groups will develop.
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