The line of development which the insect followed was comparatively
easy and its course probably rapid. Certain crustacea, aquatic
arthropoda, are among the oldest fossils, and it is possible that
insects lived on the land before the first fish swam in the sea.
They had fine structure and powers; and yet during the later
geologic periods they have scarcely advanced a step, and are now
apparently at a standstill. They ran splendidly for a time, and then
fell out of the race. What hindered and stopped them?

One vital defect in their whole plan of organization is evident. The
external skeleton is admirably suited to animals of small size, but
only to these. In larger animals living on land it would have to be
made so heavy as to be unwieldy and no longer economical. Their mode
of breathing also is fitted only for animals of small size having
an external skeleton. Whatever may be our explanation the fact
remains that insects are always small. This is in itself a
disadvantage. Very small animals cannot keep up a constant high
temperature unless the surrounding air is warm, for their radiating
surface is too large in comparison with their heat-producing mass.
At the first approach of even cool weather they become chilled and
sluggish, and must hibernate or die. They are conformed to but a
limited range of environment in temperature.

But small size is, as a rule, accompanied by an even greater
disadvantage. It seems to be almost always correlated with short
life. Why this is so, or how, we do not know. There are exceptions;
a crow lives as long as a man; or would, if allowed to. But, as a
rule, the length of an animal's days is roughly proportional to the
size of its body. And the insect is, as a rule, very short-lived. It
lives for a few days or weeks, or even months, but rarely outlasts