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 the year.  It has time to learn but little by experience.  The same experience must be passed, the same emergency arise and be met, over and over again during the lifetime of the same individual if the animal is to learn thereby.  And intelligence is based upon experience.  Hence insects can and do possess but a low grade of intelligence.  But instinct is in many cases habit fixed by heredity and improved by selection.  The rapid recurrence of successive generations was exceedingly favorable to the development of instincts, but very unfavorable to intelligence.  Insects are instinctive, the highest vertebrates intelligent.  The future can never belong to a tiny animal governed by instincts.  Mollusks and insects have both failed to reach the goal; another plan of structure than theirs must be sought if the animal kingdom is to have a future.

The future belonged to the vertebrate.  To begin with less characteristic organs the digestive system is much like that of the annelid or schematic worm, but with greatly increased glandular and absorptive surfaces.  The present mouth of nearly all vertebrates is probably not primitive.  It is almost certainly one of the gill-slits of some old ancestor of fish, such as now are used to discharge the water which is used for respiration.  The jaws are modified branchial arches or the cartilaginous or bony rods which in our present fish support the fringe of gills.  These have formed a pair of exceedingly effective and powerful jaws.  The reproductive system holds still to the old type and shows little if any improvement.  The excretory organs, kidneys, are composed primitively of nephridial tubes like those of the schematic worm