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 or annelid, but immensely increased in
number, modified, and improved in certain very important
particulars. The muscles in simplest forms are composed of heavy