rpillar they saw, but they naturally first
saw those that were least like the background on which they lived.
The only caterpillar which was effectively hidden from his enemy was
the one that was indistinguishable on the leaf. If it escaped in this
way, the probabilities are that it would produce young which would be
at least a little more likely to be green in color than the progeny of
its darker-colored brothers and sisters. By this continued process the
birds steadily weed out the darker-colored specimens. There would
result, in the course of time, a race of caterpillars, whose ancestors
for so many generations back had been light green in color, that there
is little likelihood of any of the older and darker forms turning up
again. In the course of time all dark tendencies will have disappeared
from the family and practically all of the group will be light green.
Any sport or variation in the shape of greater conspicuousness would
fall a quick prey to the enemy and its line be cut off forever.

The same sort of activity has resulted in the peculiar green color of
the katydid. This creature lives chiefly upon the leaves of trees and
shrubs. This insect is so large that, even though it is leaflike in
color, it might still be conspicuous. As a result those katydids whose
wings were most like leaves in form were least likely to be picked up
by the passing bird. This sort of protective appearance is intensified
by exactly the same means as that which brought about protective
coloration. The katydid least leaflike in appearance was eaten first.
Thus those most leaflike remain until the last, and are most likely to
produce young. Again, it was not the fact that they lived among leaves
which made them look leaflike, but it is because they look like leaves
that they escaped being devoured.

The katydid has materially assisted in its own preservation by being
active chiefly at night. In the daytime it keeps comparatively quiet.
Thus seated upon a twig, especially if hidden among the leaves, it is
almost unnoticeable. At night, however, it moves about more freely,
seeking its food and eventually its mate. At such times it becomes
distinctly more conspicuous because its wings are steadily fluttering.
The hind wings are filmy and are very light green. The creature looks
most ghost-like as it flies through the evening air.

A very similar history lies back of the coloring of the ordinary toad.
Though descended from the frog, and