in succession as the story proceeds.

Thus far, then, the insects of the Coal-forest are in entire harmony
with the principle of evolution, but when we try to trace their origin
and earlier relations our task is beset with difficulties. It goes
without saying that such delicate frames as those of the earlier insects
had very little chance of being preserved in the rocks until the special
conditions of the forest-age set in. We are, therefore, quite prepared
to hear that the geologist cannot give us the slenderest information.
He finds the wing of what he calls "the primitive bug" (Protocimex),
an Hemipterous insect, in the later Ordovician, and the wing of a
"primitive cockroach" (Palaeoblattina) in the Silurian. From these we
can merely conclude that insects were already numerous and varied. But
we have already, in similar difficulties, received assistance from the
science of zoology, and we now obtain from that science a most important
clue to the evolution of the insect.

In South America, South Africa, and Australasia, which were at one
time connected by a great southern continent, we find a little
caterpillar-like creature which the zoologist regards with profound
interest. It is so curious that he has been obliged to create a special
class for it alone--a distinction which will be appreciated when I
mention that the neighbouring class of the insects contains more than a
quarter of a million living species. This valuable little animal, with
its tiny head, round, elongated body, and many pairs of caterpillar-like
legs, was until a few decades ago regarded as an Annelid (like the
earth-worm). It has, in point of fact, the peculiar kidney-structures
(nephridia) and other features of the Annelid, but a closer study
discovered in it a character that separated it far from any worm-group.
It was found to breathe the air by means of tracheae (little tubes
running inward from the surface of the body), as the myriapods, spiders,
and insects do. It was, in other words, "a kind of half-way animal
between the Arthropods and the Annelids" ("Cambridge Natural History,"
iv, p. 5), a surviving kink in the lost chain of the ancestry of the
insect. Through millions of years it has preserved a primitive frame
that really belongs to the Cambrian, if not an earlier, age. It is one
of the most instructive "living fossils" in the museum of nature.

Peripatus, as the little animal is called, points very clearly to an
Annelid ancestor of