m to form until, in the octopus of a later age, they discard the
ancestral shell, and become the aristocrats of the Mollusc kingdom.

The last and most important line that led upward from the chaos of
Archaean worms is that of the Arthropods. Its early characteristic was
the acquisition of a chitinous coat over the body. Embryonic indications
show that this was at first a continuous shield, but a type arose in
which the coat broke into sections covering each segment of the body,
giving greater freedom of movement. The shield, in fact, became a fine
coat of mail. The Trilobite is an early and imperfect experiment of the
class, and the larva of the modern king-crab bears witness that it has
not perished without leaving descendants. How later Crustacea increase
the toughness of the coat by deposits of lime, and lead on to the
crab and lobster, and how one early branch invades the land, develops
air-breathing apparatus, and culminates in the spiders and insects, will
be considered later. We shall see that there is most remarkable evidence
connecting the highest of the Arthropods, the insect, with a remote
Annelid ancestor.

We are thus not entirely without clues to the origin of the more
advanced animals we find when the fuller geological record begins.
Further embryological study, and possibly the discovery of surviving
primitive forms, of which Central Africa may yet yield a number, may
enlarge our knowledge, but it is likely to remain very imperfect.
The fossil records of the long ages during which the Mollusc, the
Crustacean, and the Echinoderm slowly assumed their characteristic forms
are hopelessly lost. But we are now prepared to return to the record
which survives, and we shall find the remaining story of the earth a
very ample and interesting chronicle of evolution.

CHAPTER VII. THE PASSAGE TO THE LAND

Slender as our knowledge is of the earlier evolution of the Invertebrate
animals, we return to our Cambrian population with greater interest.
The uncouth Trilobite and its livelier cousins, the sluggish, skulking
Brachiopod and Mollusc, the squirming Annelids, and the plant-like
Cystids, Corals, and Sponges are the outcome of millions of years of
struggle. Just as men, when their culture and their warfare advanced,
clothed themselves with armour, and the most completely mailed
survived the battle, so, generation after generation, the thicker and
harder-skinned animals survived in the Archaean battlefield, an