The evidence suggests that both the free-moving and the stalked Echinoderms descend from a common stalked Archaean ancestor. Some primitive animal abandoned the worm-like habit, and attached itself, like a polyp, to the floor. Like all such sessile animals, it developed a wreath of arms round the open mouth. The "sea-cucumber" (Holothurian) seems to be a type that left the stalk, retaining the little wreath of arms, before the body was heavily protected and deformed. In the others a strong limy skeleton was developed, and the nerves and other organs were modified in adaptation to the bud-like or flower-like structure. Another branch of the family then abandoned the stalk, and, spreading its arms flat, and gradually developing in them numbers of little "feet" (water-tubes), became the starfish. In the living Comatula we find a star passing through the stalked stage in its early development, when it looks like a tiny sea-lily. The sea-urchin has evolved from the star by folding the arms into a ball. [*]
* See the section on Echinoderms, by Professor MacBride, in
the "Cambridge Natural History," I.
The Bryozoa (sea-mats, etc.) are another and lower branch of the primitive active organisms which have adopted a sessile life. In the shell-fish, on the other hand, the principle of armour-plating has its greatest development. It is assuredly a long and obscure way that leads from the ancestral type of animal we have been describing to the headless and shapeless mussel or oyster. Such a degeneration is, however, precisely what we should expect to find in the circumstances. Indeed, the larva, of many of the headless Molluscs have a mouth and eyes, and there is a very common type of larva—the trochosphere—in the Mollusc world which approaches the earlier form of some of the higher worms. The Molluscs, as we shall see, provide some admirable illustrations of the process of evolution. In some of the later fossilised specimens (Planorbis, Paludina, etc.) we can trace the animal as it gradually passes from one species to another. The freshening of the Caspian Sea, which was an outlying part of the Mediterranean quite late in the geological record, seems to have evolved several new genera of Molluscs.
Although, therefore, the remains are not preserved of those primitive Molluscs in which we might see the protecting shell gradually thickening, and deforming the worm-like body, we are not without indications of the process. Two unequal branches of the early wormlike organisms shrank into strong protective shells. The lower branch became the Brachiopods; the more advanced branch the Molluscs. In the Mollusc world, in turn, there are several early types developed. In the Pelecypods (or Lamellibranchs—the mussel, oyster, etc.) the animal retires wholly within its fortress, and degenerates. The Gastropods (snails, etc.) compromise, and retain a certain amount of freedom, so that they degenerate less. The highest group, the Cephalopods, "keep their heads," in the literal sense, and we shall find them advancing from form 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, and the Cambrian age opened upon the various fashions of armour that we there described. But, although half the story of life is over, organisation is still imperfect and sluggish. We have now to see how it advances to higher levels, and how the drama is transferred from the ocean to a new and more stimulating environment.
The Cambrian age begins with a vigorous move on the part of the land. The seas roll back from the shores of the "lost Atlantis," and vast regions are laid bare to the sun and the rains. In the bays and hollows of the distant shores the animal survivors of the great upheaval adapt themselves to their fresh homes and continue the struggle. But the rivers and the waves are at work once more upon the land, and, as the Cambrian age proceeds, the fringes of the continents are sheared, and the shore-life steadily advances upon the low-lying land. By the end of the Cambrian age a very large proportion of the land is covered with a shallow sea, in which the debris of its surface is deposited. The levelling continues through the next (Ordovician) period. Before its close nearly the whole of the United States and the greater part of Canada are under water, and the new land that had appeared on the site of Europe is also for the most part submerged. The present British Isles are almost reduced to a strip of north-eastern Ireland, the northern extremity of Scotland, and large islands in the south-west and centre of England.