We must now turn to the less interesting but not less important topic of the animal life of the Carboniferous period. At the beginning of the period when only a small portion of the British Isles was above the waters, and an ocean rolled from Ireland to China, the life of which the most important relics were left was that of the sea. In the early Carboniferous seas the rhizopods, some small as dust, laid down with their tiny shells the foundations of mountains yet to be; the "sea lilies" were at the height of their pride, occupying vast areas in the flowing tide; forms like the present-day nautilus began to appear, and the "lamp-shells" attained their greatest size. The trilobites, hitherto the most conspicuous and noticeable animals of the earth's childhood, were beginning to die out, vanquished in the struggle for life by more adaptable forms, and the big sea scorpions were waning fast. The king crabs and the water fleas still throve, and the fishes, though most of them not very large, were growing larger, some of them taking the appearance of the dog-fish, some of the ray, some of the shark; and, what is more important than the fact of size, the fishes were growing speedier and more capable of attacking weaker creatures.
In the course of these ages the sea invaded the land; and shores where land-snails and millipedes and centipedes, beetles and scorpions, spiders and cockroaches had found a home became entirely changed, not only in their appearance and character, but in the type which subsisted on them. It is possible (for something of the kind has been noticed in our own days in the West Indies, where a sea-crab species is showing signs of becoming a land animal) that some of the forms of water animals became used to living in shallower and shallower water as the generations went on till they became partly land and partly water animals--amphibians, as they are called. Thus small newt-like beings, moving clumsily through the swamps, made their appearance, and others with stronger limbs pushed onwards through the forest. Others in form resembling snakes crept through the mud and lived among the swamps by the side of the sea. Not much is known of the food and life-habits of any of these amphibians. From their teeth we may perhaps judge that they lived on fish, crustaceans, insects, and on one another, and their predatory life sometimes led them to climb trees in search of food. What, however, is most important about the amphibian is that they were the pioneers of the march of those creatures which had backbones--the vertebrates--from the sea to the land.