It will be understood from some of the foregoing sentences that the task of dating or classifying these early rocks is one which is far from simple, and which has given rise to many different opinions. We may here give another example. "During the years between 1831 and while Sedgwick was occupied in studying the rocks of North Wales," writes Mr. W. Jerome Harrison, "another geologist, Mr. (afterwards Sir) Roderick Murchison, was engaged in the examination of the strata which occupy the south-east of Wales and the adjoining border counties of England. To these rocks Murchison gave, in 1835, the name of Silurian, from the ancient British tribe of the Silures, who inhabited that part of the country when the Romans invaded Britain." Later in last century, in order to distinguish more clearly the periods of the rocks which began or ended in these areas, the name of another ancient British tribe was called into requisition—the Ordovics; and thus for certain strata which were neither Silurian nor Cambrian Professor Lapworth proposed the name Ordovician.
Let us, however, now leave these geological controversies, enthralling as they are to those who have taken part in them, to consider briefly what was the aspect of the earth during the ages when these rocks were being laid down. The earliest rocks do not generally contain fossils, though there is no doubt that life existed during the later part of the time when they were laid down. The few fossils that have been preserved are those of crustacea (the species from which shrimps, for example, are derived), and there are certain tracks of two kinds of burrowing worms. It is noticeable that crustacea, the oldest definite fossils yet found, belong to a family which is well up in the animal kingdom, and therefore we know that lower forms of life must have been long in existence. Since we can only draw conclusions of the climate of a period from its fossil remains, and as these fossil remains are so scarce, we cannot say really anything of value about the world's climate in the earliest eras.
When we come to the Cambrian, however, we are on firmer ground. In the Cambrian rocks there is, for the first time, a fair preservation in fossil form of the life of the period. Even here the record is far from complete, but it is an immeasurable advance on the records of previous periods. The most striking thing about this comparatively plentiful appearance of life is that while the animal kingdom is fairly well represented the plant remains are hardly to be recognised at all. Yet there must have been plants if only to feed the animals, and we have very good reasons for believing that the surface of the land was clothed with some form of vegetation. Not a few of the Cambrian animals were fixed to the bottom of the sea, and therefore there must have been enough matter of some organic kind floating in the water to bring them their daily food. Possibly many of the plants were of the minute kind which forms scum on rivers and ponds, and so would not readily leave fossil impressions. Turning to the record of animal life, it appears that nearly every division of the animal kingdom, except such as had backbones, had some kind of a representative in Cambrian times. Crustaceans, molluscs, worms, corals, jelly fish, sponges, quite a large variety of sea-animals, suddenly make their appearance, and although no traces of land animals have yet been found, we have reason to believe that some land animals may have existed. Our reason is that in the next era but one (Silurian) scorpions and insects appear, and these are such highly developed forms of land-life that they probably had some primitive ancestors in the Cambrian. No real fish have been found in the Cambrian rocks, but they appeared in the next era (Ordovician). It is the trilobite which is the characteristic animal of the Cambrian times. They were crustaceans; they had eyes; and they gave the promise of development; but there is no reason for believing that they were as high in the order of creation as the commonest lobster of the sea-shore. Nothing remains to us of them except their bony structure, but we believe that they could both swim and walk on the sea-bottom; that some were swift of movement, and that they acquired the habit of moulting their shell. They may have been sociable animals, for the shells of trilobites are sometimes found together in large numbers, occasionally closely packed, "spoon fashion," and though these may be moulted shells, we are warranted in supposing that the early trilobites lived in colonies, hunted for food, and made war like their descendants millions of years after. What were the actual conditions of life in this world of Cambrian days we do not know positively. The first beginnings of life, the simple one-celled plants, may have first dwelt in the deep ocean. The land was barren, its lakes unfitted to support life. On the other hand, it is equally likely that the first beginnings of life may have been the simple plants growing in inland waters and gradually spreading down to the sea. We do not know, but it is most probable that life began in some great body of water, where plants and insignificant animals grew together, perhaps fought together, and certainly in this environment became more and more fitted for the business of living.
In Mr. Henry R. Knipe's scholarly and well-informed volume, Nebula to Man (J. M. Dent & Co.), to which we are indebted not only for several of our illustrations but for many extremely valuable suggestions, the struggle for existence in the early ocean is well summed up:—
Thus through the brine life manifold proceeds,
Impelled to higher states by growing needs;
And all these early life-types in the seas
Will branch in time to many species;
And some amid conditions too severe,
Must, after stress and struggle, disappear.