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