Table of geological strata, with approximate thickness of the several strata in Britain. Total thickness about 23 miles. (1) THE TERTIARY OR KAINOZOIC PERIOD.-The pre-historic cave deposits found in the caves used as dens by wild beasts, who dragged into them the carcases of their prey, prove the occurrence of the Hyana, Cave-bear, Rhinoceros, Lion, Reindeer, Bison, Hippopotamus, and Beaver-with man. These forms are now living, but not in England. In the Glacial period the earlier deposits show some of these disappearing, others persisting, and animals appearing that now do not exist anywhere-viz., the Mammoth, Woolly Rhinoceros, the Sabre-toothed Tiger and the Irish Elk or deer. In the Pliocene period we find evidence of the Mastodon and Tapir; and in the Eocene period, of the Didelphys, Opossum, and Hyracotherium. (2) THE SECONDARY OR MESOZOIC PERIOD.-The entire group found at this period is extinct. The Ammonites are characteristic secondary forms, and are very abundant, but none survive into the Tertiary period. The Ichthyosaurus, Plesiosaurus, and Mosasaurus are extremely characteristic, and confined to the Secondary period. (3) THE PRIMARY OR PALÆOZOIC PERIOD.-Here、 we find fossil plants in the Coal measures, the Lepidodendron, Calamites, Sigillaria, and Stigmaria. Fish are also found abundantly. The Trilobites are exclusively Palæozoic; also the Sea-scorpions, Pterygotus, and Eurypterus. The general conclusions we arrive at are : (1) There is a general advance in organisation from the lower to the higher or more recent deposits, and an increase in the diversity of type. (2) There is no evidence of sudden breaks or cataclysms; there is no break between the Tertiary period and the present day. Some species die out and others appear, and some persist unchanged. The very evidence which Cuvier relied on to prove Catastrophism disproves it when examined more carefully and with fuller knowledge. (3) Some forms, known as persistent types, remain unchanged for great periods. This constitutes no real difficulty, for Natural Selection does not of necessity involve progression or change of any kind, and is quite consistent with a stationary condition, provided that the environment, or at least all the features of the environment affecting them, remain unchanged. These are examples of the real aristocracy of animals, for they can date back their descent not merely to the time of the appearance of man, but almost to the first appearance of animal life of which we have positive evidence. As examples of persistent types may be cited Globigerina, which shares in the formation of chalk, and is found in the Trias, or bottom of the Secondary strata; Limulus, the king-crab, which occurs in the Trias, and is found on the American coast at the present day; Dentalium, the tusk-shell, an animal in some respects between an oyster and a snail, with a tubular conical shell about two inches long, by which it burrows in the sand, is found in the Devonian and perhaps Silurian strata; the Pearly Nautilus, a rare animal, is found in the lower Silurian and at the present day; Lingula, an animal with extreme tenacity of life, is found in the lower Cambrian and at the present day, and, so far as we can see, is unchanged. (Figs. 4 and 5.) Human customs and people have often been referred to for examples of the laws of biology, and there is no need to look beyond them. As an example of a persistent type, cannot we at once call to mind a nation, a homeless nation, the members of which occur in all countries, yet have no country of their own; a nation which, in spite of persecution of unexampled severity, endured not once only, but many times repeated and in most diverse forms, has held its own; nation which, in spite of various and shifting environments to which it has been exposed, has with singular tenacity retained and preserved its language, traditions, and ceremonial observances in all essential features intact. a THE IMPERFECTION OF THE GEOLOGICAL RECORD. Undoubtedly the most interesting and important point concerning fossils yet remains to be considered. Fossils are the former inhabitants of the globe, and therefore, on the Theory of Evolution, the ancestors of the animals now living. Now, fossil forms are unlike existing ones, therefore modification, and very considerable modification, must have occurred. Do the fossils themselves show evidence of such modification? Can we point to a series of forms showing progressive modification towards the present condition? I admit at once that fossils do not give us all the evidence we could wish for; in some instances a fairly complete series can be pointed out, but in most cases we are unable to do this. This is undoubtedly at first sight a serious check, and is one often referred to. By Darwin himself it was stated to be the difficulty that would probably be most widely felt. This difficulty must be considered fully and from two main standpoints : First, of what nature is the record yielded by fossils, and how far is it reasonably complete? Secondly, are we quite certain that we know in what direction to look for intermediate forms, and are we clear that we should recognise such if we found them? The geological record is imperfect for the following reasons : 1. Only certain parts of certain animals can be preserved as fossils. Among PROTOZOA the Foraminifera and Radiolaria are well preserved, Infusoria not at all. In PORIFERA the skeletons are well preserved, and of these there is a long record. Of CELENTERATA such forms as Hydra, jelly-fish, and sea-anemones cannot be preserved, save very exceptionally; but corals are peculiarly suitable, and few classes are so well represented in a fossil state. ECHINODERMATA are well represented, excepting the Holothurians. Among VERMES there is no trace of flat or round worms, but of Annelids the jaws, tubes, and tracks are found. ARTHROPODA are well preserved, especially the Crustacea, and as a general rule aquatic forms are much more completely preserved than terrestrial ones. In MOLLUSCA the shell is well adapted for preservation, but the air-breathing terrestrial forms are rare. Among the VERTEBRATA we find the |