As the animals and plants whose remains are found in the strata of the separate systems must have lived on the earth before these strata were formed, we can obtain an approximate idea of the fauna and flora which belonged to the separate periods of the earth's history; it can only be approximate, not accurate, first because we do not know all the fossils which exist, and then because it does not follow that traces remain of all the organisms which existed. This much is certain, that the organic life on the earth has not always been the same; no trace is found in the later formations of

Lyell, by whom the name "Eocene," etc., was introduced, has modified this division in the following way: he includes the eocene, miocene, and pliocene series in the Tertiary or Cainozoic period; and he calls everything more recent than the pliocene, post-tertiary. He then divides these post-tertiary formations into post-pliocene and recent. He calls those strata recent whose fossils, shells as well as mammals, belong to the still existing species; on the other hand, he calls those deposits, in which the shells belong to existing, but the mammals for the most part to extinct species, post-pliocene. Others call the lowest strata of the miocene, "oligocene;" others, everything which lies above the eocene, "neocene," or "neogene."

several of the organisms belonging to the older formations; they must therefore have died out in the earlier period. Again, there is no trace in the earlier formations of several of the organisms belonging to the later formations, therefore they cannot have existed in the earlier period. Geological research will not justify us in assuming anything beyond this general rule. Many of the details are uncertain and disputed. Some geologists say, for instance, that every formation has its separate fauna and flora, and that the same kind of animal or plant is never, or very rarely, found in two succeeding formations; and that it must therefore be assumed that in the course of ages organic life repeatedly became extinct on the earth, and was again restored.1 Others maintain that since the first appearance of organic beings, single species have now and then become extinct, and others have come into existence, but that such gradual transformation of organic life was caused by the existence and filling up of gaps, and that the thread was never entirely broken. More recently a theory, which we shall discuss at length later on, has been much in favour, namely, that all the later plants and animals, including all at present existing, are descended from those of the earliest period, so that it is not possible to classify the fauna and flora as belonging specially to separate periods.3

2

In comparing with one another the forms of plants and animals belonging to different periods, as we get to

1 Cuvier, A. Brongniart, A. d'Orbigny, Agassiz (cf. Les animaux et les plantes aux époques géologiques, in the Revue des Cours Scientifiques, Paris 1868, No. 49 seq.), Murchison (Siluria, p. 461), and others.

2 Prevost, de Blainville, Schlotheim, Bronn, and others. 3 Cf. Zittel in the Historisch Taschenb. p. 167.

know them by means of the fossils, we find that the earliest differ most, and the later ones least, from the present fauna and flora. Speaking generally, therefore, we may assume that a development of plants and animals from more imperfect to more perfect forms has taken place, however that development may be explained. The earliest formations contain scarcely any but the remains of creatures of a low organization; flowerless plants, corals, molluscs, and articulata. There are very few signs of fish and reptiles, and as far as is known at present, no signs of birds or mammals. In the succeeding strata more highly organized plants and animals are found; in the Carboniferous period there are some conifers, many fish, a few reptiles; in the Triassic period, many reptiles, a few birds, and mammals; in the Oolitic period, a few dicotyledon plants and more mammals; and in the Tertiary period, many dicotyledon plants and mammals. In all cases the lower organisms of the great separate divisions of the animal and vegetable worlds appear first, and the higher organisms later. Thus, of the radiata, the crinoidea appeared first; of the fish, first the tailed ganoid and the placoid; of the reptiles, the saurians; of the birds, first the marsh birds and tufted birds; of the mammals, the opossums and cetacea. The organic forms differ most from those now existing in the earliest strata, and the difference diminishes steadily all through the more recent deposits. Some of the animals and plants in the older formations belong to classes which are quite extinct; later they differ from those now existing in genus only; later still only in species. Some still existing kinds of larger animals

appear first as fossils

after the chalk formation, and they increase in number gradually during the Tertiary period.'

It is no doubt unsafe to draw conclusions as to the nature of the fauna and flora of the separate periods from the fossils which are found in the separate strata ; for, on the one hand, as I have already observed, those fossils which have been preserved are not yet thoroughly known; and, on the other hand, many organisms which may have existed were either not of a kind to be petrified, as fungi, slugs, etc., which are entirely wanting in hardness,—or could not be petrified because they existed on land, on high mountains, or in the air. Many geologists have not noticed these facts, and have hastily set up systems of the history of organic beings which later discoveries have shown to be false. It was formerly thought that land animals and land plants had appeared first in the Carboniferous period; since then they have been found in the earlier Devonian system. Before the year 1824, many persons thought there could be no doubt that reptiles first existed in the Permian period; in the course of ten years it had been proved that they existed in the Carboniferous age, and even before that time. Before the year 1818 every one thought that the earliest remains of warm-blooded animals occurred in strata of the Cainozoic period; since then they have been found in the Oolitic, and

1 See Cotta, Geol. Bilder, p. 284. Cf. Pfaff, Grundriss, p. 380.

2 Vestiges of the Natural History of Creation. It is supposed that the Telerpeton Elginense-a lizard-like reptile-is a land animal; cf. Murchison, Siluria, p. 254. Lyell, Elements of Geology, p. 533. Latterly, however, it has been decided that the sandstone in which it was found belongs to the Triassic system (New Red Sandstone), and not to the Devonian system (Old Red Sandstone). Athenæum, 1863, Jan. 31, p. 153. Cf. Ausland, 1863, pp. 144, 192.

1

even in the Triassic systems, that is to say, in the Mesozoic period. It is because of this that Lyell asserts his conviction that in some ways science is only on the threshold of discovery as to the order in which the separate classes of organisms appear in the strata; and that in the second half of the century, as in the first, it will repeatedly find itself compelled to modify its previous theories.

We may also derive a good deal of information about the condition of the earth's surface in the earlier periods, about the distribution of land and water, climate, and so on, from the position and nature of the separate strata and from their organic contents; but this information must be very untrustworthy on account of the incompleteness and uncertainty of the materials on which the premises are founded. The maps which have been drawn of different parts of the earth's surface, as it appeared in the earlier periods, must in many cases be considered as merely hypothetical sketches, and fancy has more part than science

1 Lyell, Elements of Geology, ii. Fraas, Vor der Sündfluth, pp. 214, 255. The common theory, which rested principally on Murchison's investigations (Siluria, pp. 21, 469), that the Silurian system contains the remains of the first existing organisms, has also been attacked in recent years. It was supposed in 1864 that a gigantic extinct species of foraminifera, which was named Eozoon Canadense, had been discovered in Canada in the Laurentian strata, which are thought to be older than the oldest Silurian strata, and also in the gneiss of the Bohemian Forest, in Scotland, and in Sweden. See Lyell, Athenæum, 1864, Sept. 17. Murchison, Athenæum, 1865, Sept. 16. Dawson, The Dawn of Life; cf. his Nature and the Bible (Germ. tr. Gütersloh, 1877). But the organic nature of this fossil is denied on good grounds by others. Cf. Ferd. Römer, Ueber die ältesten Formen des organischen Lebens auf der Erde, Berlin 1869, p. 34. Pfaff, Grundriss, p. 224.

2 See Huxley's remark above, p. 56. Cf. S. Zaddach, Die ältere Tertiärzeit ein Bild aus der Entwicklungsgeschichte der Erde, Berlin 1869. G. Berendt, Geognostische Blicke in alt-Preussen's Urzeit, Berlin 1872.