The science of life : fully illustrated in tone and line and including many diagrams

BOOK 3 THE SCIENCE OF LIFE CHAPTER 2

MAN Q Vs 5Q B Vi 5 MODERN So BIRDS a Oo PLACENTAL MAMMALS TRUE = es} FLOWERIN: n PLANTS Oo 10 BIRDS N ° 4 4500 5 MAMMALS 45 175 NIK REPTILES © U 00 : OQ AMPHIBIA > ee 30 TREES “ SEED PLANTS INSECTS ° LAND PLANTS N FISH SO bd AQ VERTEBRATES es TRILOBITES . 500 50 50

Fig. 122. The time-scale of the record of the rocks.

The whole of Geological Time is represented in (1), except for the earlier Archeozoic (dotted), whose history is not yet known. The darker part of (1), from the Jirst well-preserved fossils to the present day, is Shown on a larger scale in (2). The relative lengths of the periods can only be approximate. The period of active Mammalian evolution, darkened in (2), is shown still larger in (3). Here the total thickness, in feet, of the strata laid down during the various periods, is added, to indicate the speed at which deposition occurs. The recent period since the last Ice Age is too short to be visible in a diagram of this scale; it is hidden by the printer's ink of the upper line. It can be seen in a less compressed diagram of the Pleistocene and recent periods, which appears in Book 5. The figures to the right hand of the columns Show the time-scale in millions of years, measured backwards from the present day. They are based on analyses of radioactive minerals. (Some authorities would make the Proterozoic of shorter duration.) The arrows in (1) indicate the great ‘* revolutions?’ or times of violent mountain-building. The dates of the earliest of these disturbances (dotted) are not certainly known ; it is generally assumed that they took place at about the same rhythm as the later ones. The first appearance as fossils of a few dominant groups of animals and plants is indicated to the left of column (2) 5 but it should be remembered that in several cases the first stages in the evolution of a group have not yet been discovered.

later to be described, and which can now be Archbishop Ussher, less than three hundred taken as accurate within about 10 per cent.) years ago, dated the creation of the world in they are indeed staggering. 4004 B.C. (and gave the day and hour, too !),

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