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synthetic treatment of climate, time and geologic change in relation to the geologic origin and the migration of the different vertebrate groups is William Diller Matthew's Climate and Evolution (1915). The subdivisions of geologic time and the successions of faunas and climates are broadly reviewed in the Textbook of Geology by Louis V. Pirsson and Charles Schuchert (1915; revised edition, vol. I, 1921). The latest summary of the geology, past physiography and palaeontology of the world is found in the French edition of the great work of Eduard Suess, Das A ntlitz der Erde, translated and annotated by Emmanuel de Margerie as La Face de la Terre (1902, 1918). The comparative evolution of the mammalia of the eastern hemi- sphere and of North America is broadly treated in Henry Fairfield Osborn's A ge of Mammals (1910); while the mammals of North and South America are compared in W. B. Scott's History of Land Mammals in the Western Hemisphere (1913). A broad treatment of the whole subject of invertebrate and vertebrate evolution is given in Richard S. Lull's Organic Evolution (1917) and a synthetic review of the earth's history, from its solar beginnings to the Age of Man, in Osborn's Origin and Evolution of Life.

Life Epochs of Geologic Time. The time scale in the accom- panying table is taken from the work of Pirsson and Schuchert of 1915, modified by the substitution of geologic time units for years. There is a growing indisposition to reckon past time in terms of years, and a growing tendency to substitute a relative term like time units, because of the enormously wide discrepancy between the older estimates of geologists, based on sedimentation and the thickness of the various assemblages of rocks, which, taken together, make up the whole geologic time scale, and the estimates of physicists, based on the slow liberation of radium from radioactive minerals. The radium estimates of the age of the earth range as high as 1,400,000,000 years for the oldest known rocks, according to Barrell, who has adopted the calcula- tions of Rutherford and others based on the " rate of disintegra- tion " of radioactive minerals. The contract between the two methods is exemplified in the following table:

by palaeontologists. In the same discussion W. J. Sollas com- ments on the expansion of time estimates proposed by physicists: " The age of the earth was thus increased from a mere score of millions to a thousand millions and more, and the geologist who had before been bankrupt in time now found himself suddenly transformed into a capitalist with more millions in the bank than he knew how to dispose of."

In this connexion we may recall the fact that as early as 1859 Charles Darwin pointed out that the high degree of evolution and specialization seen in the invertebrate fossils at the base of the Palaeozoic, namely, the Cambrian, proved that Precambrian evolution occupied a period as long as, or even longer than, that of Cambrian to Recent time (see Table I on p. n). Poulton, the leading disciple of Darwin in England (1896), declared that 400,000,000 years was none too long for the whole life evolution period; this would allow 200,000,000 years for Precambrian time and another 200,000,000 years from Cambrian to Recent time.

Walcott's Revelation of Precambrian and Cambrian Life. Charles D. Walcott (1899, 1914) has discovered the remains of life in the Precambrian (Proterozoic) rocks of North America and has been able to give us a fragmentary picture of the fauna and flora of that very ancient period. In Montana at a depth of nearly 10,000 ft. below the earliest Palaeozoic rocks (Cambrian) he found evidence of ancient reef deposits of calcareous algae, which ranged upward through 2,000 ft. of strata. Above these reefs are 3,000 ft. of shales containing worm trails and the frag- mentary remains of large crustacean-like organisms. From rocks of approximately the same age in Ontario, Canada, he has de- scribed sponge-like forms (Atikokania) which are of such general- ized structure that it is difficult to decide whether they should be regarded as sponges or as archaic corals. These few plant and animal remains are all that are known from remotely metamor-

Walcott (1893) Years

Barrell (1917) Years

Age of Man and of Mammals Cenozoic

3,000,000 9 ooo ooo

55,000,000- 65,000,000 140 ooo 000180 ooo ooo

Age of Amphibians, Fishes, Invertebrates Palaeozoic. ... Precambrian Time Evolution of Invertebrates and of Unicellular Life

18,000,000 30,000,000 90000,000 (Geikie, 1899)

360,000,000-540,000,000 600,000,000-800,000,000

1 ,2OO OOO OOO

Maximum Total

doo.ooo.ooo (Geikie. 1800)

1 .100.000.000

The most original part of Barrell's contribution was the measurement of time from the base of the Palaeozoic to Recent time by new palaeophysiographic methods, taking into account particularly the rhythms or cycles of dry and moist climates and of elevations and depressions, theories which were originally interpreted by T. C. Chamberlin and popularly treated by Ells.vorth Huntington, the physiographer of Yale University.

A few decades ago the physicists and mathematicians, es- pecially Kelvin and Tail, insisted that the earth could not be more than 10,000,000 to 20,000,000 years old; now the physicists are extending the age of the life period to 1,400,000,000 years, as estimated by Barrell (1917). The most recent determination by physicists, as reviewed by Lord Rayleigh (1921), takes into consideration the transmutation of chemical elements, for example, in the broggerite of the Precambrian rocks at Moss, Norway: " Taking the lead as all produced by uranium at the rate above given, we get an age of 925 million years. Some minerals from other archaean rocks in Norway give a rather longer age. . . The helium method is applicable in some cases to materials found in the younger formations, and proves that the ages even of these are to be reckoned in millions of years. Thus the helium in an Eocene iron ore indicated 30,000,000 years at least. . . The upshot is that radioactive methods of research indicate a moderate multiple of 1,000 million years as the dura- tion of the earth's crust as suitable for the habitation of living beings, and that no other considerations from the side of pure physics or astronomy afford any definite presumption against this estimate." Applying this estimate to the evolution of a familiar mammal like the horse, it might be said that the four- toed horse (Eohippus) existed 30,000,000 years ago, a somewhat larger estimate of the life period of the horse than that demanded

phosed rocks of Precambrian time, but the existence of annelids and possible arthropods marks a break into the hitherto unknown Precambrian. Walcott's most surprising discovery in Precam- brian time is a monad or bacterium attributed to Micrococcus sp. indet. from the Algonkian of Montana, but probably related rather to the existing Nitrosomonas, one of the prototrophic or primitive-feeding bacteria, which derives its nitrogen from ammonium salts.

In 1910 Walcott discovered in the Cambrian (Burgess) shales of Alberta, Canada, a marvellously rich fauna whose preservation is so perfect that the setae of the worms, the jointed appendages of the trilobites, the impressions of soft-bodied medusae and holothurians, and even the alimentary tract and stomach of certain of the crustaceans can be seen on the shale surfaces almost as clearly as in living forms (Plate I.). This discovery fairly revolutionizes our knowledge of the anatomy of the delicately organized as well as the chitinous-armoured forms, like the trilobites. Including the new forms contained within these Albertan shales, the Cambrian marine fauna is now known to be far more abundant than even imagined by Darwin, comprising some 1,500 species, 1,200 of which occur in North America. From Lower as well as Middle Cambrian (Burgess) faunas, it appears that the Precambrian invertebrates had probably be- come completely adapted to all the life zones of the continental and oceanic waters, excepting possibly the abyssal. All the principal phyla the jointed arthropoda (including the trilobites among the crustaceans and the merostomes among the arachnids) , segmented worms (Annelida), echinoderms, molluscs (including pelecypods, gastropods and primitive cephalopods), brachiopods, medusae and other coelenterates, and sponges were presumably established in Precambrian times.