THE EVOLUTION OP LIFE 13
cyclic history of its surface is completed by the erosion of rocks^ by the accmnrQation of sediments, and by the following subsidence of the areas loaded down by these sediments. It appears that the internal heat engine is far more active in the slowly cooling continental areas than in the rapidly cooling areas imderlying the oceans^ as manifested in the continuous outflows of igneous rocks, which, especially in the early history of the earth — at or before the time when life appeared — covered the greater part of the earth's surface. The ocean beds, being less sub- ject to the work of the internal heat engine, have always been relatively plane; except near the shores, no erosion has taken place.
The Age of the Earth
The age of the earth as a solid body affords our first instance of the very wide discordance between physical and biological opinion. Among the chief physical computations are those of Kelvin, Sir George Darwin, and King and Barus.** In 1879 Sir George Darwin allowed 56 million years as a probable lapse of time since the earth parted com- pany with the moon, and this birthtime of the moon was naturally long prior to that stage when the earth, as a cool crusted body, became the environment of living matter. Far more elastic than this estimate was that of Lord Kelvin, who, in 1862, placed the age of the earth as a cool- ing body between 20 and 400 million years, with a probability of 98 mil- lion years. Later, in 1897, accepting the conclusions of King and Barus calculated from data for the period of tidal stability, Kelvin placed the age limit between 20 and 40 million years, a conclusion very unwelcome to evolutionists.
As early as 1859 Charles Darwin led the biologists in demanding an enormous period of time for the processes of evolution, being the first to point out that the high degree of evolution and specialization seen in the invertebrate fossils at the very base of the Paleozoic was in itself a proof that pre-Paleozoic evolution occupied a period as long as or even longer than the post-Paleozoic. In 1869 Huxley renewed this demand for an enormous stretch of pre-Cambrian time ; and as recently as 1896 Poulton*' found that 400 million years, the greater limit of Kelvin's original estimate, was none too much. Later physical compu- tations greatly exceeded this biological demand, for in 1908 Ruther- ford** estimated the time required for the accumulation of the radium content of a uranium mineral found in the Glastonbury granitic gneiss of the Early Cambrian as no less than 500 milKon years.
This estimate of the age of the Early Cambrian is eighteen times as great as that attained by Walcott*" in 1893 from his purely geologic
» Becker, George F., 1910, p. 5. 2« Ponlton, Edward B., 1896, p. 808.
- Butherfopd, Sir Ernest, 1906, p. 189.
M Walcott, Charles D., 1893, p. 675.
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