THE EVOLUTION OF LIFE 21
present time. At present the amount of carbon dioxide in the atmos- phere averages about three parts in 10,000, but there is little doubt that the primordial atmosphere was richer id this compound which next to water and nitrogen is by far the most important both in the origin and in the development of living matter. The atmospheric carbon dioxide is at present continually being reduced by the absorption of carbon in living plants and the release of free oxygen; it is also washed out of the air by rains. On the other hand, the respiration of animals is continually returning it to the air. The large amount of aqueous vapor and of carbon dioxide in the primordial atmosphere served to form an atmospheric blanket which inhibited the radiation of solar heat from the earth^s surface and also prevented excessive changes of tem- perature. Thus there was on the primal earth a greater regularity of the sun's heat supply, with more moisture, while the light supply from the sun was less intense and constant than at present. This is in general accord with the fact that the most primitive organisms surviv- ing upon the earth to-day, the bacteria, are rather dependent upon heat than upon light for their energy. It is also possible that through the agency of thermal springs and the heat of volcanic regions primordial life forms may have derived their energy from the heat of the earth rather than from that of the sim.
The stable elements of the present atmosphere, for which alone estimates can be given, are essentially as follows :^®
By Weight By Volume
Oxygen 23.024 20.941
Nitrogen 75.539 78.122
Argon 1.437 .937
100.000 iMooo
Since carbon is a less essential element^® in the life-processes of the simplest bacteria, we can not agree with Henderson'® that carbon dioxide was coordinate with water as a primary compound in the origin of life. It probably was subsequently utilized in the chlorophyllic stage of plant evolution.
Atmospheric carbon dioxide (COj) which averages about three parts in every 10,000, and water (HgO) is always present in varying amounts; beside argon, the rare gases helium, xenon, neon, and krjrpton are present in traces. None of the rare gases which have been discovered in the atmosphere, such as helium, argon, xenon, neon, krypton, and niton — ^the latter a radium emanation — are at present known to have any relation to the life processes. Carbon dioxide exists in the atmos-
48 Clarke, F. W., letter of March 7, 1916.
4* Jordan, Edwin O., 1908, p. 66.
CO Henderson, Lawrence J., 1913, pp. 138, 139.
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