292 THE SCIENTIFIC MONTHLY
of chemical evolution, also adaptation to an earth and water environ- ment rather than to an atmospheric one.
In our study of the chemistry of tlie lifeless earth it has been shown how the life elements essential for the energy and nutrition of the nitri- fying bacteria, namely, sodium, potassium, calcium and magnesium, with potassium nitrite and ammonium salts as a source of nitrogen, were probably accumulated in the wattirs, pools and soilfi. These bac- teria were at once the soil-forming and the soil-nourishing agents of the primal earth ; they throve in the presence of energy-liberating com- pounds of extremely primitive character. It is important to note that water and air are essential to vigorous ammonium reactions, whether at or near the surface. In arid regions at the present time the anmionify- ing bacteria do not exist on the dry surface rocks but act vigorously in the soils, not only at the surface, but also in the lower layers at depths of from six to ten feet where moisture is constant and the porous soil well aerated,* thus giving rise to a nitrogen-nourished substratum which explains the deep rooting of desert-dwelling plants.
A second point of great significance is that these nitrifying organ- isms are heat-loving and light-avoiding ; they are dependent on the heat of the earth or of the sun, for, like all other bacteria, they carry on their activities best in the absence of sunshine, direct sunlight being generally fatal. The sterilizing effect of sunlight is due partly to the coagulation of the bacterial colloids by the rays of ultra-violet light. The sensitiveness of bacteria to sunlight cannot, however, be used as an argument against their geologic antiquity; on the contrary, their undiflEerentiated structure and their ability to live on inorganic food- stuffs even without the aid of sunshine seem to favor the idea that they represent a very primitive form of life.
The great antiquity even of higher forms of bacteria feeding on atmospheric nitrogen is proved by the discovery, announced by Wal- cott^ in 1915, of a species of pre-Paleozoic fossil bacteria attributed to ^'Micrococcus" but probably related rather to the existing Nitroso coccus which derives its nitrogen from ammonium salts. The Nitroso coccus is the form found in this country corresponding to the Ni- troso monas of Europe. Its mode of life is identical with that of the Nitroso monas. These fossil bacteria were found in a section of a chlorophyll-bearing algal plant from the Newland limestone of the Algonkian of Montana, the age of which is estimated to be about thirty- three million years. They point to a very long antecedent stage of bac- terial evolution.
6 Lipman, Charles B., 1912, pp. 7, 8, 16, 17, 20.
7 I. J. Kligler.
8 Walcott, Charles D., 1915, p. 256.
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