Page:The Scientific Monthly vol. 3.djvu/18

 12 THE SCIENTIFIC MONTHLY

upon life data has not agreed with opinion founded upon physical or chemical data. Discord has arisen especially in connection with the age of the earth and the stability of the earth^s surface. In our review of these matters we may glance at opinions of all kinds, whatever their source; but our main narrative of the chemical origin and history of life on the earth will be followed by observations on living matter as it is revealed in paleontology and as it exists to-day, and not on hypotheses and speculations upon preexisting states.

The formation of the earth's surface is a prelude to our considering the first stage of the environment of life. According to the planetesimal theory, as set forth by Chamberlin^* in the preceding lectures, the earth, instead of consisting of a primitive molten globe as postulated by the old nebular hypothesis originated in a nebulous knot of solid matter aa a nucleus of growth which was fed by the inf all or accretion of scattered nebulous matter (planetesimals) coming within the sphere of control of this knot. The temperature of these accretions to the early earth could scarcely have been high, and the mode of addition of these planet- esimals one by one explains the very heterogeneous matter and differ- entiated specific gravity of the continents and oceanic basins. The pres- ent form of the earth's surface is the result of the combined action of the lithosphere, hydrosphere, and atmosphere. Liquefaction of the rocks occurred locally and occasionally as the result of heat generated by in- creased pressure and by radioactivity; but the planetesimal hypothesis assumes that the elastic rigid condition of the earth, as at present, pre- vailed — at least in its outer half — throughout the history of its growth from the small original nebular knot to its present proportions and caused the permanence of its continents and of its oceanic basins. We are thus brought to conditions that are fundamental to the evolution of life on the earth. According to the opinion of Chamberlin cited by Pirsson and Schuchert,*® life on the earth may have been possible when it attained the present size of Mars.

According to Becker,** who follows the traditional theory of a primi- tive molten globe, the earth first presented a nearly smooth, equipo- tential surface, determined not by its mineral composition, but by its density. As the surface cooled down a temperature was reached at which the waters of the gaseous envelope united with the superficial rocks and led to an aqueo-igneous state. After further cooling the second and final consolidation followed, dating the origin of the granites and granitary rocks. The areas which cooled most rapidly and best con- ducted heat formed shallow oceanic basins, whereas the areas of poor conductivity which cooled more slowly stood out as low continents. The internal heat of the cooling globe still continues to do its work, and the

!• Chamberlin, Thomas Chrowder, 1916.

so Pirsson, Louis V., and Schnchert, Charles, 1915, p. 535.

21 Becker, George F., letter of October 15, 1915.

�� �