HYPOTHETICAL STAGES LEADING UP TO THE KNOWN ERAS. 115 



for, in the first place, the atmosphere must have been then, as now, 

 an effective cushion checking the speed of the planetesimals and 

 partially dissipating them, and, in the second place, the early organisms 

 were probably all aquatic and were further protected by their water- 

 covering. 



Changes in the atmosphere inaugurated by the introduction of life. — 

 The introduction of organic activity brought into play those chemical 

 processes that have been sketched in general terms in Chapter XI 

 of Volume I. The changes in the composition of the atmosphere are 

 especially important. It has been shown (p. 97) that the primitive 

 atmosphere probably contained a preponderance of carbon dioxide, 

 and, a little later, carried all the water-vapor that it could hold under 

 the prevailing temperatures, while the amount of nitrogen was not 

 improbably low, and that of oxygen uncertain. If only there were 

 oxygen enough to serve the functions of plant life at the outset, the 

 existing large content of oxygen could probably all have arisen from 

 subsequent plant action. It is merely necessary, therefore, to assume 

 (1) that the carbon dioxide was not too abundant to prohibit the devel- 

 opment of the early plants, (2) that the oxygen was sufficient for their 

 vital processes, and (3) that the nitrogen was much less abundant than 

 now, to give a good working basis for the evolution of the present very 

 different atmosphere. Assuming that green (photosynthetic) plants were 

 first introduced, and that, until some time later, there were no animals 

 or predaceous plants that decomposed the carbon compounds pro- 

 duced by the green plants, the effect of plant life on the atmosphere 

 would be to reduce its carbon dioxide and increase its free oxygen. 

 If there were no check or offset to this process, a relatively short time 

 would suffice for the conversion of an atmosphere of dominant carbon 

 dioxide to one of dominant oxygen. If the present vegetation can 

 remove the present content of carbon dioxide in one hundred years 

 (see pp. 639-644, 645, Vol. I), an amount of carbon dioxide as great as 

 the whole atmosphere of to-day might be changed to oxygen in about 

 300,000 years by an equally active vegetation. The early plant action 

 may have been much less efficient than that of to-day, and the requisite 

 period might be correspondingly lengthened, but it might still be 

 geologically short. Besides, the early atmosphere, by hypothesis, was 

 much less abundant than the present one, and probably much more 

 active in the carbonation of rocks. 



