THE ORIGIN AND EVOLUTION OF LIFE 



waters issuing from modern sedimentary areas. They thus 

 furnish a favorable environment for the development of such 

 low organisms (or their ancestors) as the existing diatoms, 

 radiolarians, and sponges, which have skeletons composed of 



hydrated silica, mineralogi- 

 cally of opal. 



The decomposition and 

 therefore the erosion of the 

 massive rocks was slower then 

 than at present, for none of 

 the life agencies of bacteria, 

 of algae, of lichens, and of the 

 higher plants, which are now 

 at work on granites and vol- 

 canic rocks in all the humid 

 portions of the earth, had yet 

 appeared. On the other hand, 

 much larger areas of these 

 rocks were exposed than at 

 present. 



In brief, to imagine the 

 primal lifeless earth we must 

 subtract all those portions of 

 mineral deposits which as they 

 exist to-day are mainly of organic origin, such as the organic car- 

 bonates and phosphates of lime,^ the carbonaceous shales as well 

 as the carbonaceous limestones, the graphites derived from car- 

 bon, the silicates derived from diatoms, the iron deposits made 



^ It seems improbable that organisms originally began to use carbon or phosphorus 

 in elementary form: carbonates and phosphates were probably available at the very be- 

 ginning and resulted from oxidations or decompositions. — VV. J. Gies. 



Phosphate of lime, apatite, is an almost ubiquitous component of igneous rocks, but 

 in very small amount. In more than a thousand analyses of such rocks, the average 

 percentage of P2O5 is 0.25 per cent. — F. W. Clarke. 



Fig. 2. Deep-Sea Ooze, the Forami- 



NIFERA. 



Photograph of a small portion of a cal- 

 careous deposit on the sea bottom formed 

 by the dropping down from the sea sur- 

 face of the dead shells of foraminifera, 

 chiefly Glohigerina, greatly magnified. 

 Such calcareous deposits extend over 

 large areas of the sea bottom. Repro- 

 duced from The Depths of the Ocean, by 

 Sir John Murray and Doctor Johan 

 Hjort by permission of the Macmillan 

 Company. 



