THE DAWN OF LIFE 121 



often of larger size than the pores of the cell-wall, and of 

 greater length, and branched in a complicated manner. Thus 

 there are microscopic characters by which these curious shells 

 can be distinguished from those of other marine animals ; and 

 by applying these characters we learn that multitudes of 

 creatures of this type have existed in former periods of the 

 world's history, and that their shells, accumulated in the bottom 

 of the sea, constitute large portions of many limestones. The 

 manner in which such accumulation takes place we learn from 

 what is now going on in the ocean, more especially from the 

 result of the recent deep-sea dredging expeditions. The 

 Foraminifera are vastly numerous, both near the surface and 

 at the bottom of the sea, and multiply rapidly ; and as suc- 

 cessive generations die, their shells accumulate on the ocean 

 bed, or are swept by currents into banks, and thus, in process 

 of time, constitute thick beds of white chalky material, which 

 may eventually be hardened into limestone. This process 

 is now depositing a great thickness of white ooze in the bottom 

 of the ocean ; and in times past it has produced such vast 

 thicknesses of calcareous matter as the chalk and nummulitic 

 limestone of Europe and the orbitoidal limestone of America. 

 The chalk which alone attains a maximum thickness of 1,000 

 feet, and, according to Lyell, can be traced across Europe for 

 1,100 geographical miles, may be said to be entirely composed 

 of shells of Foraminifera imbedded in a paste of smaller 

 calcareous bodies, the Coccoliths, which are probably products 

 of marine vegetable life, if not of some animal organism still 

 simpler than the Foraminifera. 



Lastly, while we have in such modern forms as the masses 

 of Polytrema attached to corals, and the Loftusa of the 

 Eocene and the carboniferous, large fossil foraminiferal 

 species, there is some reason to believe that in the earlier geo- 

 logical ages there existed much larger animals of this grade 

 than are found in our present seas ; and that these, always 



