FRESH-WATER ALG. 259 
of ships with their fleecy coverings, and protecting 
by their soft cushions the floor of the deep from 
the abrasion of the waters. 
Humble and minute although these diatoms 
may be, they are among the oldest of the living 
inhabitants of the globe, having performed their 
part in creation long ages before the first parents 
of the human race were called into existence. 
The wonderful records which they have left be- 
hind them in our rocks carry us back to a period 
when the world, now so beautiful with its verdant 
meadows and waving woods, was one dreary pes- 
tiferous bog, where calamites, sigillarias, and other 
gigantic marsh plants formed intricate jungles, in 
whose damp recesses horrid reptiles roared and 
wallowed, and made war upon each other. In 
the waters of the primeval seas they flourished in 
the greatest profusion, supplying the ultimate food 
of the pleiosauri, ichthyosauri, and the other huge 
reptiles with which they swarmed, just as their 
successors form the basis of subsistence, through 
an amazing series of links, for those mighty de- 
vourers, the whales, the seals, and the walruses of 
the Arctic and Antarctic oceans. The fiery cata- 
clysms, which extirpated whole races of plants and 
animals, left these atomies uninjured ; the physi- 
cal changes going on over the whole earth only 
served to carry them uninjured from one geologi- 
