DIATOMS. 



403 



invisible to the naked eye, they appear, when thus congregated, 

 as patches of a green or brownish slimy mass, or as little glitter- 

 ing tufts a line or two in height. Some of their numerous 

 species are natives of fresh water, but by far the majority are 

 denizens of the sea, where 

 they are found from the 

 equator to the poles. The 

 brown scum floating upon 

 the surface of the antarc- 

 tic waters near the mighty 

 ice barrier which arrested 

 Sir James Eoss's progress 

 to the south pole was 

 found to consist almost 

 solely of Diatoinaceas, and 

 they are equally abundant 

 in the Arctic Ocean. 



It is remarked by Dr. 

 Hooker that the univer- 

 sal presence of this invi- 

 sible vegetation through- 

 out the South Polar Seas 

 is a most important fea- 

 ture, since there is a marked deficiency in this region of higher 

 forms of vegetation, so that without the Diatoms there 

 would neither be food for aquatic animals nor (if it were 

 possible for these to maintain themselves by preying on one 

 another) could the ocean waters be purified of the carbonic 

 acid which animal respiration would be continually imparting 

 to it. Thus it is not in vain that they abound in the most 

 inhospitable seas, where but for them no sea-bird would flap 

 its wings, and no dolphin dart through the desert waters. 



From the indestructible nature of their flinty coverings the 

 Diatoms play a no less conspicuous part in the geological his- 

 tory of our globe than the calcareous Foraminifera. 



Extensive rocky strata, chains of hills, beds of marl — once de- 

 posited at the bottom of the ocean, and raised by subsequent 

 changes of level from the depth of the waters — contain the 

 remains of these little plants in greater or less abundance. No 

 country is destitute of such monuments, and in some they con- 



E E 



Licmophora flabellata. 

 (Highly magnified.) 



