176 VIGNETTES FROM INVISIBLE LIFE. 



valuable as fertilising agencies. In the arts and 

 manufactures they are brought into use as polish- 

 ing powder, as the finest material for delicate 

 metallic castings ; and in the manufacture of porce- 

 lain they are invaluable. 



As indicating the value and importance of the 

 DiatomacecB from a natural standpoint, or the value set 

 upon these, her favourite jewels, by Dame Nature, w r e 

 may mention the singular fact, that although found 

 in a fossil condition in connection with the dawn of 

 life on the globe, yet, in the words of Dr. Gregory, 

 " We have no evidence that any species of Diatom 

 has become extinct, as so many species, and even 

 genera and tribes, of more highly organised beings 

 have done." It is not pretended by these remarks 

 that no new species have been produced, the reverse 

 being well known, no form of life exhibiting greater 

 varieties but only as indicating their importance 

 in that, through all the mutations of other forms 

 of life, while they come and go, the Diatom, though 

 evolving an almost infinite variety through thou- 

 sands or millions of years, has been preserved, in all 

 its original specific characters to the present time. 



The multitudinous variety of figure and sculp- 



