INTRODUCTION. XXIX 



to their sustenance. Their direct uses to man are probably 

 few and unappreciable, except in the mechanical arts, where, as 

 I have before stated, the powdery debris of their frustules, from 

 its siliceous character, is profitably employed as a polishing 

 material : indirectly they doubtless contribute to the fertility of 

 the soil, and promote the growth of many of the cereal grains 

 which furnish the human family with the farinaceous elements 

 of food. This is the more probable, if it be true, as Ehrenberg 

 and other writers have asserted, that moist ground everywhere 

 exhibits the presence of these organisms. A singular instance 

 in illustration of this has been mentioned to me by Dr. Gregory, 

 who states, that upon examining the particles of earth adhering 

 to the roots of plants collected in various and widely separated 

 localities, he almost invariably detected Diatomacese. 



The result of the presence of these forms in vegetable mould 

 must be the extrication of silica from the fluids in the soil, and 

 its deposition after the death of the Diatomaceae, in another 

 condition, that may, in some unexplained manner, minister to 

 the healthy growth of larger plants. 



But even should we fail in our attempt to explain the precise 

 objects of the Diatomaceae in the economy of being, we may 

 rest assured that organisms of such infinite variety have not 

 been formed in vain ; and that the time we may occupy in the 

 study of their functions, or in the admiration of their marvellous 

 symmetry, will not have been idly spent, if it enhance our con- 

 ceptions of creative skill, and strengthen our persuasion of the 

 omnipotent power and diffusive energy of the divine Artificer 

 of Nature. 



