XXVI INTRODUCTION. 



will thus assist in the resolution of our doubts, and enable us to 

 assign to a form a systematic position and a specific name. 



These remarks, if they do not enable the inquirer to arrive at 

 certainty in the determination of specific characters, will at all 

 events, it is to be hoped, guard him from the rash conclusions 

 and hasty generalizations of the amateur microscopist, who is 

 disposed to rely upon the obvious characteristics of size and 

 form, and to regard such features as important distinctions. 



Among organisms of such simplicity, these latter characters 

 are far from sufficient to establish specific distinctions, and it 

 requires a careful examination of specimens collected in various 

 localities, and in every condition of growth, to enable the observer 

 to fix upon the size that may be taken as the average, or the 

 outline which ought to be regarded as the type. Striation is 

 the best guide; but it sometimes happens that this feature 

 is so obscure, or so alike in allied species of the simpler forms, as 

 Cocconema, Cymbella, or Navicida, that our determinations must 

 be influenced by other considerations, and the arrangement of 

 the endochrome, or the habitat of the living frustule, or even less 

 important considerations, must be taken into account. A neglect 

 of these precautions will lead to the multiplication of synonyms, 

 to vagueness of description, and to a cumbrous and unscientific 

 nomenclature. 



Section XI. 

 On the Distribution and Uses or the Diatomace.e. 



The geographical range of the Diatomaceae is so much more 

 general and uniform than that of the higher orders of plants, 

 that it would seem to be an established fact that many of the 

 commoner species are universally distributed throughout the 

 waters of the globe. There is, however, some difficulty in coming 

 at very certain conclusions on this point in reference to any large 

 number of species, both from the little attention that has hitherto 

 been paid to the subject by the generality of naturalists, and 

 from the imperfect representations given of the forms observed. 



Professor Ehrenberg has indeed discussed the subject on a 



