BRITISH DIATOMACEiE. 3 



hitherto remained involved in great obscurity, the known facts admitting of 

 nothing but loose sunnises of little scientific value. Some phenomena, 

 however, have come under the observation of Mr. Smith which seem to go 

 far towards a satisfactory determination of this important point. He has 

 rendered it, indeed, almost certain that the contents of the sporangial frus- 

 tule ultimately resolve themselves into a brood of young Diatoms, exactly 

 repeating the form of the original conjugating individuals — an observation 

 which we do not hesitate to say is one of the most important which has of 

 late years been made in the physiology of plants. 



The difficult subject of the determination of species in the Diatomacese, 

 and the selection of the characters on which the systematist must rely in 

 the construction of his natural groups, is well discussed ; and it is shown 

 that mere difference of size, and even slight difference of outline, cannot be 

 depended on as indicating a specific separation of two individuals. The 

 character of the Striation and the arrangement of the Endochrome are the 

 points on which our author chiefly relies ; and in this we entirely agree with 

 him. We cannot, however, quite make out in what rank he would place 

 habitat, when he speaks of it as assisting in the determination of species. 

 If he means that it should be considered as a character forming a "consti- 

 tuent part in a diagnosis, we cannot assent ; but it is probable, after all, 

 that what he really intends to convey is simply that the fact of two given 

 specimens being found, one in the sea, and the other in fresh water, will 

 afford a priori reasons for suspecting a difference of species, and will lead 

 the observer to seek for valid specific characters in the striation and the 

 arrangement of the endochrome. If this be his meaning, we agree with him, 

 and believe his suggestion to be one of much practical value. 



The very wide geographical distribution of the species of Diatomacese is 

 a curious fact in their history. We find them, indeed, scarcely influenced 

 by those external agencies which set such well-marked boundaries to the 

 areas occupied by organic beings generally. The following account of their 

 distribution will give the reader some idea of this feature in their economy : 



" Of fresh-water species frequent in the British Islands, the following seem 

 almost cosmopolitan, viz. : Synedra radians, Pinnularia viridis, Pinnularia borealis 

 and Cocconema lanceolatum. Gatherings from many localities in Europe, from 

 Smyrna and Ceylon, from the Sandwich Islands, New Zealand, and New York 

 from the loftiest accessible points of the Himalaya in Asia, and the Andes in 

 America, have supplied specimens of these forms. 



** Navicula serians abounds in all our mountain bogs, and is equally common in 

 the marshes of Lapland and America. 



" Epiihemia gibba is an inhabitant of the Geysers of Iceland and the lakes of 

 Switzerland. 



'• The South Sea Islands supply Stauroneis acuta, and Ceylon Synedra ulna • 



