XXV111 INTRODUCTION. 



have been carefully examined. During a tour in Languedoc 

 and the Auvergne, in the spring of 1854, I made upwards 

 of forty gatherings from the rivers, streams and lakes of the 

 district I traversed ; in these I detected one hundred and thirty 

 species, described in the present work, and but one form not 

 yet determined as indigenous to Britain. If this be the case 

 with a district, much of whose Phanerogamous flora is so dif- 

 ferent from our own, it bears out the view I have taken, that 

 these organisms enjoy a range of distribution far more general 

 than the higher orders of plant-life. 



Nor is the distribution of marine species less notable for its 

 extent and uniformity. Coscinodiscus eccentriciis and Coscino- 

 discus radiatus range from the shores of Britain to those of 

 Southern Africa. Grammatophora marina and Grammatophora 

 macilenta are found in almost every marine gathering from the 

 Arctic Ocean to the Mauritius. Stauroneis pulchetta, Cocconeis 

 Scutettum, and Biddulphia pulchetta are equally abundant on the 

 European, the American, and the African coasts ; while Bhabdo- 

 nema Adriaticum belies its name by its occurrence in the Indian, 

 Atlantic, and Pacific Oceans. During the researches already 

 mentioned in the South of Prance, I made several prolific 

 gatherings on the shores of the Gulf of Lyons ; but of thirty- 

 three forms occurring in these, Hyalosira delicatula, Kutz., was 

 the only one not familiar to me as a British species. 



Of the purposes served by the wide diffusion of these organ- 

 isms it is impossible to speak with certainty. Their minute 

 size forbids us to attribute much effect to their individual influ- 

 ences ; but when we regard them as aggregated in numbers that 

 defy enumeration, we are compelled to believe that they occupy 

 an important place and subserve necessary ends. Their nutritive 

 process, which involves the absorption of carbonic acid and the 

 extrication of oxygen, must tend to preserve the purity of the 

 water in which they are found, and to prepare it for the respira- 

 tion of aquatic animals. Their presence in the stomachs of In- 

 fusoria, Annelida, Mollusca, and Crustacea, shows that they con- 

 stitute to some extent the food of these animals ; and the vast 

 numbers of their siliceous valves which occur in Guano, prove 

 that they are swallowed in large quantities by birds, and minister 



