92 



hold and carried down to the valleys, or mouth of the stream. 

 This is the reason why we often find mountain species associated 

 with those growing in valleys, or, in some cases, even with brack- 

 ish and marine species. Many of these freshwater species will 

 undoubtedly exist for a sliprt time in brackish or marine localities, 

 often becoming much changed in their characters. It is ex- 

 tremely likely that this fact has sometimes led to mistakes by 

 inexperienced observers, and the consequence has been the erec- 

 tion of these varieties into species. Again, brackish or marine 

 species may be carried by tidal or other influences into fresh local- 

 ities, and a total change of characters result. At the present 

 time I am engaged in experimenting on this subject, and shall, I 

 hope, be able before long to lay the results before microscopists. 

 Professor Walker- Arnott has stated some curious facts, relating 

 to this subject, in the April number of the London Microscopical 

 Journal for this year (1859). It would, however, be out of 

 place for me to enter any farther into the discussion of this sub- 

 ject in the present paper, but I recommend the student to bear 

 these facts in mind when examining doubtful forms. 



Brackish species are to be looked for in swamps and marshes 

 along the coast. Whether there exist such things as true brack- 

 ish species is, to me, a matter of doubt ; such as are called so 

 being, in my opinion, either marine forms which have been car- 

 ried by the tide into their new locality, (which is the most likely,) 

 or freshwater species brought down by streams from higher 

 ground. Thus, I have found in a salt marsh in New Jersey a 

 form which looks to me like a variety of Pinnularia viridis, a 

 true freshwater species. In the same marsh, even in the least 

 salt portions, I have found many fine species, as Amphiprora 

 vitrea, Pinnularia per^egrina, Navicula. elegans, and many others, 

 in large quantities, forming masses of considerable extent, and 

 floating on the surface, buoyed up by the numerous bubbles of 

 oxygen set free from the water by the action of the sun on the 

 Diatoms. All of these are generally considered as marine spe- 

 cies. Submerged species are to be found in brackish water in 

 the same positions as in fresh. 



In the ocean, very few species are to be found floating on the 

 surface, except in rock pools, but are found in the stomachs of 

 certain microphagists. The filamentous forms, on the contrary? 



