BBITISH FEESH-WATEB 



INTRODUCTION. 



" Fresh-Water Algee J: is confessedly an artificial arrange- 

 ment which demands apology, but can scarcely receive 

 justification. The -only excuse which can be offered is, that it 

 serves the purpose of those for whom the present work was 

 written, namely, the Microscopists who desire some acquaint- 

 ance with the organisms met with in their excursions to ponds 

 and ditches. For the absolutely scientific algologist it will 

 only be fragmentary, although it is by no means a solitary in- 

 stance in which the Fresh-Water Algaa have been made the 

 sole subject of a book, to the exclusion of marine species. 

 Indeed, the restriction of Harvey's " Phycologia Britannica ' 

 to marine algse, with but very few exceptions, left at least an 

 excuse for attempting to supply the deficiency. 



The historical portion of this introduction may be speedily 

 summarized by dividing it into three epochs of about forty years 

 each, the first being limited by the publication of Dillwyn's 

 " Confervas," the second by Hassall's " Fresh-Water Alga3," 

 and the third by the present work. 



Prior to the first epoch there were but two works of sufficient 



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