BRITISH FRESH-WATER ALG. 
INTRODUCTION. 
“ Fresh-Water Alge’ 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 Algw 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 alge, 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 
‘“‘ Conferve,” the second by Hassall'’s “ Fresh- Water ey 
and the third by the present work. 
Prior to the first epoch there were but two works of sufficient 
