lv INTRODUCTION. 
the magnification employed. That there are faults no one will 
deny ; but, on the whole, we are not prepared to condemn it as 
unworthy of the time at which it appeared. A comparison of 
the Desmids with those in Ralfs’ work, of but three years 
later, will show that in execution something was left to be 
desired. 
The third epoch is one on which we must necessarily be very 
brief ; coming so near our own time we must be content to in- 
dicate what has been done, and leave conclusions to others. 
Closer relations with the Continent, cheap postage, more general 
acquaintance with foreign works, all tend to raise greater expec- 
tations for the closing work of the third epoch than of its pre- 
decessors. The works of Kutzing, the Memoirs of Pringsheim, 
De Bary, Cohn, Bornet, Thuret, Borzi, Wittrock, and many 
others, all contribute to illustrate British Fresh-Water Alge ; 
and although during forty years very little has been done in our 
own island, even in the identification of species, there has been 
considerable activity in investigation, especially in the North of 
Europe. The scattered memoranda, notes and observations of 
Professor Henfrey, Dr, Braxton Hicks, and Mr. W. Archer 
constitute the bulk of our home manufacture of the literature of 
Fresh-Water Algz for about 80 years. The later portions of 
the “ Supplement to English Botany,” containing Alge, date 
from 1843 ; and Harvey’s second Edition of the ‘‘ Manual,” in 
1849, was wholly confined to Marine species. Hence there is 
not an independent work on British Fresh-Water Alge belonging 
to this third epoch, the only contributory work being Berkeley’s 
“Introduction to Cryptogamic Botany,” in 1857. Butif there 
was an extraordinary dearth of books on this subject after 1845 
in Britain, such was not the case on the Continent. The con- 
secutive publication of the volumes of Kutzing’s “ Tabule ” 
must have been an important influence, although it was not 
until 1864 that Rabenhorst’s ‘‘ Flora Europea Algarum, Aque 
Dulcis et Submarine” was commenced. The advent of this 
work was hailed with pleasure, notwithstanding its many faults ; 
and various authors set themselves to work on different genera 
and families, such as Aldogoniaceer, Zygnemaceex, &c., so that 
in twenty years it is left far behind, As a work written in Eng- 
lish, although not containing much original observation, we 
