INTRODUCTION. v 
must mention Dr. Horatio Wood’s “ Fresh-Water Alge of 
North America,” published in 1872. The period selected, 
therefore, for a new work on the “ British Fresh-Water Alga ” 
was in all respects opportune, but we are not expected to judge 
whether good use has been made of the opportunity. 
The gradual accession to knowledge of the structure and 
development of Alge, during the past century, would extend 
this chapter far beyond the limits we have assigned to it, 
whereas an intimation of the present position is generally indi- 
cated under the several groups, in the body of the work, with 
some reference to the sources of further information. 
Classification must, of course, depend very largely upon 
investigation, and modifications necessarily follow new facts, so 
that, whilst adopting suggestions made by authors of recent 
monographs, our arrangement is not absolutely that of any 
other work, although as far as possible the orders and larger 
groups have not been disturbed. We will not venture to pre- 
dict what will be the future of such families as the Palmellacee, 
Protococcacee, and Chroococcacee during the next twenty years; 
probably they will disappear altogether, becoming merged into 
other groups of which most of them may be demonstrated to be 
only imperfect conditions. At the present time we could follow 
no other course than to include them, and leave the future to 
take care of itself. | We have preferred including too much to 
excluding too much, in confidence that for this we should be 
the more readily forgiven. 
The student will expect to find in this place some practical 
hints to guide him in his pursuit, but these must be brief, as 
experience, combined with a little personal contact with old 
hands at field-work, will supersede a volume of precepts. 
The habitats attached to the various species will indicate 
the kind of locality in which to search for them, but it may be 
remarked that, generally, any slow stream, pond, or standing 
pool will furnish something. Rapid currents will not repay the 
search. Small pools on moors, amongst Sphagnum, will usually 
prove prolific, and submerged plants, especially when dead or 
dying, often serve as a habitat for attached species. Damp 
rocks, down which the water trickles, old quarries, damp walls 
