PREFACE.

While the Preface forms the first part of every work, asthe name implies, and precedes always the descriptive matter,it is as invariably the last which is written, and the last oftento be read.

Nevertheless there are few portions of a work the endit-ing of which is undertaken with more alacrity than thewriting of this same preface — the author’s mind beingcheered by the near prospect of the completion of his task —it may be of a difficult and an arduous task — in which theobstacles to its successful issue were many, and the result prob-lematical, and the hope of a suitable reward in recompenceof his toil either in the form of approbation, if deserved, ormoney, should the undertaking be of a sufficiently popularcharacter to lead to so substantial a termination.

There is also another reason which renders the writing ofthe preface an agreeable occupation. It is the opportunitywhich it affords to the writer to render to those who haveaided him in his enquiries the acknowledgments which theirliberality and their kindness so strongly claim.

To Sir W. J. Hooker, Dr. Greville, Mr. Borrer , Mr.Ilarvey, and the Kev. M. J. Berkeley, names prominentlyinscribed in the records of the successful cultivators of naturalscience, the pursuit of which is the most peaceful and themost pleasant of all employments, I am deeply indebted.The two former gentlemen liberally placed at my disposaltheir valuable collections of freshwater Alga, and Mr. Harveyand Mr. Berkeley communicated to me numerous uniquespecimens accompanied by valuable remarks.