In the second of this series of Essays[7] we indicated the lines of inquiry which must be followed up by any reader who would become acquainted with the natural history of animals and plants. We saw that their gross and minute structure, their very varied functions, their relations to past time, and their geographical relations as well as their relations to the physical forces and to their fellow organisms, would all have to be successively considered. Obviously, however, it is impossible to make known the facts of anatomy, physiology, and hexicology[8] without constant references to animals and plants which may be expected to be either altogether unknown, or at least very incompletely known, to persons as yet unacquainted with zoological and botanical science. References to creatures so unknown or so little known would plainly be of small profit and less interest, unless the reader was already furnished with some mental images of such creatures and groups of creatures--images calculated to sustain his attention and excite his interest in the various kinds of animals and plants, otherwise unknown, which will have to be again and again referred to. Accordingly, an attempt must now be made to set before the reader a rough and general sketch, or catalogue, of what the creatures and groups of creatures are, the names of which will have so frequently to appear in the pages which are to follow. In a word, as the preceding Essay[9] was devoted to explaining what are the special characters of living beings--_i.e._, what the phrase "animals and plants" _connotes_; so the present Essay is intended to explain what that phrase _denotes_. It is not by any means intended at present to place before the reader a definitive and complete system of classification--that task must be reserved for the conclusion of the series, as it will be the expression of all the facts and inferences which will have been in the meantime brought forward. For the purpose now in view it will be well, perhaps, to follow the suggestion of the great naturalist, Buffon, and begin with creatures which are amongst the best known and most familiar, and thence proceed to speak of less and less familiar forms. In this Essay assertions will be freely made as to the natural affinities which the author believes to exist between the creatures to be enumerated, but no attempt will be made to give the reasons for such assertions. The justification of such affirmations will, it is believed, become apparent later, when the organization of living beings shall have been portrayed as far as the space and the ability at the command of the writer may enable him to portray them.