Reversing the process we may, as another illustration, select the group of articles of furniture called "chairs," which (with other co-ordinate groups, such as "tables" and "sofas") is contained within, and is subordinate to, the larger group of objects, "wooden furniture." This latter and larger group is again classifiable (together with its co-ordinate group, "metal furniture") in the yet higher and larger group of "furniture made of hard material," to which the wooden and metal groups are both subordinate. Co-ordinate with the group of "hard material" we have another group (carpets, curtains, &c.) of "furniture of soft material," and these two groups are again subordinate to the largest group of all "furniture."
It was also pointed out in the introductory Essay[5] that there are two kinds of classification, one artificial, the other natural—the latter (the kind aimed at in this Essay) being such a system of classification as leads to the association together in groups, of creatures which are really alike and which will be found to present a greater and greater number of common characters the more thoroughly they are examined.
The system of classification which zoologists and botanists adopt is a system founded upon the form, structure, number, and relations of the parts of which each living being consists. It is, therefore, a morphological system, and rests rather upon the appearances of parts and organs than upon the offices which such parts and organs fulfil. It rests, that is to say upon their forms, not upon their functions.
The mode in which animals have been arranged in zoological grouping affords an exceptionally good model for classification generally, as has been noted by the late John Stuart Mill.[6] In fact, the number of subordinate groups is very great in zoology. Thus, the kingdom of animals is subdivided into a certain number of very large groups, called sub-kingdoms. Each sub-kingdom is again divided into subordinate groups termed classes. Each class is again divided into still more subordinate groups called orders. Each order is again divisible into families; each family into genera, and each genus into species, while a zoological "species" may be provisionally defined as "a group of animals which differ only by inconstant or sexual characters."
It could be wished that the reader should pursue his further inquiries into the natural history of animals and plants, with a knowledge of biological classification already acquired. But this is, unfortunately, impossible, since biological classification reposes upon anatomical facts, and cannot, therefore, be really understood until the main facts of anatomy have been already mastered. Yet something in the way of a classification, or at least of a definitely arranged catalogue, must be even now attempted for the following reason:—
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.
As before said the object now in view is to endeavour to present a general view of living beings—of animals and plants—in the hope of fixing in the reader's memory the names of species, and of groups of species, to which names reference will have to be more or less frequently hereinafter made. At the least, such a catalogue may serve for reference whenever the reader may come upon the names of animals or plants, or of groups of animals or plants, the meanings of which names may have escaped his recollection.