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.