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:--