In the first of the present series of Essays it was pointed out[4] that the number of kinds of living creatures is so prodigious that it would be a hopeless task for any man to attempt to grasp the leading facts of their natural history, save with the help of a well-arranged system of classification. Such a system enables the student to consider the subjects of his study collectively in masses--masses arranged in a series of groups, which are successively smaller and more and more subordinate. By "subordinate groups" are meant groups which are successively contained one within the other. As an example of such subordinate grouping we may take the group of familiar objects denoted by the word "money." This group contains within it the large subordinate groups, "paper money" and "metallic money;" the latter group again contains the more subordinate and smaller groups, "gold money," "silver money," and "copper money," and these respectively contain still more subordinate and smaller groups. Thus, the group "silver money" contains the subordinate groups--(1) crowns, (2) half-crowns, (3) florins, (4) shillings, (5) sixpences, &c.; and any one of these (_e.g._, shillings) is further divisible into groups of "shillings" of the coinage of different reigns.
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.