(136.) It has been shown, in the preceding chapter, that there are three modes by which the objects of nature may be classified; and that one of these — that is, the natural system — is alone conducive to the advancement of natural history as a physical science. To this, therefore, we shall hereafter confine our attention; because the principles of this science must be discovered by a similar series of inductive generalisations to those used in every department of natural philosophy, “through which one spirit reigns, and one method of enquiry applies.”

(137.) Let us suppose, then, that an entomological student, with a well-filled cabinet of unarranged insects, having his mind well stored with those simple facts regarding their structure and economy which he is to look upon as solid data — let us suppose him to commence the arrangement of the objects before him, according to what he thinks their true affinities, and with a view of verifying or discovering their natural arrangement. He commences by placing, one after the other, those species which bear the greatest mutual resemblance; and for a time he proceeds so satisfactorily, — he finds the several links of the chain, as it were, fit into each other so harmoniously, — that he begins to think the task much easier than he at first expected; and that he will not only be able to prove, by these very examples before him, the absolute connection of one given genus to another, but also to demonstrate that the scale of nature is simple — that is, passing in a straight line from the highest to the lowest organised forms.