Many theories, which read plausibly enough, we find, on attempting to apply them, totally at variance with facts: I will, therefore, not content myself with making unsupported assertions, but endeavour to summon to my aid fragments of the great whole, and array them before the reader, in what I consider order, asking of him, as an especial favour, that he will examine and compare the genera and species which I shall mention as related to each other in corroboration of my scheme; for much as I could wish by argument to convince him that a system of circles, grouped in sevens, exists universally throughout nature, yet I should much prefer that, by actual experiment, he should convince himself. With this view I will take a rapid survey of the central class[17] of Insecta, observing in what particulars it is related to those which surround it. I have selected insecta first because I already possessed a slight knowledge of its contents; secondly, because there exists little difference of opinion as to those contents;[18] and, thirdly, because Mr. MacLeay has given it as his opinion "that it is among insects above all other groups of animals, that owing to their myriads of species, the mode in which nature's chain is linked--a mode, the knowledge of which comprises all knowledge in natural history, will be most evident, and therefore most easily detected."[19] [17] I have invariably used the term class, to designate the orders of Linnæus, and sub-class, for the next division, of which seven are supposed to exist in every class: these sub-classes may sometimes constitute natural orders, in which case a plural termination is given; thus, Blatta constitutes in itself a sub-class Blatta, a natural order Blattæ, and a genus Blatta; but generally a sub-class will contain seven natural orders; as sub-class Scarabæus contains natural orders--Lucani, Coprides, Scarabæi, Histeres, &c. [18] The only question as to the contents of insecta, is, whether the pediculi are true insects or not; the class Hemiptera is so closely related to them, that I cannot think it a great violation to place them in the outermost circle of that class; the acari may be supposed meeting them in an adjoining circle, but I have no desire to provoke controversy on this minor point. [19] Annulosa Javanica, preface, p. xi. [Illustration: THE CLASSES OF INSECTA.]