Fossil insects are unfortunately rare, there being but few strata in which the remains of this group are well preserved. Moreover, well-characterized Orthoptera and Neuroptera occur as early as the Devonian strata; Coleoptera and Hemiptera in the Coal-measures; Hymenoptera and Diptera in the Jurassic; Lepidoptera, on the contrary, not until the Tertiary. But although it appears from these facts that, as far as our present information goes, the Orthoptera and Neuroptera are the most ancient orders, it is not, I think, conceivable that the latter should have been derived from any known species of the former; on the other hand, the earliest known Neuroptera and Orthoptera, though in some respects less specialized than existing forms, are as truly, and as well characterized, Insects, as any now existing; nor are we acquainted with any earlier forms, which in any way tend to bridge over the gap between them and lower groups, though, as we shall see, there are types yet existing which throw much light on the subject. In the consideration then of this question, we must rely principally on Embryology and Development. I have already referred to the cases in which species, very unlike in their mature condition, are very similar one to another when young. Haeckel, in his “Naturliche Schöpfungsgeschichte,” gives a diagram which illustrates this very well as regards Crustacea. Pls. 1-4 show the same to be the case with Insects. The Stag-beetle, the Dragon-fly, the Moth, the Bee, the Ant, the Gnat, the Grasshopper,—these and other less familiar types seem at first to have little in common. They differ in size, in form, in colour, in habits, and modes of life. Yet the researches of entomologists, following the clue supplied by the illustrious Savigny, have proved, not only that while differing greatly in details, they are constructed on one common plan; but also that other groups, as for instance, Crustacea (Lobsters, Crabs, &c.) and Arachnida (Spiders and Mites), can be shown to be fundamentally similar. In Pl. 4 I have figured the larvæ of an _Ephemera_ (Fig. 1), of a _Meloë_ (Fig. 2), of a Dragon-fly (Fig. 3), of a Sitaris (Fig. 4), of a _Campodea_ (Fig. 5), of a _Dyticus_ (Fig. 6), of a Termite (Fig. 7), of a _Stylops_ (Fig. 8), and of a _Thrips_ (Fig. 9). All these larvæ possess many characters in common. The mature forms are represented in the corresponding figures of Plate 3, and it will at once be seen how considerably they differ from one another. The same fact is also illustrated in Figs. 48-55, where Figs. 48-51 represent the larval states of the mature forms represented in Figs. 52-55. Fig. 48 is the larva of a moth, _Agrotis suffusa_ (Fig. 52); Fig. 49 of a beetle, _Haltica_ (Fig. 53); Fig. 50 of a Saw-fly, _Cimbex_ (Fig. 54); and Fig. 51 of a Centipede, _Julus_ (Fig. 55).