of their development, and is explicable by extinction in mass. A Beetle or a Butterfly is to be recognised as such at the first glance, but only a thorough investigation can demonstrate the mutual relationships of _Termes, Blatta, Mantis, Forficula, Ephemera, Libellula,_ etc. I may refer to a corresponding remarkable example from the vegetable world: amongst Ferns the genera _Aneimia, Schizæa_ and _Lygodium,_ belonging to the group _Schizæaceæ_ which is very poor in species, differ much more from each other than any two forms of the group _Polypodiaceæ_ which numbers its thousands of species. If, from all this, it seems right to regard the Orthoptera as the order of Insects approaching most nearly to the common primitive form, we must also expect that their mode of development will agree better with that of the primitive form, than, for example, that of the Lepidoptera, in the same way that some of the Prawns (_Penéus_) approaching most closely the primitive form of the Decapoda, have most truly preserved their original mode of development. Now, the majority of the Orthoptera quit the egg in a form which is distinguished from that of the adult Insect almost solely by the want of wings; these larvæ then soon acquire rudiments of wings, which appear more strongly developed after every moult. Even this perfectly gradual transition from the youngest larva to the sexually mature Insect, preserves in a far higher degree the picture of an original mode of development, than does the so-called complete metamorphosis of the Coleoptera, Lepidoptera, or Diptera, with its abruptly separated larva-, pupa- and imago-states. The most ancient Insects would probably have most resembled these wingless larvæ of the existing Orthoptera. The circumstance that there are still numerous wingless species among the Orthoptera, and that some of these (_Blattidæ_) are so like certain Crustacea (Isopods) in habit that both are indicated by the same name (“_Baratta_”) by the people in this country, can scarcely be regarded as of any importance. The contrary supposition that the oldest Insects possessed a “complete metamorphosis,” and that the “incomplete metamorphosis” of the Orthoptera and Hemiptera is only of later origin, is met by serious difficulties. If all the classes of Arthropoda (Crustacea, Insecta, Myriopoda and Arachnida) are indeed all branches of a common stem (and of this there can scarcely be a doubt), it is evident that the water-inhabiting and water-breathing Crustacea must be regarded as the original stem from which the other terrestrial classes, with their tracheal respiration, have branched off. But nowhere among the Crustacea is there a mode of development comparable to the “complete metamorphosis” of the Insecta, nowhere