In one sense, indeed, this is true. We separate the living Cockroaches from other kinds of Orthoptera as a “family” group, and “Cockroaches” have existed since the Coal Measures at least; yet the structure of every one of the older types is really so peculiar that none of them can be brought within the limits of the family as it now exists. We recognise ours, indeed, as the direct descendants of the ancient forms, but so changed in structure as to form a distinct group. A parallel case is found in the Walking-sticks, and is even more obvious. The recent researches of M. Charles Brongniart have brought to view a whole series of forms in Carboniferous times, which are manifestly the progenitors of living Walking-sticks, with their remarkably long and slender stick-like body, attenuated legs, and peculiar appendages at the tip of the abdomen. Existing forms are either wingless or else have opaque elytron-like front wings, and very ample, gauzy, fan-like hind wings; while the Carboniferous species are furnished with four membranous wings, almost precisely alike, and so utterly different from those of existing types that, before the discovery of the bodies, these wings were universally classed as the wings of Neuropterous Insects (sensu Linneano). Thus Gerstaecker, in the very place already quoted, says of these same wings, known under the generic name _Dictyoneura_, that they show at least a very close relationship to the _Ephemeridæ_ of to-day. One principal difference here alluded to--the exact resemblance, except in minor details, of the front and hind wings, and, as consequent therewith, _equal diaphaneity in both_--is found indeed in all palæozoic insects, with exceedingly few exceptions;[196] it is one of their most characteristic and pervading peculiarities. It marks one phase of the movement in all life from homogeneity to heterogeneity--from the uniform to the diverse. In the Cockroaches of to-day a few are found in which the tegmina are nearly as diaphanous as the hind wings; but in the great mass the texture of the tegmina, as in Orthoptera generally (excepting most Gryllides), is decidedly coriaceous; and in some, _e.g._, _Phoraspis_, the veins are nearly obliterated in the thickness and opacity of the membrane, so as to resemble many Coleopterous elytra. [196] A few elytra of Coleoptera are recently announced from the Silesian “culm.” Three principal differences have been noticed between the ancient and modern forms of Cockroaches. Doubtless others could be found were we able to compare the structure of all parts of the body; and perhaps future research and more happy discovery may yet bring them to light; at present, however, we are compelled to restrict our comparisons to the wings alone.