statement by examples taken from the same classes as in the former table, with the exception of Fishes (F), which are here substituted for Cetacea. For comparison with the tables just given of species and genera still living traced backward in time, we may in like manner select species and genera living in the older strata, and trace them upward in the rocks, or forward in time, as in the following examples. There being no Fishes in the * * lower parts of the Palæozoic Strata, Zoophytes are introduced (Z) instead. We have added also a column for the Cephalopoda (Cc). In the same manner we may select genera which occur in the older strata, and trace upward those which have the greatest range in the strata, in other words, the longest duration in time. Again, if we consider not genera but greater groups as families, their extension is greater, their antiquity higher; while some classes of the radiated and molluscous and articulated divisions occur in all the strata. Thus the general result is to shew that only one general plan of creation is to be traced in all the range of geological time, though the terms in which this plan is expressed vary from period to period. There is a succession of the forms of life to be traced in the strata; we shall shew hereafter that the relative proportions of the families, orders, and classes vary so as, in this limited sense, to justify our speaking of different systems or combinations of life; but they are all included in one general plan of the Omniscient mind, comprehending the past, the present, and the future. Perhaps this subject may be yet further illustrated by means of tables shewing on the one hand the numerical distribution in time of some genera of marine animals now living and remarkably prevalent in the sea, and of others as abundant in the more ancient strata. In each case the relative abundance of species is intended to be shewn by the numerals corresponding to each period. I confine the illustrations to the Molluscous animals, because of their being the most uniformly plentiful in all periods, and to the British Islands, as affording the best or most convenient terms of comparison. 32 14 Recent .... 200 200 100 100 170 60 117 269 430 70 350 150 Cænozoic.. 22 34 43 1 3 9 50 32 8 14 The numbers representing the several genera in the Cænozoic groups of strata would bear a much nearer proportion to those of the now living species if we had employed the data of Deshayes, and included European species. The analogy of the Cænozoic to the living marine fauna, and the contrast between these and the older systems of life are especially striking in regard to the Gasteropodous genera placed to the right. VARIETY OF THE FORMS OF LIFE IN SUCCESSIVE PERIODS. If we select among the marine classes of animals those which are represented in all the great periods of Geology, count the number of species yet discovered in them in the British strata1, and refer them at present to only three great periods, we have the following result: 1 See Morris's excellent Catalogue of British Fossils, 2nd Edit. Cænozoic 27 41 15 8 63 394 662 12 1222 The absolute number of marine species appears thus to be greatest in the Palæozoic Strata; but when the thickness of the deposits-which represents elapsed time is taken into account, the variety of forms in a given thickness or given period of time is very much less. For if we take account of the thickness according to the maximum scale for Britain proposed by Ramsay, on data collected by the Geological Survey, the following table may be constructed : The ratio thus found by dividing the number of species by the maximum thickness, represents. the |