Cardium. 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. 200 200 100 100 170 60 117 269 430 Recent 34 43 7 Mesozoic.. 34 5 Paleozoic. 42 Conus. Pleurotoma. The numbers representing the several genera in the Cenozoic 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 1 See Morris's excellent Catalogue of British Fossils, 2nd Edit. them at present to only three great periods, we have the following result: Cenozoic ... 27 41 15 8 63 394 662 12 1222 Mesozoic 103 245 65 165 308 499 389 396 2170 Paleozoic 379 225 218 632 196 342 401 336 2729 ... The absolute number of marine species appears thus to be greatest in the Paleozoic 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 variety of life by the relative number of species to be expected on an average in searching a given thickness of strata in each of the great periods. Also since it is known that the species are not uniformly arranged through the deposits of each period, but occupy many distinct stages in each, we may say that on the average, they represent the rate of change in the forms of marine life, or the number of different species to be expected in searching successive equal thicknesses in each of these systems of strata. According to Morris's estimates of thickness (p. 52), the relative numbers would have been 432, 251, 59. Had we instead of the figures which represent the thicknesses and number of species found in Britain employed the data given by D'Orbigny1, counting all the species in all the classes of Mollusca and Radiata, viz. we should have nearly the same result as with Morris's numbers; a nearer approximation of the Mesozoic and Cenozoic ratios, but the same remarkable inferiority of the Paleozoic series. 1 Palæontologie et Geologie, II. Thus it appears certain that the variety of life, estimated by the marine tribes existing in a given period, is greater in the more recent periods; but the number of individuals, or the abundance of life, is not measured by the same proportions. Periods of extraordinary abundance alternate in every great series of strata with other periods of comparative scarcity; and though sometimes this may be explained by the well-known fact that red peroxide of iron in sedimentary strata is very unfavourable to marine invertebral life, while grey protoxidated rocks of the same series contain organic remains in abundance; and sometimes requires attention to the unequal conservative conditions, or originally unequal feeding circumstances of Calcareous, Argillaceous and Arenaceous sea-bottom; still it is a very impressive phenomenon in the continuously grey Cambrians and Silurians, in the continuously grey Carboniferous rocks, in the continuously protoxidated Oolitic strata, and in the almost uniform deposits of Lias, Oxford Clay, Kimmeridge Clay, Gault, London Clay and Barton Clay. An illustration is subjoined from the Lower Palæozoic Strata of Britain. Fig. 3. |