on the surface, we are still without a shadow of evidence that this interior heat has exercised the least perceptible effect on the climatology of the globe since the deposition of the fossiliferous strata.* On the contrary, all that we know of the nature and thickness of pre-Cambrian sediments, and all that we have learned of the Cambrian rocks themselves, preclude the supposition of such an influence beyond the most infinitesimal degree, and compel us to believe that the physical conditions of life have been much the same throughout every period of its existence. It is true that during the Carboniferous period, the Oolite, and the earlier. Tertiaries, certain latitudes of the northern hemisphere appear to have enjoyed a more genial climate than they do now; but the explanation of this we are to seek in the varying distributions of sea and land, the existence of warmer oceanic currents and other geographical conditions, rather than in any perceptible influence derived from the earth's interior. Nay, as we have warmer and colder regions in space, in virtue of the earth's relations to the solar system, so we are inclined to believe we have had warmer and colder periods in time, in virtue of some great but unknown cosmical law. The existence of the Glacial or boulder epoch over the greater portion of the northern hemisphere (say up to the 40th or 42d parallel of latitude) is now admitted on all hands; and as we cannot entertain the idea of * According to Fourier, Hopkins, and other physicists, the internal heat of the globe, which increases at the rate of 1 degree for every 60 feet of depth, does not at present affect the mean superficial temperature more than 1-20th of a degree; and to have had any sensible effect on external climates-say to the exent of 10 degrees-this interior heat must have been two hundred times its present amount. At that rate the melting point of lavas would have been reached at a depth of 580 feet, instead of 116,000 feet as presently estimated, and all the deeper seated strata must have been fused or rendered crystalline-a condition in which they do not occur even to the depth of 30,000 feet, as many of the Cambrian and Silurian slates are merely hardened and cleaved, but in no degree metamorphic. cataclysmal irregularity in Creation, so we are led to infer the prior occurrence of such glacial periods at determinate times and over determinate areas. The existence of such glacial recurrences has been surmised by several geologists as characterising the periods of the old red sandstone and Permian;* and we may venture to extend them to other systems as probable features of a great cosmical plan. Thus, looking at the Cambrian strata of the northern hemisphere-their angular grits and conglomerates, their extreme paucity of fossil forms, and other features—we are at once reminded of the action of ice and the presence of ungenial conditions. This is followed over the same areas by the more genial and exuberant period of Siluria; which is in turn succeeded by the old red sandstone, whose grits and bouldery conglomerates, as well as paucity of vegetable forms, once more suggest the recurrence of colder influences. Following the old red we have the exuberant flora and fauna of the coal period, again to be succeeded by the scanty life-forms and grits and conglomerates of Permia. Again, the trias and oolite of the northern hemisphere are characterised by life-forms that betoken warm and genial conditions; while the chalk that succeeds imbeds water-worn blocks of granite and lignite, which would seem to imply the presence of ice-drift and deposit in seas that were open to boreal influences. Next the early tertiaries occur over the same areas, marked by plants and animals that indicate a warm and genial climate; and this in turn gives place to the well-known glacial or boulder-drift epoch; once more to be succeeded by the milder influences of the post-tertiary or current era. Throwing these recurrences into diagrammatic form, we appear to have had an alter * Mr Cumming, in History of the Isle of Man (1848); Mr GoodwinAusten, Geological Journal (1850); Professor Ramsay, Ibid. (1855); and the author, in his Advanced Text-Book. nation of colder and warmer cycles over the northern hemi sphere, at least; and if such has really been the case, we must seek the explanation not in revolutions and cataclysms, but in some fixed and continuously operating law.* Whether the phenomena may depend on operating on and within the globe itself, so as to change the axis of rotation, or whether it may not more causes *This idea of colder and warmer cycles as affecting the northern hemisphere was indicated some years ago to the Literary and Philosophical Society of St Andrews. Since then the author has endeavoured to establish the fact, partly by the character and composition of the rocks of the colder periods, and partly by the nature of their fossil contents. Much, however, still remains to be done, and he would earnestly solicit the attention of geologists to the subject, and this altogether apart from the cosmical causes to which the recurrences may be due. On this latter aspect of the question some discussion took place in the Athenæum of 1860, on the suggestion of Colonel James of the Ordnance Survey, that former changes of climate may be due to changes in the inclination of the earth's axis, brought about by alterations in the crust that gradually affect the centre of gravity. Whatever the cause-whether it is to be sought for on or within the globe itself, or in purely astronomical influences-this is not the place to discuss; but most unmistakably the gradual uprise of land that now taking place in the arctic regions, the shifting of volcanic areas in the northern hemisphere since the tertiary period, and the approach and departure of the boulder epoch over the same latitudes, all point to the operation of some determinate law of secular succession. May it Current. Tertiary. Boulder Drift. Old Red. Permian. Cambrian. COLDER CYCLES. not be, that in the periodicity of this law we may yet discover the key to the expression of geological chronology in years and centuries? Enough likely depend on forces purely astronomical, are questions that lie beyond the scope of the present Sketch. for our purpose to have indicated the probability of such recurrences, and derive therefrom the conclusion that the conditions of life have been very much the same through all geological periods-successively varying in different areas, but never presenting, any more than they do now, a universal similitude, and that least of all through the influence of the earth's internal temperature. [Introduction of New Life-forms.] As each geological epoch is characterised by its own peculiar plants and animals, the question naturally arises, Whether these are independent creations, or whether there is in nature some law of development by which, during the lapse of ages and under the change of physical conditions, the lower may not be developed into the higher species, and the simpler into the more complex? On this topic much has been said and written, but after all, geology is not in a position to solve the problem of vital gradation and progress. It cannot tell, for instance, why trilobites should have flourished so profusely during the silurian epoch and died out before the deposition of the oolite; why chambered cephalopods, like the ammonite, should have come to their meridian, as it were, during the liassic era; reptilian life during the oolite and chalk; or why mammalian development should have been reserved to the tertiary and current epochs. It cannot explain why the palæotherium should not continue to inhabit the same forest with the tapir of South America, or the ichthyosaurus gambol in the same waters with the alligator of the Amazon. It can discover no physical condition in the oolitic seas to have prevented the continuance of trilobites; nothing in the geography or climate of the coal period to have prevented the huge terrestrial reptiles of the Weald from browsing on its vegetation, or marine species, like those of the lias, from preying on its fishes. The appearance and preponderance of certain races during certain geological epochs is a problem which lies as yet beyond the solution of science. That this succession occurs regularly as regards time, space, and biological sequence, we clearly perceive; but how, or by what means of causation, we are altogether unable to determine. We can often trace the extinction of races to a change of external condition; and as vitality is endowed with a certain amount of elasticity and adaptability, we may account for modifications within the limits of what naturalists term varieties; but we appeal in vain to physical conditions for the first introduction or creation of species. In the PAST LIFE of the globe we only see dimly and broadly the outline of a great scheme of gradation and progress-a progress on which we may rest as a matter of FAITH, but the terms of whose LAW lie far, as yet, beyond the grasp of exact scientific demonstration. In vain we turn to "external conditions" and "unlimited time;" to the doctrines of "embryology" and "morphology;" or to "natural selection in the struggle for existence." These are oracles to which theorists have often appealed, but they fail, as yet, to utter an intelligible response. That each of them has some portion of the mystery in keeping, all the tendencies of modern science do, no doubt, appear to indicate, but how much, and in what order of connection, our highest determinations are little better than a train of ingenious guess-work. As far as geological evidence goes, all the great types of life began simultaneously and independently. All the subsequent introductions of new genera |