122 



NA TURE 



[August 4, 1892 



As might be expected, the activities involved in the lowering 

 of the surface of the land are not everywhere equally energetic. 

 They are naturally more vigorous where the rainfall is heavy, 

 where the daily range of temperature is large, and where frosts 

 are severe. Hence they are obviously much more effective in 

 mountainous regions than on plains ; and their results must 

 constantly vary, not only in different basins of drainage, but 

 even, and sometimes widely, within the same basin. Actual 

 measurement of the proportion of sediment in river water shows 

 that while in some cases the lowering of the surface of the land 

 may be as much as ^\-^ of a foot in a year, in others it falls as 

 low as iT^Vo- In other words, the rate of deposition of new sedi- 

 mentary formations, over an area of sea-floor equivalent to 

 that which has yielded the sediment, may vary from one foot in 

 730 years to one foot in 6,800 years. 



If now we take these results and apply them as measures of 

 the length of time required for the deposition of the various sedi- 

 mentary masses that form the outer part of the earth's crust, we 

 obtain some indication of the duration of geological history. On 

 a reasonable computation these stratified masses, where most 

 fully developed, attain a united thickness of not less than 100,000 

 feet. If they were all laid down at the most rapid recorded rate 

 of denudation, they would require a period of seventy-three 

 millions of years for their completion. If they were laid down 

 at the slowest rate they would demand a period of not less than 

 680 millions. 



But it may be argued that all kinds of terrestrial energy are 

 growing feeble, that the most active denudation now in progress 

 is much less vigorous than that of bygone ages, and hence that 

 the stratified part of the earth's crust may have been put together 

 in a much. briefer space of time than modern events might lead 

 us to suppose. Such arguments are easily adduced and look 

 sufficiently specious, but no confirmation of them can be 

 gathered from the rocks. On the contrary, no one can thought- 

 fully study the various systems of stratified formations without 

 being impressed by the fulness of their evidence that, on the 

 whole, the accumulation of sediment has been extremely slow. 

 Again and again we encounter groups of strata composed of thin 

 paper-like laminae of the finest silt, which evidently settled down 

 quietly and at intervals on the sea bottom. We find successive 

 layers covered with ripple-marks and sun-cracks, and we recog- 

 nize in them memorials of ancient shores where sand and mud 

 tranquilly gathered as they do in sheltered estuaries at the present 

 day. We can see no proof whatever, nor even any evidence 

 which suggests, that on the whole the rate of waste and sedi- 

 mentation was more rapid during Mesozoic and Palaeozoic time 

 than it is to-day. Had there been any marked difference in 

 this rate from ancient to modern times, it would be incredible 

 that no clear proof of it should have been recorded in the crust 

 of the earth. 



But in actual fact the testimony in favour of the slow accumu- 

 lation and high antiquity of the geological record is much stronger 

 than might be inferred from the mere thickness of the stratified 

 formations. These sedimentary deposits have not been laid 

 down in one unbroken sequence, but have had their continuity 

 interrupted again and again by upheaval and depression. So 

 fragmentary are they in some regions, that we can easily demon- 

 strate the length of time represented there by still existing sedi- 

 mentary strata to be vastly less than the time indicated by the 

 gaps in the series. 



There is yet a further and impressive body of evidence fur- 

 nished by the successive races of plants and animals which have 

 lived upon the earth and have left their remains sealed up within 

 its rocky crust. No one now believes in the exploded doctrine 

 that successive creations and universal destructions of organic 

 life are chronicled in the stratified rocks. It is everywhere ad- 

 mitted that, from the remotest times up to the present day, there 

 has been an onward march of development, type succeeding type 

 in one long continuous progression. As to the rate of this 

 evolution precise data are wanting. There is, however, the im- 

 portant negative argument furnished by the absence of evidence 

 of recognizable specific variations of organic forms since man 

 began 10 observe and record. We know that within human 

 experience a few species have become extinct, but there is no 

 conclusive proof that a single new species has come into exist- 

 ence, nor are appreciable variations readily apparent in forms 

 that live in a wild state. The seeds and plants found with 

 Egyptian mummies, and the flowers and fruits depicted on 

 Egyptian tombs, are easily identified with the vegetation of 

 modern Egypt. The embalmed bodies of animals found in that 



NO. II 88, VOL. 46] 



country show no sensible divergence from the structure or pro- 

 portions of the same animals at the present day. The human 

 races of Northern Africa and Western Asia were already as dis- 

 tinct when portrayed by the ancient Egyptian artists as they are 

 now, and they do not seem to have undergone any perceptible 

 change since then. Thus a lapse of four or five thousand years^ 

 has not been accompanied by any recognizable variation in such 

 forms of plant and animal life as can be tendered in evidence. 

 Absence of sensible change in these instances is, of course, na 

 proof that considerable alteration may not have been accom- 

 plished in other forms more exposed to vicissitudes of climate 

 and other external influences. But it furnishes at least a pre- 

 sumption in favour of the extremely tardy progress of organic 

 variation. 



If, however, we extend our vision beyond the narrow range of 

 human history, and look at the remains of the plants and animals 

 preserved in those younger formations which, though recent 

 when regarded as parts of the whole geological record, must be 

 many thousands of years older than the very oldest of human 

 monuments, we encounter the most impressive proofs of the 

 persistence of specific forms. Shells which lived in our sea& 

 before the coming of the Ice Age present the very same 

 peculiarities of form, structure, and ornament which their 

 descendants still possess. The lapse of so enormous an interval 

 of time has not sufficed seriously to modify them. So too with 

 the plants and the higher animals which still survive. Some 

 forms have become extinct, but few or none which remain dis- 

 play any transitional gradations into new species. We must 

 admit that such transitions have occurred, that indeed they have 

 been in progress ever since organized existence began upon our 

 planet, and are doubtless taking place now. But we cannot 

 detect them on the way, and we feel constrained to believe that 

 their march must be excessively slow. 



There is no reason to think that the rate of organic evolution 

 has ever seriously varied ; at least no proof has been adduced of 

 such variation. Taken in connection with the testimony of the 

 sedimentary rocks, the inferences deducible from fossils entirely 

 bear out the opinion that the building up of the stratified cru^t 

 of the earth has been extremely gradual. If the many thousands 

 of years which have elapsed since the Ice Age have produced! 

 no appreciable modification of surviving plants and animals, 

 how vast a period must have been required for that marvellous 

 scheme of organic development which is chronicled in the rocks ! 



After careful reflection on the subject, I affirm that the geo- 

 logical record furnishes a mass of evidence which no arguments 

 drawn from other departments of Nature can explain away, and 

 which, it seems to me, cannot be satisfactorily interpreted save 

 with an allowance of time much beyond the narrow limits which 

 recent physical speculation would concede. 



I have reserved for final consideration a branch of the history 

 of the earth which, while it has become, within the lifetime of 

 the present generation, one of the most interesting and fasci- 

 nating departments of geological inquiry, owed its first impulse 

 to the far-seeing intellects of Hutton and Playfair. With the 

 penetration of genius these illustrious teachers perceived that if 

 the broad masses of land and the great chains of mountains owe 

 their origin to stupendous movements which from time to time 

 have convulsedjthe earth, their details of contour must be mainly 

 due to the eroding power of running water. They recognized 

 that as the surface of the land is continually worn down, it is 

 essentially by a process of sculpture that the physiognomy of 

 every country has been developed, valleys being hollowed out 

 and hills left standing, and that these inequalities in topographi- 

 cal detail are only varying and local accidents in the progress of 

 the one great process of the degradation of the land. 



From the broad and guiding outlines of theory thus sketched 

 we have now advanced amid ever-widening multiplicity of de- 

 tail into a fuller and nobler conception of the origin of scenery. 

 The law of evolution is written as legibly on the landscapes of 

 the earth as on any other page of the Book of Nature. Not 

 only do we recognize that the existing topography of the con- 

 tinents, instead of being primeval in origin, has gradually been 

 developed after many precedent mutations, but we are enabled 

 to trace these earlier revolutions in the structure of every hill 

 and glen. Each mountain-chain is thus found to be a memorial 

 of many successive stages in geographical evolution. Within 

 certain limits, land and sea have changed places again and 

 again. Volcanoes have broken out and have become extinct in 

 many countries long before the advent of man. Whole tribes 



