III. THE RATE OF GEOLOGICAL CHANGE. By H. M. JENKINS, F.G.S., Secretary of the Royal Agricultural Society of England. PUBLIC opinion on questions of theoretical geology grows slowly, and usually precedes the statement of important speculations. The progress of discovery leads up to the induction, which, after floating more or less hazily in the minds of geologists for a certain time, is at last enunciated piecemeal at irregular intervals by the more daring theorists. Finally the scattered fragments are collected and arranged, bound together by the idea which connects them, and placed on record as a complete whole. This last is the task which I propose to attempt in reference to the progress of public opinion on the subject at the head of this article. But first, let me clear the way by a short summary of the ideas which prevail amongst English geologists, so far as they bear on this subject. The prominent feature of the favourite modern school of geology in England-Uniformitarianism-is the belief that the forces in operation at the surface of the earth in former times differed in no appreciable degree from those now in action. This article of faith is, however, commonly restricted to what, for convenience of expression, has been termed "Geological time”—a period which is entirely represented by the rocks found on the earth's surface, from the oldest to those now in course of formation. According to this school, the rate of geological change has been approximately equal throughout the vast period which has elapsed since the deposition of the oldest stratified rock. Local variations of this law would doubtless be admitted by even the most thorough advocate of Uniformity, but the broad general principle characteristic of the creed is equality in the rate of change throughout all geological time. Catastrophism, which is the name usually given to the other great school of geologists, is distinguished broadly by the tenet that in past times the forces in operation at the surface of the globe were of far greater intensity than they are now, and that great physical changes were then produced by more or less violent cataclysms. Probably there are geologists who now-a-days reject the catastrophes, but still cling to the belief that modern forces are much less intense, and modern changes much less rapid and extensive, than those which occurred in former geological periods. Therefore, whichever view we take of this theory, it is clear that its advocates believe that the rate of geological change was greater in past times than it is now; and the inference appears fair that, according to this school, the rate of change has, on the whole, progressively diminished from the earliest down to modern times. In his Anniversary Address to the Geological Society last year, Professor Huxley defined a "third phase of geological speculation, namely, Evolutionism." This doctrine, in the words of the author, "embraces all that is sound in both Catastrophism and Uniformitarianism, while it rejects the arbitrary assumptions of the one, and the, as arbitrary, limitations of the other." To my mind it cannot well be distinguished from ordinary Theoretical Geology, unfettered by the trammels of any school; but obviously, the Evolutionist is prepared to accept whatever theory on the rate of geological change can be shown to be consistent with those known facts which can fairly be quoted as evidence. The title of this article is capable of more than one interpretation, and in its various meanings it has already been investigated by speculative geologists. The late Professor Edward Forbes, in his lecture before the Royal Institution, "On the Manifestation of Polarity in the Distribution of Organized Beings in Time," endeavoured to show that the rate of development of generic types reached its maximum intensity, firstly, during the earlier Palæozoic periods, and secondly, during the later Neozoic periods; that is to say, near the beginning and the end of the geological scale. Again, the rate of development was shown to be at its minimum during the later Palæozoic (Permian) and earlier Neozoic (Triassic) periods, from which contiguous zero-points the development of generic types was asserted to increase in both directions.† This relation Professor Forbes termed "Polarity," and he showed how in several of the great divisions of the animal kingdom, two of their groups appeared to exercise a kind of "reciprocity," as, for instance, our old friends the Paleozoic four-starred corals versus the Neozoic six-starred. But Professor Forbes was careful to make the reservation that "the numbers of species in a group or genus at any given epoch is to be excluded, not being an element in the discussion of the question, though apt to be introduced through mistake of the nature of the generalization attempted to be attained." Indeed, the relations of individuals, species, and genera were favourite subjects of speculation with this poetic and philosophical paleontologist; and the generalization we have just sketched was a sequel to some other inquiries, in recording which he defines a genus as "an abstraction—an idea-but an idea impressed on nature, and not arbitrarily dependent on man's conceptions." Again, "a genus consists of more or fewer of these manies resulting from one [species] linked together, not by a relationship of descent, but by *Notices of the Meetings of the Royal Institution,' vol. i., p. 428. This view may be correct; but at present, as at the time when it was advanced, we have only negative evidence in support of it; and it is still very possible that Permian and Triassic rocks, rich in generic types, may be discovered in some hitherto unexplored region of the earth. 'Notices,' &c., vol. i., p. 196. an affinity dependent on a divine idea."* It is necessary to bear in mind these definitions of a genus in order to understand the author's generalization of "Polarity" in the "development of generic types," and to prevent confusion with other ideas which I shall attempt to elucidate in the following pages. Before estimating the rate of geological change in successive epochs, we must clearly understand the means by which that rate is measured. Our geological chronology is divided into epochs of greater or less extent, distinguished and characterized by certain forms of animal and vegetable life, either peculiar to them, or preponderating in number and variety during their continuance. We can conceive that in a previously unexplored country, far away from any region whose geology is known, the explorer may meet with diverse geological formations, each formation being characterized by a sufficiently numerous and distinctive fauna. If formation A contains 1000 species, and is 10,000 feet thick, and formation B contains 5000 species, but is only 1000 feet thick, and if the range of zoological rank in the 5000 species is approximately the same as in the 1000 species, we should be justified in saying of formation A, that, in comparison with formation B, (1) It was deposited very quickly; or, (2) During its deposition species changed very slowly. Further investigation by our hypothetical explorer might possibly furnish him with evidence that, during the deposition of formation A, the conditions of climate and physical geography had remained more or less stationary, and that the strata were deposited slowly. On the other hand, formation B and its fossils might yield evidence of great changes in climate and physical geography, and of comparatively rapid deposition. Under these circumstances, he would be justified in the conclusion that, during the epoch represented by formation A, the rate of geological change was much less rapid than during the period represented by formation B. This hypothetical contrast will assist the reader in appreciating the significance of the following synopsis of the Paleozoic and Mesozoic rocks and their fossils, and will enable him to estimate how far these epochs agree with those which we have supposed to be represented by formations A and B respectively. Professor Phillips, in his Rede lecture, delivered before the University of Cambridge in 1860,† remarks that 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 British strata, and refer them at present to only three great periods, we find that the Paleozoic rocks contain 2729 species, *Loc. cit. Afterwards published under the title of 'Life on the Earth.' Macmillan. 1860. See pp. 59-62. the Mesozoic 2170, and the Cainozoic 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." This conclusion he illustrates by the following Table : And he again observes (p. 62), "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."‡ *Should be 93. † Should be 47. The editor of this Journal (Mr. James Samuelson) has asked me whether I may not have overlooked the biological aspect of the case, and whether this may not be the result of the laws of selection in the lower forms of life varying from those in the higher forms, and the rate of change being perhaps more rapid in the latter; e. g. low molluscs or acalephs having predominated in earlier times, might go on multiplying for ages without material change, their conjugation being, like that of plants, regulated by the elements, whilst in the fishes and higher molluscs, sexual selection and destruction of each other would be very active and would produce rapid change of species. It appears to me that at least two powerful arguments may be advanced against this interpretation. First, geologically considered, the interpretation would be erroneous because we are contrasting the abundance and variety of life characteristic of the different great periods, and it cannot be said of any one great period that it is characterized, for instance, exclusively by low molluscs, nor of any other that it is characterized exclusively by higher orders of that class, as is shown in the following Table by Professor Phillips : Zoo- Echino- Crus-Brachio- Mono- Dimy- Gaste- Cepha- Total. Thus the Paleozoic rocks abound both in the highest and the lowest orders of the testaceous Mollusca, viz. the Cephalopoda and the Brachiopoda, while the Tertiary deposits are characterized by the intermediate groups of Gasteropoda and Lamellibranchiata. The oldest fishes are not regarded by the best authorities as of uniformly lower types than the most recent; and with regard to the power of sexual selection, the advantage is doubtless on the side of the Paleozoic and Mesozoic representatives of the modern Claspers. But the weightiest argument, to my mind, is to be derived from the improbability that sexual selection in such animals (for, owing to our imperfect record, we are necessarily compelled to deal almost exclusively with animals of no high zoological grade) was to any great extent the result of direct volition, or that sexual selection was the determining cause of change of species, although it no doubt was one means to that end. It appears to me far more probable that changes in species have been rapid These quotations show that so keen an observer and so thoughtful a philosopher as Professor Phillips had not allowed this subject to escape him; but I am not aware that any other geologist has discussed it from precisely the same point of view. The figures given by Professor Phillips show that the rate of geological change, with regard to the relation between the deposition of strata and the changes in fauna, was neither uniform throughout all geological times, as the Uniformitarians would have it, nor more intense in the earlier periods, as the Catastrophists contend. On the contrary, these figures prove, to the extent which they go, that the rate of change was marvellously more rapid in the more recent periods, and that the increase in rapidity was rapidly progressive, from the earliest to the latest times. There are four times as many species belonging to the eight classes which are persistent through all geological periods, per 1000 feet, in the Mesozoic strata than in the Palæozoic, and there are three times the number † of such species per 1000 feet in the Cainozoic strata that there are in the Mesozoic. In the same year (1860), Professor Phillips, as President of the Geological Society, delivered the Anniversary Address to the Fellows, and again adverted to this subject, and the following quotation gives his conclusions, as delivered before one of the most critical geological audiences in Europe: * "In the earlier periods of the world's history, the changes of life in the sea were accomplished at a rate much less rapid than that which prevailed in later times, which agrees with the acknowledged very wide distribution of Paleozoic forms in geographical space. Admitting the changes of life on the whole to be equal from the Paleozoic to the Mesozoic, and from these to the Cainozoic periods, we find the rate of progressive change § 7th for Paleozoic, 18th for Mesozoic, and 3rd for Cainozoic time, a conclusion of great importance, and probably indicative of the greater influence and or slow, in proportion as changes in climate and physical geography have been frequent or seldom. In the Paleozoic periods we have reason to believe that changes in physical geography were rare, while the climate and the nature of the earth's surface were very slightly diversified. In the Tertiary epoch, on the contrary, climate and physical conditions have been very diverse, and have frequently varied; and these variations have been accompanied by proportionate changes of species. If periods of small duration had been compared by Professor Phillips, it is quite certain that the chances of error would have been very great; but by putting in contrast only the three great epochs into which Geological Time has been divided, I believe that those chances of error have been reduced to a minimum, according to the well-known law of averages.-H. M. J. With all deference to so eminent a geologist as Professor Phillips, and to the author of this essay, it appears to me that such a table is an unsafe guide in the present state of the palæontological record. Here, for example, the fishes-the most important group of all, biologically-are entirely omitted.-EDITOR. *According to my calculation, twice as many. † I make it six times. The italics are mine.-H. M. J. § Taking the unit of thickness such that it shall be th of the ascertained strata in which life-traces occur. |