Part 11

William Thomson: 'On the Secular Cooling of the Earth,' Transact. R. S. Edinb., xxiii., 1864, pp. 157-169.

Wallace, in his 'Island Life,' by making use of Professor A. Geikie's results as to the rate of denudation of matter by rivers from the area of their basins, and estimating the average rate of deposition, concludes that 'the time required to produce this thickness of rock [Professor Haughton's maximum of 177,000 feet] at the present rate of denudation and deposition is only 28,000,000 years.' Our lower assumption of 130,000 feet thickness would give only 20,000,000 years--a rate of 1 foot in 154 years.

Again, if we prefer round numbers to start with, we have only to assume that the age of the whole Tertiary period, with its 3,000 feet thickness, is 3,000,000 years (i.e., 1,000 feet in 1,000,000 years, or 1 foot in 1,000 years, surely an excessively slow rate); then 130,000,000 years would bring us to the bottom of the Laurentian or pre-Cambrian deposits. Of course, it is a pure assumption that the same rate of destruction and sedimentation applies to the whole of the strata; but we know nothing to the contrary, especially if we consider the average periods, the quick periods of extra activity, taken with the slow periods or those of standstill.

Dana estimated the length of the whole Tertiary period at one-fifteenth of the Mesozoic and Palæozoic combined. If we take the duration of the Tertiary period, as before, as 3,000,000 to 4,000,000 years, the total will amount to from 45,000,000 to 60,000,000 years.

Lastly, Walcott has estimated the duration of the Palæozoic, Mesozoic, and Cænozoic or Tertiary epochs at about 17,000,000, 7,000,000 and 3,000,000 years respectively, giving 27,700,000 years from the beginning of the Cambrian; and Williams has calculated the relative duration of the smaller epochs. See the table on p. 149.

The results of all these calculations fall surprisingly well within the limits of Lord Kelvin's allowance. Of course they are based upon assumptions, but none of them is inherently unreasonable; and it was my purpose to draw attention to the surprising coincidence in the closeness of these results, perhaps too good to be true. Such calculations are considered close enough if they range within a few multiples of each other.

'Geological Time as indicated by the Sedimentary Rocks of North America.' Proc. Amer. Assoc. Adv. Sci., xlii., 1893, pp. 129-169.

Henry Shaler Williams, 'Geological Biology.' New York, 1895.

Zoologists have fallen into the habit of requiring enormous lengths of time for the evolution of the animal kingdom. We know that Evolution is at best a slow process, and the conception of the changes necessary to evolve man from monkey-like creatures, these from the lowest imaginary mammals, these from some reptilian stock, thence descending to Dipnoan fish-like creatures, and so on back into Invertebrata, down to the simple Monera--this conception is indeed gigantic. Innumerable, almost endless, slow changes require seemingly unlimited time, and as time is endless, why not draw upon it ad libitum?

Huxley pointed out that it took nearly the whole of the Tertiary epoch to produce the horse out of the four-toed Eohippos, and that, if we apply this rate to the rest of its pedigree, enormous times would be required. This is, however, a very misleading statement, which necessitates considerable reduction, in conformity with our increased palæontological knowledge. Animals of the genus Equus--namely, Ungulata, with one toe, and with a certain tooth pattern--from the Upper Miocene of India are now known. Moreover, it is not simply a question of the gradual loss of the side-toes. The change from the fox-sized little Eohippos and Hyracotherium, so far as skull, teeth, vertebral column, and limbs are concerned (about the soft parts we know next to nothing), is a very great one indeed.

Elephants and mammoths seem to have developed very rapidly. None are known from Eocene strata; but towards the end of the Miocene they had spread over Asia, Europe, and North America, and that in great numbers. The Eocene Amblypoda are still so different that we hesitate to connect them ancestrally with the elephants.

The Pinnipedia (seals and walruses) are strongly modified fissiped Carnivora, and have existed since at least the Upper Miocene; the transformation must have been accomplished within the Miocene period.

We cannot shut our eyes to the fact that various groups have from the time of their first appearance burst out into an exuberant growth of modifications in form, size, and numbers, into all possible--and one might almost say impossible--shapes; and they have done this within comparatively short periods, after which they have died out not less rapidly. It seems almost as if these go-ahead creatures had, by accepting every possible modification and carrying the same to the extreme, too quickly exhausted their plasticity--which, after all, must have limits--thereby becoming unable to meet successfully the requirements of further changes in their surroundings. The slowly developing groups, keeping within main lines of Evolution, and not being tempted into aberrant side-issues, had, after all, a much better chance of onward evolution.

A good example of the former are the Dinosaurs. We do not know their ancestors; but we have here to deal only with their range of transformation. The oldest known forms occur in the Upper Trias; they attain their most stupendous development in the Upper Jurassic and in the Wealden; and they have died out with the Cretaceous epoch. But already some of their earliest forms had assumed bipedal gait, and the Oolitic Compsognathus had developed almost bird-like hind-limbs.

On the other hand, there are many instances of extremely slow development--facts which raise the difficult question of 'persistent types.' Are these due to a state of perfection which cannot be improved upon? Or are they due to a kind of morphological consolidation (not necessarily specialization) which can no longer yield easily, so that therefore through changes in their surroundings they may come to an end sooner than more plastic groups?

Struthio, the ostrich; Orycteropus, the Cape ant-eater; Tapirus, and many others, existed in the Miocene age practically as they are now; but pre-Pliocene dolphins, cats, monkeys, stags, all belong to closely-allied and well-defined 'genera,' but different from the living forms.

Alligators and crocodiles are known from the Upper Chalk; Tomistoma since the Miocene; Gavialis since the Pliocene.

The oldest surviving reptile is Sphenodon, the Hatteria of New Zealand, a fair representative of what generalized reptiles of the later Triassic period seem to have been like; and to the same period belongs Ceratodus, the Australian mud-fish, hitherto the oldest known surviving genus of a very ancient and low type so far as Vertebrata are concerned.

Now let us see if the above estimates of geological time are so utterly inapplicable to animal evolution. On purpose we take one of the lowest estimates, about 28,000,000 years, and apportion them equally to the various strata or epochs.

The original owner of the famous Trinil skull, a Pithecanthropus erectus, lived, according to some, in the Late Pliocene, according to others in the Early Plistocene, period--that is to say, somewhere about the beginning of our last Glacial epoch, some 270,000 years ago. Assuming that he and his like reached puberty at sixteen to twenty years of age, about 17,000 generations would lie between him and ourselves, or, to put it more forcibly, between him and the lowest living human races--say the Ceylonese Veddahs. Only 250 generations, at twenty years, carry us back to 3000 B.C. (i.e., beyond the ken of history); and if it be objected that the differences between the oldest inhabitants of Egypt, the Naquada, and the present Fellahin are very slight, we are welcome to multiply these differences sixty or seventy fold, in order to arrive at the Pithecanthropus level. But these Naquada had no metal implements, and there cannot be the slightest doubt that the development of the human race went on by leaps and bounds after certain discoveries had been made--to wit, the use of implements and that of fire. That creature which first took up a stone or a branch and wielded it thereby got such an enormous advantage over his fellow-creatures that his mental and bodily development went on apace. The same applies to the improvement of speech. We assume the single, monophyletic origin of mankind at one place, in one district; and the differences between some of the races of man are great enough to constitute what we might call species. Compare the Venus of Milo, that noble expression of the ancient Greeks' notion of female beauty, with the 'products of art' of the Veddahs or the dwarfs of Central Africa, or think of the beau-idéal which a Michael Angelo could possibly have evolved if he had never seen any but such people.

TIME AND EVOLUTION

====================================================================== I. |II.| III. | IV. | V. |VI.| VII. | | | | | |Generations. -----------+---+-----------+----------+--------------+---+------------ |} |} |} |Adam and Eve | | 250 Recent |} 5|} |} |Man, contem- | | 3,500 Plistocene |} |} |} 270,000| porary with | | | |} |} | Reindeer | | | |} |} | in France | | Pliocene -|} |} 3,000,000| |Pithecanthro-| 16| 17,000 |} |} |} 600,000| pus erectus| | Miocene -|}10|} |} |Anthropoid | 10| 60,000 |} |} |}2,100,000| Apes | | Eocene -|} |} |} |Lemures | 5| 420,000 | | | | | | Cretaceous | 10|} | 3,600,000| | | Jurassic - | 5|} | 1,800,000| | | Rhætic -|} |} |} |Prototheria, | 3| 1,800,000 |} |} |} | or first | | |} |} 7,200,000|} | Mammalia | | Keuper -|} |} |}1,800,000| | | Muschel- |} 5|} |} | | | kalk |} |} |} | | | New Red |} |} |} |Theromorpha | 4| 425,000 Sandstone| | | | | | Magnesian |} |} |} | | | Limestone|} |} |} | | | Lower Red |} |} |} |Proreptilia | 4| 250,000 Sandstone|} |} |}4,000,000| | | Coal- |}15|} |} |Eotetrapoda | 4| 500,000 measures |} |} |} | | | Mountain |} |}17,500,000|} | | | Limestone | |} | | | | Devonian -| 15|} | 4,000,000|Dipnoi and | 5| 1,000,000 | |} | |Crossopterygii| | Silurian -| 10|} | 2,700,000|First fishlike| 3| 900,000 | |} | | creatures | | Ordovician | 10|} | 2,700,000| | | Cambrian -| 15|} | 4,000,000| Sum total of| | Laurentian | | | | generations| | --------- Archæan | | | | (about) | | 5,375,000 or Meta- | | | | | | morphic | | | | | | ======================================================================

EXPLANATION OF THE TABLE ON P. 149.

Column I. contains the names of the successive sedimentary strata.

" II. contains the percentage of the duration of the various epochs, according to Williams, the time from the Cambrian until recent times being taken as 100.

" III. gives the estimated duration in years of the Palæozoic, Mesozoic, and Cænozoic periods, according to Walcott.

" IV. gives in years the duration of the various smaller epochs, as computed from Walcott and Williams' statements.

" V. Representatives of stages of the ancestral line of man. The names stand in the level of the stratum in which they have made their first appearance.