One million years is a stretch of time beyond our conception. We can
arrive at a more or less adequate understanding of what a million
individuals or concrete things means. Several Continental nations
can put more than a million men into the field. We can gaze at a
building which contains as many bricks; and we know that our own body
is composed of millions of millions of cells. No such help applies to
time, because that itself is an entirely relative, abstract conception.
We can imagine what one hundred years are like--a span of time
seemingly short to the hale and hearty octogenarian, enormous to the
child, totally inapplicable to certain animals whose whole life is
crowded into one single day.
Astronomers have long ceased to reckon distances by miles or any
other understandable unit. They express the distances between us and
the stars and nebulæ by 'years of light.' Try to imagine a unit of
length equal to that which is passed through by light (186,000 miles
per second) in one year. Not so very long ago the enormous distances
resulting from astronomical calculations were looked upon as the most
serious objection to the correctness of the astronomers' views as to
the distances which separate our globe from the nearest fixed stars.
We have not yet accustomed ourselves to reckoning time by some similar
broadly-conceived standard--say æons of so many thousand years each.
Unfortunately, we possess no data whatever for calculating the age
of the successive geological strata. Thanks to Lyell, the theory of
violent universal cataclysms has been done away with. It is more
probable that the same agencies have acted which are now changing
the aspect of the globe; and these changes are slow, as far as we
know them--at least, as far as the formation of sedimentary strata is
concerned, and these alone we have to deal with. Various calculations
have been made, based upon the denudation of the mountains, the
filling up of the valleys by the débris, the formation of deltas,
etc. The results give enormous stretches of time, but all of them
unsatisfactory, because the methods are so very local in their
application.
The least objectionable attempt is that which, based upon astronomical
calculations, tried to fix the height of the last Glacial epoch[26] at
about 200,000 years ago, and asserted that since its beginning in the
Pliocene epoch as many as 270,000 years have elapsed. The duration
of the whole Tertiary period has by the same authorities been fixed
approximately at 3,000,000 to 4,000,000 years. Beyond this we cannot
venture without the wildest speculation; but we know to a certain
extent the thickness of the various sedimentary strata, which amount
in all to from 100,000 to 175,000 feet--on the average perhaps 130,000
feet, or about twenty miles.
[26] James Croll: 'On Geological Time, and the Probable Date of the
Glacial and Upper Miocene Period,' _Philos. Magazine_, xxxv., 1868, pp.
363-384; xxxvi., pp. 141-154; 362-386.
Unless we prefer giving up all attempt at calculation as absolutely
hopeless, and thus resign the whole problem, we must at least try to
arrive at some results, and then see if these cannot reasonably be made
use of.
Neither geologist nor physicist, and no zoologist, would accept the
suggestion that these 130,000 feet of stratified rocks have been
deposited within only as many years, although the average rate of
deposit would in that case be not more than 1 foot per year. On the
other hand, an indignant protest is raised against the assumption of
1,000,000,000 years.
Lord Kelvin[27] has come to the conclusion (from data which various
other authorities regard as very unsatisfactory) that not much more
than 100,000,000 years can have elapsed since the molten globe acquired
a consolidated crust. Further time must have passed before the surface
had become stable and cool enough to allow the temperature of the
collecting oceans to fall below boiling-point, and it is obvious that
life cannot possibly have begun until after this had happened.
[27] 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[28] 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[29] 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.
[28] 'Geological Time as indicated by the Sedimentary Rocks of North
America.' _Proc. Amer. Assoc. Adv. Sci._, xlii., 1893, pp. 129-169.
[29] 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 | | | | | |
======================================================================