definite conclusion. One thing, however, must be remarked. Both parties
assume--the geologist tacitly, the physicist avowedly--the nebular
hypothesis of the origin of the solar system, and therefore the early
incandescent _fluid_ condition of the earth as the basis of all his
reasonings. Now, while this is probably the most reasonable view, it is
not so certain that it can be made the basis of complex mathematical
calculation. There is a possible alternative theory--viz., the meteoric
theory--which is coming more and more into favor. According to this
view, the planets may have been formed by aggregation of meteoric
swarms, and the heat of the earth produced by the collision of the
meteors in the act of aggregation. According to the one view (the
nebular), the heat is all primal, and the earth has been only losing
heat all the time. According to the other, the aggregation and the
heating are both gradual, and may have continued even since the earth
was inhabited. According to the one, the spendthrift earth wasted
nearly all its energy before it became habitable or even a crust was
formed, and therefore the habitable period must be comparatively short.
According to the other, the cooling and the heating, the expenditure
and the income, were going on at the same time, and therefore the
process may have lasted much longer.
The subject is much too complex to be discussed here. Suffice it to
say that on this latter view not only the age of the earth, but many
other fundamental problems of dynamical geology, would have to be
recalculated. The solution of these great questions must also be left
to the next century. In the meantime we simply draw attention to two
very recent papers on the subject--viz., that of Lord Kelvin,[3] and
criticism of the same by Chamberlin.[4]
ANTIQUITY AND ORIGIN OF MAN.
Even after the great antiquity of the earth and its origin and
development by a natural process were generally accepted, still man
was believed, even by the most competent geologists, to have appeared
only a few thousand years ago. The change from this old view took
place in the last half of the present century--viz., about 1859--and,
coming almost simultaneously with the publication of Darwin's Origin of
Species, prepared the scientific mind for entertaining, at least, the
idea of man's origin by a natural process of evolution.
Evidences of the work of man--flint implements, associated with the
bones of extinct animals and therefore showing much greater age than
usually accepted--had been reported from time to time, notably those
found in the river Somme by Boucher de Perthes. But the prejudice
against such antiquity was so strong that geologists with one accord,
and without examination, pooh-poohed all such evidence as incredible.
It was Sir Joseph Prestwich who, in 1859, first examined them
carefully, and published the proofs that convinced the geological world
that early man was indeed contemporaneous with the extinct animals of
the Quaternary period, and that the time must have been many times
greater than usually allowed.[5]
Since that time confirmatory evidence has accumulated, and the earliest
appearance of man has been pushed back first to the late glacial, then
to the middle glacial, and finally, in Mr. Prestwich's Plateau Gravels,
to the early glacial or possibly preglacial times.
Still, however, in every case earliest man was unmistakably man. No
links connecting him with other anthropoids had been found. Very
recently, however, have been found, by Du Bois, in Java, the skull,
teeth, and thigh bone of what seems to be a veritable _missing_ link,
named by the discoverer _Pithecanthropus erectus_. The only question
that seems to remain is whether it should be regarded as an ape
more manlike than any known ape, or a man more apelike than any yet
discovered. The age of this creature was either latest Pliocene or
earliest Quaternary.
BREAKS IN THE GEOLOGICAL RECORD AND THEIR SIGNIFICANCE.
From the earliest times of geologic study there have been observed
unconformities of the strata and corresponding changes in the fossil
contents. Some of these unconformities are local and the changes of
organic forms inconsiderable, but sometimes they are of wider extent
and the changes of life system greater. In some cases the unconformity
is universal or nearly so, and in such cases we find a complete
and apparently sudden change in the fossil contents. It was these
universal breaks that gave rise to the belief in the occurrence of
violent catastrophes and corresponding wholesale exterminations and
re-creations of faunas and floras.
It is evident, however, on a little reflection, that every such
unconformity indicates a land period at the place observed, and
therefore a time unrecorded in strata and fossils at that place--i.
e., a lost interval--certain leaves missing from the book of time. And
if the unconformity be widespread, the lost interval is correspondingly
great. It is therefore probable that change of species went on slowly
and uniformly all the time, although not recorded at that place.
Intermediate strata may be and often are found elsewhere, and the
supposed lost interval filled. The record was continuous and the
changes uniform, but the record is not all found in one place. The
leaves of the book of Time are scattered here and there, and it is the
duty of the geologist to gather and arrange them in proper order, so
that the record may read continuously.
This is the uniformitarian view, and is undoubtedly far truer than
the catastrophic. But the objection to it is that in the case of very
widespread unconformities, such as occurred several times in the
history of the earth, the changes of organisms are so great that if the
rate of change was uniform the lost interval must have been equal to
all the rest of the history put together. Therefore we are compelled
to admit that in the history of the earth there have been periods of
comparative quiet (not fixedness) during which evolutionary changes
were slow and regular, and periods of revolution during which the
changes were much more rapid, but not catastrophic. This is exactly
what we ought to expect on the idea of gradual evolution of earth
forms by secular cooling, for in the gradual contraction of the earth
there must come times of general readjustment of the crust to the
shrinking nucleus. These readjustments would cause great changes in
physical geography and climate, and corresponding rapid changes in
organic forms. In addition to this, the changes in physical geography
and climate would cause extensive migrations of species, and therefore
minglings of faunas and floras, severer struggles of competing forms,
and more rapid advance in the steps of evolution. Among these changes
of organic forms there would arise and have arisen _new dominant
types_, and these, in their turn, would compel new adjustment of
relations and still further hasten the steps of evolution. Such
changes, whether geographic, or climatic, or organic, would _not_
be simultaneous all over the earth, but propagated from place to
place, until quiet was re-established and a new period of comparative
stability and prosperity commenced.
This view is a complete reconciliation of catastrophism and
uniformitarianism, and is far more rational than either extreme.
_Critical Periods in the History of the Earth._--Such periods of rapid
change may well be called _critical periods or revolutions_. They are
marked by several characteristics: (1) By widespread oscillations of
the earth's crust, and therefore by almost universal unconformities.
(2) By widespread changes of physical geography, and therefore by
great changes in climate. (3) By great and widespread changes in
organic forms, produced partly by the physical changes and partly by
the extensive migrations. (4) By the evolution of new dominant types,
which are also the cause of extensive changes in species. (5) Among the
physical changes occurring at these times is the formation of great
mountain ranges. The names of these critical periods or revolutions are
often taken from the mountain range which form their most conspicuous
features.
There have been at least four of these critical periods, or periods of
greatest change: (1) The pre-Cambrian or Laurentide revolution; (2)
the post-Paleozoic or Appalachian; (3) the post-Cretaceous or Rocky
Mountain; (4) the post-Tertiary or glacial revolution.
Now, as these critical periods separate the primary divisions of
time--the eras--it follows that the _Present_--the Age of Man--is an
era. It may be called the _Psychozoic Era_. These views have been
mainly advocated by the writer of this sketch, but I believe that, with
perhaps some modification in statement, they would be accepted by most
geologists as a permanent acquisition of science.[6]
GEOLOGICAL CLIMATES.
Attention was first drawn to this subject by the apparently unique
phenomena of the Glacial epoch.
For nearly a century past Alpine glaciers, their structure, their
mysterious motion, and their characteristic erosive effects, have
excited the keenest interest of scientific men. But until about 1840
the interest was purely physical. It was Louis Agassiz who first
recognized ice as a great _geological agent_. He had long been familiar
with the characteristic marks of glacial action, and with the fact
that Alpine glaciers were far more extensive formerly than now, and
had, moreover, conceived the idea of a Glacial epoch--an ice age in
the history of the earth. With this idea in his mind, in 1840 he
visited England, and found the marks of glaciers all over the higher
regions of England and Scotland. He boldly announced that the whole
of northern Europe was once covered with a universal ice sheet. A few
years later he came to the United States, and found the tracks of
glaciers everywhere, and again astonished the world by asserting that
the whole northern part of the North American continent was modeled
by a moving ice sheet. This idea has been confirmed by all subsequent
investigation, especially here in America.
But it would be strange, indeed, if the cold of the Glacial epoch
should be absolutely unique. Attention was soon called to similar marks
in rocks of other geological periods, especially in the Permian of the
southern hemisphere. This opened up the general question of _geological
climates and their causes_.
Perhaps no subject connected with the physics of the earth is more
obscure and difficult than this. The facts, as far as we know them,
are briefly as follows: (1) All the evidence we have point to a
high, even an ultra-tropical, climate in early geological times;
(2) all the evidence points to a uniform distribution of this early
high temperature, so that the zonal arrangement of temperatures,
such as characterizes present climates, did not then exist; (3)
temperature zones were apparently first introduced in the late Mesozoic
(Cretaceous) or early Tertiary times, and during the Tertiary the
colder zones were successively added, until at the end there was formed
a polar ice-cap as now.
Thus far all might be explained by progressive cooling of the earth
and progressive clearing of the atmosphere of its excess CO_[2] and
aqueous vapor. But (4) from time to time (i. e., at critical periods)
there occurred great oscillations of temperature, the last and probably
the greatest of these being the Glacial period. The cause of these
great oscillations of temperature, and especially the cause of the
glacial climate, is one of the most interesting and yet one of the
obscurest and therefore one of most hotly disputed points in geology.
Indeed, the subject has entered into the region of almost profitless
discussion. We must wait for further light and for another century.
Only one remark seems called for here. It is in accordance with a true
scientific method that we should exhaust terrestrial causes before we
resort to cosmical. The most usual terrestrial cause invoked is the
oscillation of the earth's crust. But recently Chamberlin, in a most
suggestive paper,[7] has invoked oscillations in the composition of the
atmosphere, especially in its proportion of CO_[2], as the _immediate_
cause, although this in turn is due to oscillations of the earth's
crust.
THE NEW GEOLOGY.
Heretofore the geological history of the earth has been studied only
in the record of stratified rocks and their contained fossils. But in
every place there have been land-periods in which, of course, erosion
took the place of sedimentation. This kind of record is very imperfect,
because there are no fossils. Until recently no account was taken of
these erosion-periods except as breaks of indefinite length in the
record--as lost intervals. But now, and mainly through the work of
American geologists, interpretation of these erosion-periods has fairly
commenced, and so important has this new departure in the study of
geology seemed to some that it has been hailed as a new era in geology,
connecting it more closely with geography. Heretofore _former_ land
periods were recognized by unconformities and the amount of time by
the degree of change in the fossils, but now the amount of time is
estimated in _existing_ land surfaces by topographic _forms_ alone.
This idea was introduced into geology by Major J. W. Powell, and has
been applied with success by William Morris Davis, W. J. McGee, and
others.
The principle is this: Land surface subject to erosion and standing
still is finally cut down to gently sweeping curves, with low, rounded
divides and broad, shallow troughs. Such a surface is called by Davis
a Peneplain. Such a peneplain is characteristic of old topography.
If such a surface be again lifted to higher level, the rivers again
dissect it by ravines, which are deep and narrow in proportion to the
amount and rate of the uplift. If the land again remains steady, the
sharply dissected surface is again slowly smoothed out to the gentle
curves of a peneplain. If, on the contrary, the surface be depressed,
the rivers fill up the channels with sediment which, on re-elevation,
is again dissected. Thus the whole _ontogeny_ of land surfaces have
been studied out, so that their age may be recognized at sight.
Thus, while heretofore the more recent movements of the crust were
supposed to be readable only on coast lines and by means of the old
sea strands, now we read with equal ease the movements of the interior
by means of the physiognomy of the topography, and especially the
structure of the river channels. Moreover, while heretofore the history
of the earth was supposed to be recorded only in stratified rock and
their contained fossils, now we find that recent history is recorded
and may be read also in the general topography of the land surfaces.
Geography is studied no longer as mere description of earth forms, but
also as to the causes of these forms, no longer as to present forms,
but also as to the history of their becoming. Thus geography, by its
alliance with geology, has become a truly scientific study, and as
such is now introduced into the colleges and universities. It is this
alliance with geology which has caused the dry bones of geographic
facts to live. It is this which has created a soul under the dry "_ribs
of this death_." This mode of study of the history of the earth has
just commenced. How much will come of it is yet to be shown in the next
century.
In this connection it is interesting to trace the effect of environment
on geological reasonings in different countries. Heretofore, especially
in England, what we have called peneplains were usually attributed to
marine denudation--i. e., to cutting back of a coast line by constant
action of the waves, leaving behind a level submarine plateau, which
is afterward raised above sea level and dissected by rivers. American
geologists, on the contrary, are apt to regard such level surfaces as
the final result of aërial degradation or a base level of rain and
river erosion. The same difference is seen in the interpretation of
glacial phenomena. Until recently, English geologists were inclined
to attribute more to iceberg, Americans more to land ice. Again, in
England coast scenery is apt to be attributed mainly to the ravages
of the sea, while in America we attribute more to land erosion
combined with subsidence of the coast line. In a word, in the tight
little sea-girt island of Great Britain, where the ravages of the
sea are yearly making such serious inroads upon the area of the
land, it is natural that the power of the sea should strongly affect
the imagination and impress itself on geological theories, and tend
perhaps to exaggeration of sea agencies, while the broad features
of the American continent and the evidences of prodigious erosion
in comparatively recent geological time tend to the exaggeration of
erosive agency of rain and rivers. These two must be duly weighed and
each given its right proportion in the work of earth sculpture.
PALEONTOLOGY.
Paleontology at first attracted attention mainly by the new and strange
life forms which it revealed. It is the interest of a zoölogical
garden. This interest is of course perennial, but can hardly be called
scientific. Geology at first was a kind of wonder book.
Next fossils, especially marine shells, were studied as characteristic
forms denoting strata of a particular age. They were coins by which we
identify certain periods of history. They were "medals of creation."
It was in this way chiefly that William Smith, the founder of English
stratigraphic geology, used them. It was in this way that Lyell and
all the older geologists, until the advent of evolution, were chiefly
interested in them.
It was Cuvier, the great zoölogist and comparative anatomist, who, in
the beginning of the present century, first studied fossils, especially
mammalian fossils, from the _zoölogical_ point of view--i. e., as
to their affinities with existing animals. Cuvier's studies of the
vertebrates of the Paris basin may be said to have laid the foundation
of scientific paleontology from this point of view.
Thenceforward two views of paleontology and two modes of study
gradually differentiated from one another, the one zoölogical, the
other geological. In the one case we study fossils in _taxonomic_
groups--i. e., as species, genera, families, orders, etc.--and trace
the gradual evolution of each of these from generalized forms to their
specialized outcomes, completing as far as possible the genetic chain
through all time. In the other we study fossils in _faunal groups_,
as successive geological faunas, and the geographic diversity in
each geological period--i. e., the evolution of geologic faunas and
the causes of geographic diversity in each. In a word, we study the
laws of distribution of faunas in time geologically and in space
geographically, and the causes of these laws in each case. The first
is strictly a branch of zoölogy and botany, and we leave it to these
specialists. The second alone belongs properly to geology. In this
purely geologic paleontology, as seen from its scope given above, there
are many questions of widest philosophical interest which are only now
attracting the attention they deserve. I only touch lightly two which
have been brought forward in these very last years of the century.
I. GENERAL LAWS OF FAUNAL EVOLUTION.--The evolution of the organic
kingdom from this strictly geological point of view may be briefly
formulated as follows:
1. Throughout all geological time there has been a general movement
upward and onward, as it were abreast, everywhere. If this were all,
there would be only geological progress, but no geographical diversity.
Geological history would be the same everywhere. A time horizon would
be easily determined by identity of fossil species. This we know is not
true. Therefore there are other elements besides this.
2. In different countries, isolated from one another and under
different conditions, evolution takes different _directions_ and
different _rates_, producing geographical diversity in each geological
period. This diversity increases with time as long as the isolation
continues. If this were all, the geographical diversity by continued
divergence would have become so great that it would be impossible even
approximately to determine any geological horizon. The history of each
country must be studied for itself. A general history of the earth
would be impossible. But this also is not true. There is therefore
still another element.
3. From time to time, at long intervals--i. e., _critical
periods_--there are widespread readjustments of the crust to internal
strain, determining changes of physical geography and of climate,
and therefore wide migrations of species with mingling and conflict
of faunas. This would produce more rapid movement of evolution, but
at the same time more or less complete obliteration of geographical
diversity.
4. After these periods of migrations and minglings there would be
re-isolations in new localities, and the process of diversification
would recommence and increase as long as the isolation continues.
The last of these critical periods of migrations and minglings and
struggles for life among competing species was the _Glacial epoch_ or
ice age. Therefore the present geographical distribution of species was
largely determined by the extensive migrations of that time.
II. COSMOPOLITAN AND PROVINCIAL FAUNAS.--There are apparently in the
history of the earth periods of widespread or cosmopolitan faunas,
alternating with localized or provincial faunas. The cosmopolitan
periods are usually times of prevalence of limestones or organic
sediments, and the fossils are very abundant. The provincial periods
are usually characterized by sandstones and shales or mechanical
sediments, and are comparatively poor in fossils. Moreover, it
is believed that the cosmopolitan limestone periods are oceanic
periods--i. e., periods of wide oceans and lower and smaller continents
and little erosive activity, while the sandstone periods, characterized
by provincial faunas, are periods of higher and larger continents, and
therefore of great erosion and abundant mechanical sedimentation.
Now, according to Chamberlin, these remarkable alternations are due
to oscillations of the crust, in which the continents are alternately
lifted and depressed. It must be remembered that abyssal faunas
are almost unknown among fossils. This is the necessary result of
substantial permanency of oceanic basins. The whole geological record
is in shallow-water faunas. These shallow waters are along continental
shore lines and in interior continental seas. According to Chamberlin
again, during a period of continental depression all the flat
continental margins are submerged, forming broad submarine platforms,
and the lower interior portions of the continents are also submerged,
forming wide and shallow interior seas. Under these conditions
continental waste, and therefore sand and clay sediments, are reduced
to a minimum. Life, animal and vegetal, abounds, and therefore much
limestone is formed. The oceans are widely connected with one another,
and therefore the faunas are widespread or cosmopolitan. During the
period of elevation, on the contrary, the continents are extended to
the margin of the deep oceanic basins, the broad, shallow submarine
platforms are abolished, the interior seas are also abolished, the
shallow-water areas are reduced to isolated bays, and their faunas are
peculiar or provincial. Also, elevated and enlarged continents give
rise to maximum erosion, and therefore abundant sediments of sandstone
and clay, and comparative poverty of life and therefore of limestone.
Chamberlin also gives reasons why the oceanic periods should be warm,
humid, equable in temperature, and the atmosphere highly charged
with CO_[2], and therefore highly favorable to abundant life, both
vegetal and animal, while land periods would be drier and cooler, the
atmosphere deficient in CO_[2], and therefore cold from that cause and
in many ways unfavorable to abundant life.
These extremely interesting views, however, must be regarded as still
on trial, as a provisional hypothesis to be sifted, confirmed, or
rejected, or in any case modified, in the next century.