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Henry Edward Crampton - The Doctrine of Evolution

H >> Henry Edward Crampton >> The Doctrine of Evolution




The word "palaeontology" means literally the "science of living things of
long ago." It deals directly with the remains of animals and plants found
as fossils, and it interprets them through its knowledge of the way modern
animals are constructed and of the changes the earth's crust has
undergone. A skull-like object may be found in a coal field and may come
into the hands of the palaeontologist: from his acquaintance with the head
skeletons of recent types he will be able to assign the extinct creature
which possessed the skull to a definite place in the animal scale and to
understand its nearer or wider affinities with other animals of later
times and of earlier epochs. In doing these things palaeontology employs
the methods of comparative anatomy with which we have now become familiar.
In the performance of its other tasks, however, palaeontology must work
independently. It is necessary to know when a fossilized animal lived, not
that its time need be measured by an absolute number of a few thousands or
millions of years antedating our own era, for that is impossible. But the
important thing is to know its relative age, and whether it preceded or
followed other similar animals of its own group or of different divisions.
The rocks themselves must be understood, how they have been formed and how
they are related in mineralogical nature and in historical succession.
Palaeontology also deals with a number of subjects that are not in
themselves biological, such as the combination of circumstances necessary
for the adequate preservation of fossil relics. In so far as it is
concerned with physical matters, as contrasted with strictly biological
data, it is one with geology. Indeed, the investigators in these two
departments must always work side by side and render mutual assistance to
one another in countless ways, for each division needs the results of the
other in order to accomplish its own distinct purposes. It must be evident
to every one that it is impossible to understand the meaning of fossils
and the place of the testimony of the rocks in the doctrine of evolution
without knowing much about the geological history of the earth and the
influences at work in the past. For these reasons palaeontology differs
somewhat from the other divisions of zooelogy where direct observation
gives the materials for arrangement and study; in this case the individual
data, that is, the fossil fragments themselves, can be made available only
through a knowledge of their exact situations, of the reasons for their
occurrence in particular places in the rock series and of the way rocks
themselves are constructed and worked over by natural agencies. Our task
is therefore twofold: certain physical matters of a geological nature must
first be investigated before the biological facts can be described.

No doubt most people feel justified in believing that the whole doctrine
of evolution must stand or fall according to the cogency of the
palaeontological evidences. Plain common sense says that the owners of
shelly or bony fragments found in the deeply-laid strata of the earth must
have lived countless years ago, and if the evolutionist asserts that
primitive organic forms of ancient times have produced changed descendants
of later times, it would seem that fossil evidence would be supremely and
overwhelmingly important. It is true, of course, that this evidence is
peculiarly significant, because in some ways it is more direct than that
of the other categories already outlined. But it must not be forgotten
that the doctrine is already securely founded upon the basic principles of
anatomy and embryology. Science must treat the data of this category by
different methods and must view them in different ways. Therefore we are
interested in palaeontology because of the way it tells the story of
evolution in its own words, and because we are justified in expecting that
its account should include a description of some such order of events as
that revealed by the developing embryos of modern organisms and that
demonstrated by the comparative anatomy of the varied species of adult
animals.

It is true that palaeontology gives direct testimony about the evolutionary
succession of animals in geologic time. But we now know that embryology is
even more direct in its proof that organic transformation is natural and
real; while at the same time there is a completeness in the full series of
developmental stages connecting the one-celled egg with the adult creature
that must be forever lacking in the case of the fossil sequence of
species. If paragraphs and pages are missing from the brief embryonic
recapitulation, whole chapters and volumes of the fossil series have been
lost for all time. The investigators whose task it has been to decipher
the story of the earth's evolution have had to meet numerous and
exasperating difficulties which do not confront the embryologist and
anatomist who study living materials. Nevertheless the library of
palaeontological documents is one which has been founded for over a
century, and it has grown fast during recent decades, so that consistent
accounts may now be read of the great changes in organic life as the earth
has altered and grown older. And in all this record, there is not a single
line or word of fact that contradicts evolution. What definite evidence
there is tells uniformly in favor of the doctrine, for it is possible, in
the first place, to work out the order of succession of many of the great
groups of animals, and this order is found to be the same as that
established by the other bodies of evidence. Secondly, some fossil groups
are astonishingly complete, so that the ancient history of a form like the
horse can be written with something approaching fullness. Finally, the
remains of certain animals have been found so situated in geological ways,
and so constructed anatomically, that the zooelogist is justified in
denoting them "missing links," because they seem to have been intermediate
between groups that have diverged so widely during recent epochs as to
render their common ancestry scarcely credible.

With these general results in mind, we must now become acquainted with
such subjects as the interpretation of fossils, the causes for the
incompleteness of the series, the conditions for fossilization, the forces
of geological nature, and other matters that make the fossils themselves
intelligible as scientific evidence.

* * * * *

Many views have been entertained regarding the actual nature of the relics
of antiquity exhumed from the rocks or exposed upon the surface by the
wear and tear of natural agencies. In earliest times such things were
variously considered as curious freaks of geological formation, as sports
of nature, or as the remains of the slain left upon the battle-ground of
mythical Titans. Some of the Greeks supposed that fossils were parts of
animals formed in the bowels of the earth by a process of spontaneous
generation, which had died before they could make their way to the
surface. They were sometimes described as the bones of creatures stranded
upon the dry land by tidal waves, or by some such catastrophe as the
traditional flood of the scriptures. In medieval times, and even in our
own day, some people who have been opposed to the acceptance of any
portion of the doctrine of evolution have actually defended the view that
the things called fossils were never the shells or bones of animals living
in bygone times, but that they only simulate such things and have been
created as such together with the layers of rock from which they may have
been taken. If we employed the same arguments in dealing with the broken
fragments of vases and jewelry taken from the Egyptian tombs or from the
buried ruins of Pompeii, we would have to believe that such pieces were
created as fragments and that they were never portions of complete
objects, just because no one alive to-day has ever seen the perfect vessel
or bracelet fashioned so long ago. Common sense directs us to discard such
a fantastic interpretation in favor of the view that fossils are what they
seem to be--simply relics of creatures that lived when the earth was
younger.

Until this common sense view was adopted there was no science of
palaeontology. Cuvier was the first great naturalist to devote particular
attention to the mainly unrelated and unverified facts that had been
discovered before his time. He was truly the originator of this branch of
zooelogy, for he brought together the observations of earlier men and
extended his own studies widely and surely, emphasizing particularly the
necessity for noting carefully the geological situation of a fossil in
rocks of an older or later period of formation. His great result was the
demonstration that many groups of animals existed in earlier ages that
seem to have no descendants of the same nature to-day, and also that many
or most of our modern groups are not represented in the earliest formed
sedimentary rocks, although these recent forms possess hard parts which
would surely be present somewhere in these levels if the animals actually
existed in those times. But the meaning of these facts escaped Cuvier's
mind. He was a believer in special creation, like Linnaeus and all but a
few among his predecessors, and he explained the diversity of the faunas
of different geological times in what seems to us a very simple and naive
way. In the beginning, he held, when the world was created, it was
furnished with a complete set of animals and plants. Then some great
upheaval of nature occurred which overwhelmed and destroyed all living
creatures. The Creator then, in Cuvier's view, proceeded to construct a
new series of animals and plants, which were not identical with those of
the former time, but were created according to the same general working
plans or architectural schemes employed before. Another cataclysm was
supposed to have occurred, which destroyed the second series of organisms
and laid a new covering of rocks over the earth's surface for a subsequent
period of relative quiet; and so the process was continued. By this
account, Cuvier endeavored to reconcile the doctrine of supernatural
creation and intervention with the obvious facts that organisms have
differed at various times in the earth's history. Although he saw that
animals of successive periods displayed similar structures, like the
skeleton of vertebrates, which testified to some connection, Cuvier could
not bring himself to believe that this connection was a genealogical one.

Mainly through the influence of the renowned English man of science,
Charles Lyell, the students of the earth came to the conclusion that its
manifold structures had developed by a slow and orderly process that was
entirely natural; for they found no evidence of any sudden and drastic
world-wide remodeling such as that postulated by the Cuvierian hypothesis
of catastrophe. The battle waged for many years; but now naturalists
believe that the forces, of nature, whose workings may be seen on all
sides at the present time, have reconstructed the continents and ocean
beds in the past in the same way that they work to-day. The long name of
"uniformitarianism" is given to Lyell's doctrine, which has exerted an
influence upon knowledge far outside the department of geology. Darwin
tells us how much he himself was impressed by it, and how it led him to
study the factors at work upon organic things to see if he could discern
evidence of a biological uniformitarianism, according to which the past
history of living things might be interpreted through an understanding of
their present lives.

* * * * *

What, now, are the reasons why the palaeontological evidence is not
complete and why it cannot be? In the first place the seeker after fossil
remains finds about three fifths of the earth's surface under water so
that he cannot explore vast areas of the present ocean beds which were
formerly dry land and the homes of now extinct animals. Thus the field of
investigation is seriously restricted at the outset, but the naturalist
finds his work still more limited, in so far as much of the dry land
itself is not accessible. The perennial snows of the Arctic region render
it impossible to make a thorough search in the frigid zone, and there are
many portions of the temperate and torrid zones that are equally
unapproachable for other reasons. But even where exploration is possible,
the surface rocks are the only ones from which remains can be readily
obtained, for the layers formed in earlier ages are buried so deeply that
their contents must remain forever unknown in their entirety. Only a few
scratches upon the earth's hard crust have been made here and there, so it
is small wonder that the complete series of extinct organisms has not been
produced by the palaeontologist.

A brief survey of the varied groups of animals themselves is sufficient to
bring to light many biological reasons which account for still more of the
vacant spaces in the palaeontological record. We would hardly expect to
find remains of ancient microscopic animals like the protozoa, unless they
possessed shells or other skeletal structures which in their aggregate
might form masses like the chalk beds of Europe. Jellyfish and worms and
naked mollusks are examples of the numerous orders of lower animals having
no hard parts to be preserved, and so all or nearly all of the extinct
species belonging to these groups can never be known. But when an animal
like a clam dies its shell can resist the disintegrating effects of
bacteria and other organic and inorganic agencies which destroy the soft
parts, and when a form like a lobster or a crab, possessing a body
protected by closely joined shell segments, falls to the bottom of the
sea, the chances are that much of the animal's skeleton will be preserved.
Thus it is that corals, crustacea, insects, mollusks, and a few other
kinds of lower forms constitute the greater mass of invertebrate
palaeontological materials because of their supporting structures of one
kind or another. Perhaps the skeletal remains of the vertebrates of the
past provide the student of fossils with his best facts, on account of the
resistant nature of the bones themselves, and because the backboned
animals are relatively modern; then, too, the rocks in which their remains
occur have not been so much altered by geological agencies, or buried so
deeply under the strata formed later. Of course only the hardest kinds of
shells would remain as such after their burial in materials destined to
turn into rock; in the majority of cases, an entombed bone is infiltrated
or replaced by various mineral substances so that in time little or
nothing of the original thing would remain, though a mold or a cast would
persist.

But even if an animal of the past possessed hard structures, it must have
satisfied certain limited conditions to have its remains prove serviceable
to students of to-day. A dead mammal must fall upon ground that has just
the right consistency to receive it; if the soil is too soft, its several
parts will be separated and scattered as readily as though it had fallen
upon hard ground where it would be torn to pieces by carnivorous animals.
The dead body must then be covered up by a blanket of silt or sand like
that which would be deposited as the result of a freshet. If a skeleton is
too greatly broken up or scattered, it may be difficult or even impossible
for its discoverer to piece together the various fragments and assemble
them in their original relations. Very few individuals have been so buried
and preserved as to meet the conditions for the formation of an ideal
fossil. To realize how little may be left of even the most abundant of
higher organisms, we have only to recall that less than a century ago
immense herds of bison and wild horses roamed the Western plains, but very
few of their skulls or other bones remain to be enclosed and fossilized in
future strata of rocks. When we appreciate all these difficulties, both
geological and biological, we begin to see clearly why the ancient lines
of descent cannot be known as we know the path and mode of embryonic
transformation. The wonder is not that the palaeontological record is
incomplete, but that there is any coherent and decipherable record at all.
Yet in view of the many and varied obstacles that must be surmounted by
the investigator, and the adverse factors which reduce the available
evidence, the rapidly growing body of palaeontological facts is amply
sufficient for the naturalist to use in formulating definite and
conclusive principles of evolution.

* * * * *

For the purposes of palaeontology, the most essential data of geology are
those which indicate the relative ages of the strata that make up the hard
outer crust of the earth, for only through them can the order of animal
succession be ascertained. It does not matter exactly how old the earth
may be. While it is possible to determine the approximate length of time
required for the construction of sedimentary rocks like those which
natural agencies are producing to-day, there are few definite facts to
guide speculation as to the mode or duration of the process by which the
first hard crystalline surface of the earth was formed. But palaeontology
does not care so much about the earliest geological happenings, for it is
concerned with the manifold animal forms that arose and evolved after life
appeared on the globe. Questions as to the way life arose, and as to the
earliest transformations of the materials by which the earth was first
formed are not within the scope of organic evolution, although they relate
to intensely interesting problems for the student of the process of cosmic
evolution.

According to the account now generally accepted, the original material of
the earth seems to have been a semi-solid or semi-fluid mass formed by the
condensation of the still more fluid or even gaseous nebula out of which
all the planets of the solar system have been formed and of which the sun
is the still fiery core. As soon as the earth had cooled sufficiently its
substances crystallized and wrinkled to form the first mountains and
ridges; between and among these were the basins which soon filled with the
condensing waters to become the earliest lakes and oceans. The wear and
tear of rains and snows and winds so worked upon the surfaces of the
higher regions that sediments of a finer or coarser character like sand
and mud and gravel were washed down into the lower levels. These sediments
were afterwards converted into the first rocks of the so-called stratified
or sedimentary series, as contrasted with the crystalline or plutonic
rocks like the original mass of the earth and the kinds forced to the
surface by volcanic eruptions. Later the earth wrinkled again in various
ways and places so that new ridges and mountains were formed with new
systems of lakes and oceans and rivers; and again the elements continued
to erode and partially destroy the higher masses and to lay down new and
later series of sedimentary rocks upon the old.

It seems scarcely credible that the apparently weak forces of nature like
those we have mentioned are sufficiently powerful to work over the massive
crust of the earth as geology says they have. Our attention is caught, as
a rule, only by the greater things, like the earthquakes at San Francisco
and Valparaiso, and the tidal waves and cyclones of the South Seas; but
the results of these sporadic and local cataclysms are far less than the
effects of the persistent everyday forces of erosion, each one of which
seems so small and futile. When we look at the Rocky Mountains with their
high and rugged peaks, it seems almost impossible that rain and frost and
snow could ever break them up and wear them down so that they would become
like the rounded hills of the Appalachian Mountain chain, yet this is what
will happen unless nature's ways suddenly change to something which they
are not now. A visitor to the Grand Canon of the Colorado sees a
magnificent chasm over a mile in depth and two hundred miles long which
has actually been carved through layer after layer of solid rock by the
rushing torrents of the river. Perhaps it is easier to estimate the
geological effects of a river in such a case as Niagara. Here we find a
deep gorge below the famous falls, which runs for twenty miles or so to
open out into Lake Ontario. The water passing over the brim of the falls
wears away the edge at a rate which varies somewhat according to the
harder or softer consistency of the rocks, but which, since 1843, has
averaged about 104 inches a year. Knowing this rate, the length of the
gorge, and the character of the rocky walls already carved out, the length
of time necessary for its production can be safely estimated. It is about
30,000 to 40,000 years, not a long period when the whole history of the
earth is taken into account. A similar length of time is indicated for the
recession of the Falls of St. Anthony, of the Mississippi River, an
agreement that is of much interest, for it proves that the two rivers
began to make their respective cuttings when the great ice-sheet receded
to the north at the end of the Glacial epoch.

What has become of the masses washed away during the formation of these
gorges? As gravel and mud and silt the detritus has been carried to the
still waters of the lower levels, to be laid down and later solidified
into sandstone and slate and shale. All over the continents these things
are going on, and indefatigable forces are at work that slowly but surely
shear from the surface almost immeasurable quantities of earth and rock to
be transported far away. In some instances it is possible to find out just
how much effect is produced in a given period of time, especially in the
case of the great river systems. For example, the mass of the fine
particles of mud and silt carried in a given quantity of the water of the
Mississippi as it passes New Orleans can be accurately measured, and a
satisfactory determination can also be made of the total amount of water
carried by in a year. From these figures the amount of materials in
suspension discharged into the Gulf of Mexico becomes known. It is
sufficient to cover one square mile to the depth of 269 feet; in twenty
years it is one cubic mile, or five cubic miles in a century. Turning now
to the other aspect of this process, and the antecedent causes which
produce these effects, it appears that the area of the Mississippi River
basin is 1,147,000 square miles--about one third of the total area of the
United States. Knowing this, and the annual waste from its surface, it is
easy to demonstrate that it will take 6000 years to plane off an average
of one foot of soil and rock from the whole of this immense area. Of
course only an inch or a few inches will be taken from some regions where
the ground is harder or rockier, or where little rain falls, while many
feet will be washed away from other places. The waters of the Hoang-ho
come from about 700,000 square miles of country, from which one foot of
soil is washed away in 1464 years. The Ganges River, draining about
143,000 square miles, carries off a similar depth of eroded materials from
its basin in 823 years! Should we add to the above figures those that
specify the bulk of the chemical substances in solution carried by these
waters, the total would be even greater. We know that in the case of the
Thames River, calcareous substances to the amount of 10,000 tons a year
are carried past London, and all this mineral has been dissolved by
rain-water from the chalky cliffs and uplands of England, so that the land
has become less by this amount. Thus we learn that vast alterations are
being made in the structure of great continents by rain and rivers, as well
as by glaciers and other geological agencies. And at the same time that old
strata are undergoing destruction new ones are in process of construction
at other places, where animal remains can be embedded and preserved as
fossils. The forces at work seem weak, but they continue their operations
through ages that are beyond our comprehension and they accomplish results
of world-building magnitude.

Thus the whole process of geological construction is such that older
exposed strata continually undergo disintegration, but this involves the
destruction of any fossils that they might contain. The very forces that
preserve the relics of extinct animals at one time undo their work at a
later period. There are many other influences besides that destroy the
regularity of rock layers or change their mineralogical characters by
metamorphosis. It is easier to see how volcanic outbursts alter their
neighboring territory. The intense subterranean heat and imprisoned steam
melt the deeper substances of the earth's crust, so that these materials
boil out, as it were, where the pressure is greatest, and where lines of
fracture and lesser resistance can be found. Because so much detritus is
annually added to the ocean floors--enough to raise the levels of the
oceans by inches in a century--it is natural that greater pressures should
be exerted in these areas than in the slowly thinning continental regions.
These are some of the reasons why volcanoes arise almost invariably along
the shores or from the floors of great ocean beds. The chain that extends
from Alaska to Chili within the eastern shore of the Pacific Ocean, and
the many hundreds of volcanoes of the Pacific Islands bring to the surface
vast quantities of eruptive rocks which break up and overlie the
sedimentary strata formed regularly in other ways and at other times. The
volcanoes of the Java region alone have thrown out at least 100 cubic
miles of lava, cinders, and ashes during the last 100 years--twenty times
the bulk of the materials discharged into the Gulf of Mexico by the
Mississippi River in the same period of time.

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