Henry Edward Crampton - The Doctrine of Evolution

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

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From these and similar facts, the naturalist finds how agencies of the
present construct new rocks and alter the old; and so in the light of this
knowledge, he proceeds with his task of analyzing the remote past,
confident that the same natural forces have done the work of constructing
the lower geological levels because these earlier products are similar to
those being formed to-day. After learning this much, he must immediately
undertake to arrange the strata according to their ages. This might seem a
difficult or even an impossible task, but the rocks themselves provide him
with sure guidance.

Wherever a river has graven its deep way through an area of hard rocks, as
in the case of Niagara, the walls display on their cut surfaces a series
of lines and planes showing that they are superimposed layers formed
serially by deposits that have differed some or much at different times
according to the circumstances controlling the erosion of their
constituent particles. A layer of several feet in thickness may be
composed of compact shale, while above it will be a zone of limestone, and
again above this another layer of shale. Successive strata like these,
where they are parallel and obviously undisturbed, are evidently arranged
in the order of their formation and age. But by far the most impressive
demonstration of the basic principle of geology employed for the
determination of the relative ages of rocks is the mighty Canon of the
Colorado. As the traveler stands on the winding rim of this vast chasm,
his eye ranges across 13 miles of space to the opposite walls, which
stretch for scores of miles to the right and left; upon this serried face
he will see zone after zone of yellow and red and gray rock arranged with
mathematical precision and level in the same order as on the steep slopes
beneath him. Plain common sense tells him that the great sheets of rock
stretched continuously at one time between the now separate walls, and
that the various strata of sandstone and limestone were deposited in
successive ages from below upwards in the order of their exposure. When
now he extends his explorations to another state like Utah or Wyoming, he
may find some but not all of the series exhibited in the Grand Canon,
overlaid or underlaid by other strata which in their turn can be assigned
to definite places in the sequence. By the same method, the geologist
correlates and arranges the rocks not only of different parts of the same
state, or of neighboring states, but even those of widely separated parts
of North America and of different continents. But he learns that he must
refrain from over-hasty conclusions, for he soon finds that the
sedimentary rocks have not been constructed at the same rate in different
places during one and the same epoch, and that rocks formed even at one
period are not always identical in nature. But his guiding principle is
sensible and reasonable, and by employing it with due caution he provides
the palaeontologist with the requisite knowledge for his special task,
which is to arrange the extinct animals whose remains are found as fossils
of various earth ages in the order of their succession in time.

CONDENSED TABLE OF PALAEONTOLOGICAL FACTS

__________________________________________________________________________
| | | |
YEARS | NUMBER OF | | | ORDER OF
NECESSARY FOR | FEET IN | GEOLOGICAL | GEOLOGICAL | APPEARANCE OF
FORMATION | THICKNESS | AGE | EPOCH | CHARACTERISTIC
| | | | GROUPS
______________|___________|______________|_______________|________________
| | | |
| | | | M B R A F I
| | | | a i e m i n b
| | | | m r p p s v r
| | Recent | | m d t h h e a
| | or | | a s i i e r t
| | Quaternary | | l l b s t e
| | | | s e i e s
| | | | s a -
______________|___________|______________|_______________|_|_|_|_|_|_|____
| | | | | | | | | |
| | | Pleistocene | | | | | | |
| | Cenozoic | Pliocene | | | | | | |
5,000,000 | 25,000 | or | Miocene | | | | | | |
| | Tertiary | Oligocene | | | | | | |
| | | Eocene | | | | | | |
______________|___________|______________|_______________|_|_|_|_|_|_|____
| | | | | | | | | |
| | Mesozoic | Cretaceous | | | | | | |
4,000,000 | 23,000 | or | Jurassic | | | | | | |
| | Secondary | Triassic | | | | | |
______________|___________|______________|_______________|_____|_|_|_|____
| | | | | | | |
| | | Permian | | | | |
| | Palaeozoic | Carboniferous | | | |
21,000,000 | 106,000 | or | Devonian | | |
| | Primary | Silurian | | |
| | | Cambrian | | |
______________|___________|______________|_______________|________________
| | | |
20,000,000 | 30,000 | Azoic | Archaen |
______________|___________|______________|_______________|________________

After what seems an unduly long preparation, we now come to the actual
biological evidence of evolution provided by the results of this division
of zooelogical science. But all of the foregoing is fundamentally part of
this department of knowledge and it is absolutely essential for any one
who desires to understand what the fossils themselves demonstrate.

The oldest sedimentary rocks are devoid of fossil remains and so they are
called the Azoic or Archaean. They comprise about 30,000 feet of strata
which seem to have required at least 20,000,000 years for their formation.
This period is roughly two-fifths of the whole time necessary for the
formation of _all_ the sedimentary rocks, and this proportion holds true
even if the entire period of years should be taken as 100,000,000 instead
of 50,000,000 or less. The earth during this early age was slowly
organizing in chemical and physical respects so that living matter could
be and indeed was formed out of antecedent substances--but this process
does not concern us here. The important fact is that the second major
period, called the Palaeozoic, or "age of ancient animals," saw the
evolution of the lowest members of the series,--the invertebrates,--and
the most primitive of the backboned animals, like fishes and amphibia. The
rocks of this long age include about 106,000 feet of strata, demanding
some 21,000,000 or 22,000,000 years for their deposition. Thus it is
proved that the invertebrate animals were succeeded in time by the higher
vertebrates, which is exactly what the evidences of the previous
categories have shown. When we remember that the lower animals are devoid
as a rule of skeletal structures that might be fossilized, and when we
recall the fact that the strata of the palaeozoic provided the materials
out of which the upper layers were formed afterwards, we can understand
why the ancient members of the invertebrate groups are not known as well
as the later and higher forms like vertebrates. Yet all the fossils of
these relatively unfamiliar creatures clearly prove that no complex animal
appears upon a geological horizon until after some simple type belonging
to a class from which it may have taken its origin; in brief, there are no
anachronisms in the record, which always corresponds with the record
written by comparative anatomy, wherever the facts enable a comparison to
be made.

But the extinct animals of the third and fourth ages are more interesting
to us, because there are more of them and because they are more like the
well-known organisms of our present era. These two ages are called the
Mesozoic or Secondary, and the Cenozoic or Tertiary. The former is so
named because it was a transitional age of animals that are intermediate
in a general way between the primitive forms of the preceding age and
those of the next period; the latter name means the "recent-animal" age,
when evolution produced not only the larger groups of our present animal
series, but also many of the smaller branches of the genealogical tree
like orders and families to which the species of to-day belong.

Confining our attention to the large vertebrate classes, the testimony of
the rocks proves, as we have said, that fishes appeared first in what are
called the Silurian and Devonian epochs, where they developed into a rich
and varied array of types unequaled in modern times. At that period, they
were the highest existing animals--the "lords of creation," as it were. To
change the figure, their branch constituted the top of the animal tree of
the time, but as other branches grew upwards to bear their twigs and
leaves, as the counterparts of species, the species of the branch of
fishes decreased in number and variety, as do the leaves of a lower part
of a tree when higher limbs grow to overshadow them.

Following the fishes, the amphibia arose during the coal age or
Carboniferous, usurping the proud position of the lower vertebrate class.
The reptiles then appeared and gained ascendancy over the amphibia, to
become in the Mesozoic age the highest and most varied of the existing
vertebrates. At that time there were the great land dinosaurs with a
length of 80 feet, like _Brontosaurus_; aquatic forms like _Ichthyosaurus_
and _Plesiosaurus_, whose mode of evolution from terrestrial to swimming
habits was like that of seals and penguins of far later eras. Flying
reptiles also evolved, to set an example for the bats of the mammalian
class, for both kinds of flying organisms converted their anterior limbs
into wings, although in different ways.

During the Triassic and Jurassic periods of the Mesozoic age, the first
birds and mammals appeared to follow out their diverging and independent
lines of descent. Palaeontology makes it possible to trace the origin and
development of many of the different branches that grew out of the
mammalian limb from different places and at different times during the
Mesozoic and the following age, called the Cenozoic, or age of recent
animals. It is unnecessary, however, for us to review more of the details:
the main result is obvious; namely, that the appearance of the great
classes of vertebrates is in the order of comparative anatomy and
embryology. Not only, then, is the fact of evolution rendered trebly sure,
but the general order of events is thrice and independently demonstrated
to be one and the same. Surely we must see that no reasonable explanation
other than evolution can be given for these basic facts and principles.

Turning now to the second division of palaeontological evidence, we come to
those groups where abundant materials make it possible to arrange the
animals of successive epochs in series that may be remarkably complete.
For the reasons specified, the backboned animals provide the richest
arrays of these series, and such histories as those of horses and
elephants have taken their places in zooelogical science as classics. But
even among the invertebrates significant cases may be found. For example,
in one restricted locality in Germany the shells of snails belonging to
the genus _Paludina_ have been found in superimposed strata in the order
of their geological sequence. The ample material shows how the several
species altered from age to age by the addition of knobs and ridges to the
surface of the shell, until the fossils in the latest rocks are far
different from their ancestors in the lowermost levels. Yet the
intervening shells fill in the gaps in such a way as to show almost
perfectly how the animals worked out their evolutionary history. This
example illustrates the nature of many other known series of mollusks and
of brachiopods, extending over longer intervals and connecting more widely
separated ages like the Secondary and the present period.

Since the doctrine of evolution and its evidences began to occupy the
thoughts of the intellectual world at large, no fossil forms have received
more attention than the ancient members of the horse tribe. As we have
learned, a modern horse is described by comparative anatomy as a one-toed
descendant of remote five-toed ancestors. When the hoofed animals of
modern times were reviewed as subjects for comparative anatomical study,
the odd-toed forms arranged themselves in a series beginning with an
animal like an elephant with the full number of five digits on each foot
and ending at the opposite extreme with the horse. A reasonable
interpretation of these facts was that the animals with fewer toes had
evolved from ancestors with five digits, of which the outer ones had
progressively disappeared during successive geological periods, while the
middle one enlarged correspondingly. The facts provided by palaeontology
sustain this contention with absolutely independent testimony.
Disregarding some problematical five-toed forms like _Phenacodus_, the
first type of undoubted relationship to modern horses is _Hyracotherium_,
a little animal about three feet long that lived during the Eocene period
of the Cenozoic epoch. Its forefeet had four toes each, and its hinder
limbs ended with three toes armed with small hoofs, but one of its
relatives of the same time has a vestige of another digit on the hind
foot. By the geological time mentioned, therefore, the earliest true
horses had already lost some of the toes that their progenitors possessed.
In the Miocene the extinct species, obviously descended from the Eocene
forms, had lost more of their toes; still higher, that is, in the rocks
formed during succeeding periods of time, the animals of this division are
much larger and each of their feet has only three toes, of which the
middle one is the largest while the ones on the sides are small and
withdrawn from the ground so as to appear as useless vestiges. To produce
modern horses and zebras from these nearer ancestors, few additional
changes in the structure of the feet are necessary, for the lateral toes
need only to become a little more reduced and the middle one to enlarge
slightly to give the one-toed limb of modern types, with its splint-like
vestiges still in evidence to show that the ancestor's foot comprised more
of these terminal elements. Comparing the animals of successive periods,
these and other skeletal structures demonstrate that the ancestry of each
group of species is to be found in the animals of the preceding epoch, and
that the whole history of horses is one of natural transformation,--in a
word, of evolution.

No less interesting in their own way are the remains of other hoofed forms
that lead down to the elephants of to-day and to the mammoth and mastodon
of relatively recent geologic times. Common sense would lead to the
conclusion that a form like a modern tapir was the prototype from which
these creatures have arisen, and common sense would lead us to expect that
if any fossils of the ancestors of the modern group of elephants occurred
at all they would be like tapirs. Thus a fossil of much significance in
this connection is _Moeritherium_, whose remains have been found in the
rocks exposed in the Libyan desert, for this creature was practically a
tapir, while at the same time its characters of muzzle and tusk mark it as
very close to the ancestors of the larger woolly elephants of later
geological times, when the trunk had grown considerably and the tusks had
become greatly prolonged. Again the fossil sequence confirms the
conclusions of comparative anatomy, regarding the mode by which certain
modern animals have evolved.

The fossil deer of North America, as well as many other even-toed members
of the group of mammalia possessing hoofs, provide the same kind of
conclusive evidence. The feature of particular interest in the case of
their horns, is a correspondence between the fossil sequence and the order
of events in the life-history of existing species,--that is, between the
results of palaeontology and of embryology. Horns of the earliest known
fossil deer have only two prongs; in the rocks above are remains of deer
with additional prongs, and point after point is added as the ancient
history of deer is traced upwards through the rocks to modern species. We
know that the life-history of a modern species of animals reviews the
ancestral record of the species, and what happens during the development
of deer can be directly compared with the fossil series. It is a matter of
common knowledge that the year-old stag has simple spikes as horns, and
that these are shed to be replaced the following year by larger forked
horns. Every year the horns are lost and new ones grow out, and become
more and more elaborately branched as time goes on, thus giving a series
of developmental stages that faithfully repeats the general order of
fossil horns. Even Agassiz, who was a believer in special creation and an
opponent of evolution, was constrained to point out many other instances,
mainly among the invertebrata, where there was a like correspondence
between the ontogeny of existing species and their phylogenetic history as
revealed by the fossil remains of their ancestors.

* * * * *

In the last place, we must give more than a passing consideration to some
of the extinct types of animals that occupy the position of "links"
between groups now widely separated by their divergence in evolution from
the same ancestors. Perhaps the most famous example is _Archaeopteryx_
found in a series of slates in Germany. This animal is at once a
feathered, flying reptile, and a primitive bird with countless reptilian
structures. Its short head possesses lizard-like jaws, all of which bear
teeth; its wings comprise five clawed digits; its tail is composed of a
long series of joints or vertebrae, bearing large feathers in pairs; its
breastbone is flat and like a plate, thus resembling that of reptiles and
differing markedly from the great keeled breastbone of modern flying
birds, whose large muscles have necessitated the development of the keel
for purposes of firm attachment. In brief, this animal was close to the
point where reptiles and birds parted company in evolution, and although
it was a primitive bird, it is in a true sense a "missing link" between
reptiles and the group of modern birds. Other fossil forms like
_Hesperornis_ and _Ichthyornis_, whose remains occur in the strata of a
later date, fill in the gap between _Archaeopteryx_ and the birds at the
present time, for among other things they possess teeth which indicate
their origin from forms like _Archaeopteryx_, while in other respects they
are far nearer the birds of later epochs. That these links are not unique
is proved by numerous other examples known to science, such as those which
connect amphibia and reptiles, ancient reptiles and primitive mammals, as
well as those which come between the different orders of certain
vertebrate classes.

In summarizing the foregoing facts, and the larger bodies of evidence that
they exemplify, we learn how surely the testimony of the rocks establishes
evolution in its own way, how it confirms the law of recapitulation
demonstrated by comparative embryology, and how it proves that the greater
and smaller divisions of animals have followed the identical order in
their evolution that the comparative study of the present day animals has
independently described.

* * * * *

The facts of geographical distribution constitute the fifth division of
zooelogy, and an independent class of evidences proving the occurrence of
evolution. This department of zooelogy assumed its rightful status only
after the other divisions had attained considerable growth. Many
naturalists before Darwin and Wallace and Wagner had noticed that animals
and plants were by no means evenly distributed over the surface of the
globe, but until the doctrine of evolution cleared their vision they did
not see the meaning of these facts. As in the case of all the other
departments of zooelogy the immediate data themselves are familiar, but
because they are so obvious the mind does not look for their
interpretation but accepts the facts at their face value. While the
phenomena of distribution are no less fascinating to the naturalist, and
no less effective in their demonstration of evolution, their comprehensive
treatment would demand more space than the whole purpose of the present
description of organic evolution would justify. Thus a brief outline only
can be given of the salient principles of this subject in order that their
bearing upon the problem of species may be indicated.

Even as children we learn many facts of animal distribution; every one
knows that lions occur in Africa and not in America, that tigers live in
Asia and Malaysia, that the jaguar is an inhabitant of the Brazilian
forests, and that the American puma or mountain lion spreads from north to
south and from east to west throughout the American continents. The
occurrence of differing human races in widely separated localities is no
less familiar and striking, for the red man in America, the Zulu in
Africa, the Mongol and Malay in their own territories, display the same
discontinuity in distribution that is characteristic of all other groups
of animals and of plants as well. As our sphere of knowledge increases, we
are impressed more and more forcibly by the diversity and unequal extent
of the ranges occupied by the members of every one of the varied divisions
of the organic world. Another fact which becomes significant only when
science calls our attention to it is the absence from a land like
Australia of higher mammals such as the rabbit of Europe. The hypothesis
of special creation cannot explain this absence on the assumption that the
rabbit is unsuited to the conditions obtaining in the country named, for
when the species was introduced into Australia by man, it developed and
spread with marvelous rapidity and destructive effect. It may seem
impossible that facts like these could possess an evolutionary
significance, but they are actual examples of the great mass of data
brought together by the naturalists who have seen in them something to be
interpreted, and who have sought and found an explanation in the
formularies of science.

The general principles of distribution appear with greatest clearness when
an examination is made of the animals and plants of isolated regions like
islands. The Galapagos Islands constitute a group that has figured largely
in the literature of the subject, partly because Darwin himself was so
impressed by what he found there in the course of his famous voyage around
the world in the "Beagle." They form a cluster on the Equator about six
hundred miles west of the nearest point of the neighboring coast of South
America. Although the lizards and birds that live in the group differ
somewhat among themselves as one passes from island to island, on the
whole they are most like the species of the corresponding classes
inhabiting South America. Why should this be so? On the hypothesis of
special creation there is no reason why they should not be more like the
species of Africa or Australia than like those of the nearest body of the
mainland. The explanation given by evolution is clear, simple, and
reasonable. It is that the characteristic island forms are the descendants
of immigrants which in greatest probability would be wanderers from the
neighboring continent and not from far distant lands. Reaching the
isolated area in question the natural factors of evolution would lead
their offspring of later generations to vary from the original parental
types, and so the peculiar Galapagos species would come into being. The
fact that the organisms living on the various islands of this group differ
somewhat in lesser details adds further justification for the evolutionary
interpretation, because it is not probable that all the islands would be
populated at the same time by similar stragglers from the mainland. The
first settlers in one place would send out colonies to others, where
independent evolution would result in the appearance of minor differences
peculiar to the single island. In this manner science interprets the
general agreement between the animals of the Azores Islands and the fauna
of the northwestern part of Africa, the nearest body of land, from which
it would be most natural for the ancestors of the island fauna to come.

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