Henry Edward Crampton - The Doctrine of Evolution

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

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If we should visit a woodland pond in early spring, we would find
somewhere among the leaves and sticks in the water large masses of a clear
jellylike consistency enclosing hundreds of little black spheres about an
eighth of an inch in diameter. These are the egg masses and eggs of a
common frog. Watching them day by day we see the small one-celled egg
spheres divide into more and more numerous portions which are the
daughter-cells, destined to form by their products the many varied tissues
and organs of the developing larva and adult frog. After three or four
days the egg changes from its globular form into an oval or elliptical
mass, and from one end of this a small knob projects to become a flattened
waving tail a few days later. On the sides of the larger anterior portion
shallow grooves make their appearance and soon break through from the
throat or pharynx to the exterior as gill-slits. Shortly afterwards the
little embryo wriggles out of its encasing coat of jelly, develops a
mouth, and begins its independent existence as a small tadpole, with eyes,
nasal and auditory organs, and all other parts that are necessary for a
free life. Thus the one-celled egg has transformed into something that it
was not at first, and in doing this it has proved the possibility and the
reality of organic reconstruction.

The tadpole breathes by means of its gills, and it is at first entirely
devoid of the lungs which the adult frog possesses and uses. When we speak
of the larval respiratory organs as gills we imply that they are like the
organs of a fish which have the same name; they are truly like those of
fishes, for the blood-vessels which go to them are essentially the same as
in the lower types and they are supported by simple skeletal rods like the
gill-bars of the fish. In a word, they are the same things.

The animal feeds and grows during the months of its first summer, and
hibernates the following winter; with the warmth of spring it revives and
proceeds further along the course of its development. Near the base of the
tail two minute legs grow out from the hinder part of the body, and while
these are enlarging two front legs make their appearance a little behind
the gills. The tadpole now rises more frequently to the surface where it
takes small mouthfuls of air. Meanwhile great changes are effected inside
the body where the various systems of fishlike organs become remodeled
into amphibian structures. A sac is formed from the wall of the esophagus,
and this enlarges and divides to form the two simple lungs. The legs
increase in size, the tail dwindles more and more, the gills close up, and
soon the animal hops out on land as a complete young frog. From this time
on it breathes by means of its lungs instead of gills, even though it
returns to the water to escape its foes, to seek its prey, and to
hibernate in the mud of the lake bed during the winter months.

All these changes are familiar and natural, but until science places them
and similar facts in their proper relations their significance is lost to
us. The tadpole is essentially a fish in its general structure and mode of
life, even though its heritage is such that it can develop into a higher
animal. When it does become a frog it proves beyond a doubt that there is
no impassable barrier between fishes and amphibia. Our earlier comparison
of the structures of these two classes of vertebrates led to the
conclusion that the latter had evolved from antecedents like the former,
and had thus followed them upon the earth; now that sequence seems to have
some connection with the method by which a tadpole, obviously not a fish
but nevertheless actually fishlike, changes into a frog, a member of a
higher class of vertebrates. This method is employed by developing frogs
apparently because it follows the ancestral order of events, and because,
so to speak, the only way a frog knows how to become a frog is to develop
from an egg first into a fishlike tadpole and then to alter itself as its
ancestors did during their evolution in the past. We begin to see, then,
that in addition to the impressive fact of development itself, the mode of
organic transformation is far more conclusive evidence of evolution,
because it reveals an order of events which parallels the order
established by comparative anatomy as the evolutionary sequence.

However it is well to review some of the changes by which a chick comes
into existence before attempting to comprehend fully the fundamental
principle of development that the tadpole's history discloses to us. The
egg of a common fowl is certainly not a chick. Within the calcareous shell
are two delicate membranes that enclose the white or albumen; within this,
swung by two thickened cords of the albumen, is the yellow yolk ball
enclosed by a proper membrane of its own. In the earliest condition, even
before the albumen and the shell are added and before the egg is laid, on
one side of the yolk-mass there is a tiny protoplasmic spot which is at
first a single cell and nothing more. The hen's egg is relatively
enormous, but nevertheless, like that of the frog, it starts upon its
course of development as a single unitary biological element--a cell.
During the earliest subsequent hours the first cell divides again and
again to form a small disk upon the surface of the yolk. Soon the cells
along the middle line of this small sheet become rearranged to make an
obvious streak or band, and about this line a simple tube is constructed
which is destined to become the future brain and spinal cord. The whole
disk continues to enlarge by further division of its constituent elements
so that it encloses more and more of the yolk mass, but the little chick
itself is made out of the cells along the central line of the original
plate, from which it folds at the sides and in front and behind so as to
lie somewhat above and apart from the flatter enclosing cell layers which
partly surround the yolk.

At the sides of the primitive nerve-tube small blocks of cells arise to
develop into primitive muscles and other structures. As nourishment is
brought to the embryo from the surrounding layers enclosing the nutrient
yolk, one system after another takes its shape and builds its several
parts into organs which can be recognized as elementary structures of a
chick. Among the more interesting ones are small clefts or slits formed in
the side walls of the rudimentary throat or pharynx. Blood-vessels go
forward from the simple heart to run up through the intervening bars
exactly as in the tadpole and the fish. In brief, the young chick
possesses a series of gill-slits, for these structures are the same in
essential plan and relations as the clefts of tadpoles and fishes. Does
this mean that even birds have descended from gill-breathing ancestors?
Science answers in the affirmative, because evolution gives the only
reasonable explanation of such facts as these. The case seems different
from that of the frog, because gills are used by the tadpole, but
gill-slits and gill-bars can have no conceivable value for the chick as
organs concerned with the purification of the blood. None the less, if the
transition from a gilled tadpole to the adult with lungs means an
evolution of amphibia from fishlike ancestors, then the change of a chick
embryo with gill-clefts into the fledgling without them is most reasonably
interpreted as proof that birds as well as amphibia have had ancestors as
simple as fishes.

As development progresses four small pads make their appearance; two of
these lie on either side of the body back of the head and the other two
arise near the posterior end. They are far from being wings and legs, but
as day follows day they become molded into somewhat similar limbs, as much
alike in general plan as the four legs of a lizard; subsequently the ones
at the front change into real wings and the hinder ones become legs.
Meanwhile the internal organs slowly transform from fishlike structures
into things that display the characteristics of reptilian counterparts,
and only later do they become truly avian. Last of all the finishing
touches are made, and the whole creature becomes a particular kind of a
bird which picks its way out of the shell and shifts for itself as a
chick.

Only a few of the countless details have been mentioned which demonstrate
the resemblance of the successive stages first to fishes, and later to
amphibia and reptiles. We have a wide choice of materials, but even the
foregoing brief list of illustrations shows that the order in which the
stages follow is the one which comparative anatomy independently proves to
be the order of the evolution of fishes, amphibia, reptiles, and birds.
Why, now, should it be necessary for a developing bird to follow this
order? The answer has been found in the immense array of embryological
facts that investigators have verified and classified, that all tell the
same story. It is, that birds have arisen by evolution from ancestors
which were really as simple as the members of these lower classes. It
seems then that the only way a bird of to-day can become itself is to
traverse the path along which its progenitors had progressed in evolution.
Stating its conclusions precisely, science formulates the principle in the
following words: _individual development is a brief resume of the history
of the species in past times_, or, more technically, _ontogeny
recapitulates phylogeny_. To be sure, the full history is not reviewed in
detail, for the chick embryo does not actually swim in water and breathe
by means of gills. Only a condensed account of evolution of its kind is
presented by an embryo during its development; as Huxley and Haeckel have
put it, whole lines and paragraphs and even pages are left out; many false
passages of a later date are inserted as the result of peculiar larval and
embryonic needs and adjustments. But in its major statements and as a
general outline, the account is a trustworthy natural document submitted
as evidence that higher species of to-day have evolved from ancestors
which must have been like some of the present lower animals.

Coming now to the mammalia, it might seem that we have reached forms so
highly developed that they would not exhibit the same kind of
developmental history, but would have their own mode of growing up. This
is not so, for like the adult fish, the larval tadpole, and the embryo
chick, an embryo of a cat or a man is at one time constructed with a
series of gill-clefts and with blood-vessels and skeletal supports of
fishlike nature that are everywhere associated with gills. The embryos of
wildcats and dogs, rabbits and rats, pigs, deer, and sheep, and of all
other mammalia, possess similar structures. Thus they all pass through a
stage which is found also in the development of reptiles, birds, and
amphibia,--a stage which corresponds to the fish throughout its life.
Unless these facts mean that the great classes of vertebrates have
originated together from the same or closely similar ancestors, they are
unintelligible; for we cannot see why a cat or a chick should have to be
essentially fishlike at any time unless this is so. Comparative anatomy
states as we have learned that the amphibia as a class have evolved from
and have out-developed the fishes, that reptiles have progressed still
higher, and that birds and mammals have originated from reptilian
ancestors along roads that have diverged beyond the immediate parent
class. Because the members of each class have to pass along the same path
trodden by their many varied ancestors, although at express speed, as it
were, the similarity of the earliest stages in their development is
explained, for during these periods they are traversing a path over which
their ancestors passed together.

The places where the developing embryos depart from the common mode show
where the several divisions took leave of one another in their
evolution,--a point that comes out with great clearness when the facts of
mammalian development are broadly compared. The embryos of carnivora and
rodents and hoofed animals are alike in their earlier development, and
their agreement means a community of origin. At a certain point the cat and
dog depart from the common mode, but they remain alike up to a far later
stage than the one in which they are similar to the embryos of rats and
sheep. The rat and squirrel and rabbit, on their part, remain together
until long after they take leave of the carnivora and ungulates; while the
sheep and cattle and pigs have their own branch line, which they follow in
company after leaving the embryos of the other orders. The reasons for
these facts seem to be that the members of the three orders exemplified
have evolved from the same stock, which accounts for their embryonic
similarity for a long time after they collectively come to differ from
amphibia and reptiles, while the members in each order became
differentiated only later, wherefore their embryonic paths coincide for a
longer period. Thus the degree of adult resemblance which indicates the
closeness of relationship corresponds with the degree of embryonic
agreement; that is, the cat and dog are much alike and their modes of
development are essentially the same to the latest stages, while the cat
and horse agree only during the earliest and middle stages, and their lines
diverge before those of the cat and dog on the one hand, or those of the
horse and pig on the other.

* * * * *

Like the fundamental principle of comparative anatomy in its sphere, the
Law of Recapitulation, formulated as a summary description of the
foregoing and similar facts, is one that holds true throughout the entire
range of embryology and for every division of the animal series, however
large or small. We have discussed its broader application, and now we may
take up some of the more or less special cases mentioned in the earlier
section of the present chapter, to see how it may work in detail.

The flounder was noted as a variant of the fish theme which seemed to be a
descendant of a symmetrical ancestor because its structural plan was like
that of other bony fishes. If this be true, and if in its development a
flounder must review its mode of evolution as a species, the young fish
ought to be symmetrical; and it actually is. The grotesque skate and
hammerhead shark were demonstrated to be derivatives of a simpler type of
shark; their embryos are practically indistinguishable from those of
ordinary dogfish and sharks.

Among the jointed animals a wealth of interesting material is found by the
embryologist. All crabs seemed to be modified lobsterlike creatures; to
confirm this interpretation, based solely upon details of adult structure,
young crabs pass through a stage when to all intents and purposes they are
counterparts of lobsters. Even the twisted hermit crab, which has a
soft-skinned hinder part coiled to fit the curve of the snail shell used as
a protection, is symmetrical and lobster-like when it is a larva.

Among the insects many examples occur that are already familiar to every
one. The egg of a common house-fly hatches into a larva called a maggot;
in this condition the body destined to become the vastly different fly is
composed of soft-skinned segments very much alike and also similar to the
joints of a worm. Comparative anatomy demonstrates that the fly and all
other insects have arisen from wormlike ancestors, whose originally
similar segments later differentiated in various ways to become the
diverse segments of adult insects; the embryonic history of flies of
to-day corroborates these assertions, in so far as every individual fly
actually does become a wormlike larva before it changes into the final and
complete adult insect. The other kinds of insects are equally striking in
their life-histories. All beetles, such as the potato bug and June bug,
develop from grubs which, like the maggots of flies, are similar to worms
in numerous respects. Butterflies and moths pass through a caterpillar
stage having even more striking resemblances to worms. All the larvae of
insects are therefore like one another, and like worms also, in certain
fundamental characters of internal and external structure; so the
conclusion that the whole group of insects has arisen by evolution from
more primitive ancestors resembling the worms of to-day is based upon
mutually explanatory details of comparative anatomy and embryology.

* * * * *

Let us now turn back to some of the earlier pages of the embryological
record which we passed over in order that we might translate the later
portions dealing with more familiar and intelligible structures like
gills. Before the egg of the frog becomes an elliptical mass of cells, it
is at one time a double-walled sac enclosing a central cavity; in this
stage it is called a _gastrula_. Tracing back the mode of its formation,
we find that it is produced from a hollow sphere of fewer cells that are
essentially alike; this stage also is so important that the special term
_blastula_ is applied to it. Still earlier, there are fewer cells--128 or
thereabouts, 64, 32, 16, 8, 4, 2, and 1. In other words, the starting
point in the development of the frog is a _single biological unit_; this
divides and its products redivide to constitute the many-celled blastula
and the double-walled gastrula. All the other animals we have mentioned
begin like the frog, as eggs which are single cells and nothing more; they
too pass on to become blastulae and gastrulae, similar to those of the frog
in all essential respects, particularly as regards the nature of the
organs produced by each of the two primary layers, and the mode of their
formation. Does the occurrence of blastulae and gastrulae and one-celled
beginnings mean that the higher animals composed of numerous and much
differentiated cells have evolved in company from two-layered saccular
ancestors which were themselves the descendants of spherical colonies of
like cells, and ultimately of one-celled animals?

Comparative anatomy has asserted that this is so, as we have already
learned, for it finds that adult animals array themselves at different
levels of a scale beginning at the bottom with the protozoa, continuing on
to the two-layered animals like _Hydra_ and jellyfish and sea-anemones,
and then extending upwards to the region of the more complicated
invertebrates and vertebrates. It was difficult perhaps to believe that
these successive grades of organic structure indicated an order of
evolution, because it seemed impossible that an animal so simple as a
protozoan could produce offspring with the complex organization of a frog
or a cat, even in long ages. But development delivers its evidence
relating to this matter with telling and impressive force. How can we
doubt the possibility of an evolution of higher animals from ancestors as
simple as _Hydra_ and _Amoeba_ when a frog and a cat, like all other
complicated organisms, begin individual existence as single cells, and
pass through gastrula stages? If we deny it, we contradict the evidence of
our senses, for the development is actually accomplished by the
transformation of a single cell into a double-walled sac, and of this into
different and more intricate organic mechanisms. The process _can_ take
place, for it _does_ take place. Not until the investigator becomes
familiar with a wide range of diverse animals and the peculiar qualities
of their similar early stages, can he estimate the tremendous weight of
the facts of comparative embryology. Were the statement iterated and
reiterated on every page and in every paragraph, there would be no undue
emphasis put upon the astounding fact that the apparently impassable gap
between a one-celled animal like _Amoeba_ and a mammal like a cat is
actually compassed during the development of the last-named organisms from
single cells. The occurrence of gill-slits in the embryos of lizards,
birds, and mammals now seems a small thing when compared with the
correspondences disclosed by the earliest stages of development. But in
spite of their complexity, all the changes of "growing up" are explained
and understood by the simple formula that the mode of individual
development owes its nature primarily to the hereditary influence of
earlier ancestors back to the original animals which were protozoa.

* * * * *

Embryology as a distinct division of zooelogy has grown out of studies of
classification and comparative anatomy. Its beginnings may be found in
medieval natural history, for as far back as 1651 Harvey had pointed out
that all living things originate from somewhat similar germs, the terse
dictum being "Ex ovo omnia." By the end of the eighteenth century many had
turned to the study of developing organisms, though their views by no
means agreed as to the way an adult was related to the egg. Some, like
Bonnet, held that the germ was a minute and complete replica of its
parent, which simply unfolded and enlarged like a bud to produce a similar
organism. Even if this were true, little would be gained, for it would
still remain unknown how the germinal miniature originated to be just what
it was conceived and assumed to be. Wolff was the originator of the view
that is now practically universal among naturalists, namely, that
development is a real process of transformation from simpler to more
complex conditions.

The subject of comparative embryology grew rapidly during the nineteenth
century as the field of comparative anatomy became better known, and when
naturalists became interested in animals, not only as specific types, but
also as the finished products of an intricate series of transformations.
When life-histories were more closely compared, the meaning of the
resemblances between early stages of diverse adult organisms was read by
the same method which in comparative anatomy finds that consanguinity is
expressed by resemblance. The great law of recapitulation, stated in one
form by Von Baer and more definitely by Haeckel in the terms employed in
the foregoing sections, was for a time too freely used and too rigidly
applied by naturalists whose enthusiasm clouded their judgment. A strong
reaction set in during the latter part of the nineteenth century, when
attention was directed to the anachronisms of the embryonic record and to
the alterations that are the results of larval or embryonic adaptation as
short cuts in development. Nevertheless, it is not seriously questioned, I
believe, that the main facts of a single life-history owe their nature to
the past evolution of the species to which a given animal belongs.

Nowadays the problems in this well-organized department are concerned not
only with more accurate accounts of the development of animals, but also
with the mechanics of development, with the relative value of external and
internal influences, and above all with the physical basis of inheritance.
It is clear that the factors that direct the development of a wood frog's
egg so that it becomes a wood-frog and not a tree-toad must lie in the egg
itself, as derivatives from the two parent organisms. Weismann and his
followers have proved that a peculiar substance in the nuclei of the egg
and its daughter-products contains the essential factors of development,
whatever these may be. Experiments dealing with the phenomena of heredity
in pure and mixed breeds have largely confirmed Weismann's doctrine, and
they have prepared the way for a deeper investigation of the marvelous
process of biological inheritance.

However much he may be interested in the details of embryological science,
the general student of natural history is more concerned with the bearing
of its primary laws upon the great problem of evolution. In the foregoing
brief review of the fundamental facts and principles of this subject, the
purpose has been to show how the phenomena of development are viewed by
men of science, and how they take their place in the doctrine of organic
evolution. And it has also been made plain that comparative anatomy and
comparative embryology support and supplement one another in countless
ways and places, although each in itself is a complete demonstration that
evolution is a real and a natural process.

III

THE EVIDENCE OF FOSSIL REMAINS

Few natural objects appeal to the interest and imagination of the student
with more force than the fragments of animals and plants released from the
rocks where they have been entombed for ages. Our lives are so brief that
it is impossible for us to comprehend the full duration of the slow
process which constructed the burial shrouds of these creatures of long
ago. We try to picture the earth and its inhabitants as they were when
lizards were the highest forms of animals, and we wonder how life was
lived in the dense forests of the coal age. Science can never learn all
about the ancient history of the earth and of the organisms of bygone
times; yet it has been able to accomplish much through its endeavors to
reconstruct the past, for its method is one by which sure results can
always be obtained whenever there are definite facts with which it can
work. In our present study of evolution we reach the point when we must
examine the testimony of the rocks, and the results and methods of that
department of knowledge called palaeontology, which is concerned with
fossils and their interpretation.

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