the results provided by the comparative study of development. In the first
place it is an obvious fact that every living thing changes in the course
of its life-history, and if as an adult it occupies a high place in the
animal scale, its embryological transformation is more elaborate and
intricate than in the case of a lower form. Every one knows that organisms
do develop, and yet I believe that few appreciate the tremendous
significance of the mere fact that this is true, while still fewer are
aware that the peculiar and characteristic early stages through which an
animal passes in becoming an adult are even more striking than the fact of
development itself. We shall learn something of these earlier conditions
in the development of some of our most familiar animals, but at the outset
nothing can be more important than an appreciation of the first great
lesson of this department of natural history--namely that organic
transformation is real and natural. We do not need to employ the methods
of formal logic to know that in growing up a human infant undergoes the
changes of childhood and adolescence, that kittens become cats, and that
an oak tree is produced by an acorn, for we know these things directly by
observing them. It is natural for development to take place under normal
conditions, and if it does not, then something has interfered with nature.
Inasmuch as "growing up" is accomplished by the alteration of an organic
mechanism with one structure into an individual with a changed plan of
body, it is in essence the actual process of evolution which the
comparative study of grown animals of to-day demonstrates in the way we
have learned. The study of animal structure discovers the process of
evolution because the most reasonable interpretation of the similarities
and minor differences exhibited everywhere by the various groups of
animals is that descent with adaptive and divergent modification has taken
place; the result is reached by inference, it is true, but by scientific
and logical inference. With development it is otherwise. No reasoning is
necessary to tell us that organic transformation is a real and a natural
process. We see it everywhere about us and we ourselves have come to be
what we are by a natural history of change. Can we consistently deny that
it is possible for a species to alter in the long course of time when a
few brief weeks are sufficient for the new-laid egg of the fowl to develop
into a fledgling? Many indeed strain at the gnat of the longer process in
the past when without hesitation they recognize the real and obvious fact
of individual development in a brief period.
I have said that development is a "natural" process. We employ this word
for the familiar and everyday occurrence or thing; it does not imply that
everything is known about the object or phenomenon, because science knows
that complete and final knowledge is impossible. We say that it is natural
for rain to fall to the earth, and we speak of the law of gravitation
according to which this takes place as a natural principle, but it may not
have occurred to many to inquire _what_ makes rain fall and _why_ do
masses of matter everywhere behave toward one another in the consistent
manner described by the law in question. Sunshine is natural, but we do
not know _why_ light travels as it does from the sun to the earth, and
this is another question which, like the inquiry into the ultimate cause
of the familiar and natural phenomenon of gravitation, has not yet been
answered. But it is still regarded as natural for the rain to fall and for
the sun to shine. In the same way does science view development, denoting
it natural because it is an ordinary everyday matter. And we are under no
more obligation to postulate supernatural control for the changing forms
in the life-history of a chick or a cat than we need to assume that
gravitation and the radiation of light demand immediate supernatural
direction. The embryology of no form is fully understood or described or
explained, but no intelligent person would be willing to assert that
because complete knowledge is lacking, it is unnatural for organic
transformation to take place during growth. Whatever may be the ultimate
origin and nature of the directing powers behind gravitation and
development and other phenomena, we have no concern with such matters
because they cannot be handled by scientific methods and one belief about
them is on the same plane with any other. Our task is to deal with the
everyday phenomena of life and the production of living species.
* * * * *
It is not necessary to go far afield to find an animal which will
introduce us to the general principles of embryology. In the present
instance as in the case of comparative anatomy almost any form will
disclose the meaning of development, for animate nature is uniform and
consistent in its methods of operation throughout its wide range. We shall
begin with the familiar frog which every one knows is a product of a
tadpole; passing on to the chick we will learn more facts that will enable
us to formulate the main principle of comparative embryology in definite
terms; we will then be prepared to extend our survey so as to include
somewhat less familiar facts and animals that are even more significant
than the first illustrations.
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 résumé 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 larvæ 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 blastulæ and gastrulæ, 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 blastulæ and gastrulæ 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?