—
APR. 4, 1923 CLARK : ORIGIN OF VERTEBRATES 129
were analyzed. The quality of these waters and the source of their
mineral content are discussed in the report.
It is thus apparent that the reconnaissance has yielded results that
are important contributions to the geologic history of the West Indian
region. It has also yielded results that in many ways have an intimate
bearing on the welfare of the inhabitants of the Republic^—results
based on an understanding of the geologic features derived from ob-
servations that at first glance may seem to be of purely scientific in-
terest only.
BIOLOGY.' The origin of the vertebrates. Austin H. Clark, Na-
tional Museum.
Heretofore all the writers on the subject of the evolution of the
vertebrates have approached the problem with the complex vertebrate
structure admittedly or unconsciously dominating the perspective
within which all other types of animal structure should fall. Under
the influence of this preconceived though unconfessed idea either a
devious line was traced from the vertebrates through progressively
simpler forms eventually ending in the protozoans, or a line was drawn
from the protozoans to the vertebrates from which more or less numer-
ous side branches were given off terminating blindly in supposedly
anomalous types.
It never seems to have been noticed that animals and plants are
but slightly different manifestations of the same organic phenomena,
and that therefore there is no reason to suppose that the evolutionary
line in one kingdom would be in its broader features greatly different
from that in the other.
In the following pages I shall attempt to show that if we consider
the phanerogam-like radially symmetrical colonial coelenterate type
as representing the culmination of animal evolution properly so called,
and the bilateral animals as having arisen through the disruption of
this type and the gradual geometrical recombination of the characters
of the forms resulting from this disintegration, we shall have an ex-
planation of the origin of all the different animal types by which each
and every one is allocated and shown to be a necessary element in the
general plan.
The embryological processes common to all animals show that the
egg develops into a blastula which subsequently becomes a gastrula; \\
but from this point onward the developmental processes exhibit no
features common to all animal types.
130 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 13, NO. 7
The gastrula, present in the ontogeny of all animals, is the last
structural complex which is of universal occurrence throughout the
animal kingdom and the last common bond between the various animal
t>Tpes, and it therefore must in some way represent the starting point
for all subsequent divergence.
An egg typically divides into two, four and eight cells in three planes
each of which is at right angles to the other two and, equal cell division
continuing, a hollow sphere is formed, the blastula, which collapses,
forming a more or less hemispherical structure with two layers of
cells, an outer and an inner, the gastrula.
The gastrula possesses a single axis which runs through the center
of the opening resulting from the collapse of the blastula; but since
the walls of the gastrula are everywhere the same there is a perfect
radial sj^mmetry about this axis.
Since the gastrula, though usually in a considerably modified form,
is common to all animal types and forms the starting point for the
divergence of the various major groups, it is important to determine
what its real significance is.
From the egg through the blastula to and including the gastrula
there is a direct geometrical continuity leading to the formation of a
body radially symmetrical about a single axis. The logical termination
of such development would be the formation of an animal type in
which the gastrula axis persists to the adult and the body of the adult
is radially symmetrical about it.
If the facts presented by a study of embryology are significant in
indicating the phylogenj^ of animals, it is clear that the last common
ancestor of all the bilaterally synnnetrical animal types was a radially
symmetrical form, or a sort of adult gastrula.
There are two such animal forms. In one of these, the sponges, the
body consists of a community of cells imperfectly integrated and show-
ing relatively little devision of labour or unified life. The sponges
continue to grow throughout life, and their increase is always radial.
In the other, the coelenterates, the body is a distinct unit of more
or less definite size with a gastrovascular cavity and a well developed
muscular system. Growth in the coelenterates as in the sponges is
continuous throughout life; but since the complexity of the organiza-
tion imposed a definite maximum size on the individuals, the growth
impetus results in the continued formation of new individuals which
bud off from those preceding, typically resulting in an arborescent or
mass colony comparable to that seen in the phanerogams. While in
many coelenterates the budded individuals become free, and in some
:
APR, 4, 1923 CLARK : ORIGIN OF VERTEBRATES 131
there is no budding at all, there can be no doubt that fundamentally
the coelenterates are phanerogam-like colonial animals.
There is nothing that can be assumed to connect the sponges with
any other animal type except, perhaps, with certain of the Protozoa.
It is evident that the gastrula stage in the development of the bilateral
animals cannot represent any sponge-like progenitor. It is possible,
however, to interpret the bilateral animals in terms of the colonial
coelenterate. Indeed, it is not possible to interpret them in any other
way, for any other explanation of their origin would assume the
presence of a fundamental bilateral tendency, an unknown and un-
determinable variable not common to all animal types.
If a colonial coelenterate with radially symmetrical polyps should
develop a persistent defect in the ontogeny whereby the units became
bilaterally symmetrical, bilateral animals of four main types would
at once appear:
1. Bilateral animals in the form of a linear more or less unified
colony.
2. Bilateral animals in which the colony formation was inverted,
the budding of the new elements taking place within the original unit.
3. Bilateral solitary animals each representing a dissociated coelen-
terate unit; and
4. Bilateral animals with the colonial habit, though independent
of each other.
These four main types, between which there would be numerous
intergrades, all represent definite types occurring among the coelen-
terates themselves, and therefore none of them can be said actually
to represent anything new in animal structure other than the novelty
consequent on the developmental defect which resulted in the loss of
the radial and the assumption of the bilateral body form.
Among the animals of today all four of these main types are
represented
1. The tape-worms or segmented cestodes form a linear colony of
continuous growth so like a partially unified strobila as to leave little
doubt of the fundamental similarity of type. The scolex of the ces-
todes is radially symmetrical, but the proglottides are strongly flat-
tened and bilaterally symmetrical, though the difference between the
dorsal and the ventral surface is but little marked.
2. The flukes have a peculiar development which is essentially
similar to strobilization, except that the buds are formed within the
original unit instead of in a linear series.
132 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 13, NO. 7
In those coelenterates with division of labour the polyps are of three
sorts, (a) nutritive, or sac-like, (b) reproductive, and (c) excretory,
or "defensive." If in strobilization of the fluke type buds of each
of these sorts were formed internally, this would furnish the elements
necessary for the creation of the so-called coelome, which is divided
into three parts, (a) the perivisceral, or sac-like, (b) the gonadial,
and (c) the excretory or nephridial.
The flukes and their allies always retain distinct traces of radial
symmetry, especially in their digestive system and in the arrangement
of their nerves.
3. The turbellarians and nematodes are bilateral solitary animals,
the individuals each comparable to a single coelenterate polyp. All
of them show distinct traces of radial symmetry in their nervous sys-
tem, and the turbellarians also in their digestive system.
4. Such turbellarians as Microstomum are single animals each
comparable to a single coelenterate polyp ; but they divide in such a way
as to produce chains of similar attached animals each of which is
independent of the others and not a part of a more or less unified entity
as in the case of the proglottides of the tape worms.
The cestodes, the flukes, the turbellarians and Microstomum are
all flat worms and all more or less closely related to each other. They
all retain to a very considerable degree traces of radial symmetry and
of other coelenterate features. Being intermediate between radially
symmetrical and bilateralh'- symmetrical types it requires very little
imagination to assume that they represent the four original t>T)es into
which the coelenterates disintegrated upon the appearance of that
developmental defect which resulted in bilateral symmetry.
If the preceding suppositions are logical it is evident that the so-
called evolution of the bilateral animals cannot be evolution in the
sense of the progressive development of higher types from lower, but
instead must have been a recombination and reassortment of the four
diverse features characteristic of the four types into which the radially
symmetrical colonial coelenterate type disintegrated. In other words
the so-called evolution of animals is in reahty a convergence toward a
common centre from four equidistant points, and the progressive
economic efficiency does not indicate any real phylogenetic progress,
but results merely from a more and more intimate intermingling of,
and a progressively better balance between, the main features indi-
cated by the tape worms, flukes, turbellarians and Microstomum
standing at the four corners of the original square.
APR. 4, 1923 CLARK : ORIGIN OF VERTEBRATES 133
The four corners of this square are marked by four animal typves
which are closely related to each other, yet at the same time are funda-
mentally distinct. One of them indicates the commencement of
the segmented body; another shows the beginnings of the coelomic
structures; a third is simple, with no indications of segmentation or of
a coelome; while the fourth is a colonial form of the third.
From these four points there would proceed evolution of two kinds.
Each type would give rise to all economically possible variants
through a process of continuous development which could be approx-
imately represented by a branching tree-like figure; but all of the
the different forms arising in this fashion would fall strictly within
limits of its proper tj'^pe.
As examples of such evolution may be mentioned the insects, crus-
taceans, molluscs, annelids, etc., within which groups all the included
types may be more or less successfully represented as branches of a
tree-like figure at the base of which lies a generalized or primitive
form; but this and all the others however much they may diverge
in details of structure always agree in their fundamental features.
Since they all have arisen from the same colonial coelenterate-like
ancestor which has, so to speak, exploded into four different types,
each of these four points represents an animal complex in a state of
unstable equilibrium; for each one has latent within it the fundamental
features of the other three.
Such an unstable equilibrium, in effect an explosive force, would
presumably result in a sudden readjustment of the somatic balance
whereby intermediate types would appear, one between each two of
the four corners (fig. 1) ; while these intermediate types, each a dis-
tinct re-creation and not genetically connected with either of its
neighbors, would show a distinct economic advance, this economic
advance would in no way represent real evolutionary progress, for it
would be merely the result of the combination of the advantages
inherent in the structure of the types on either side.
Thus there would suddenly appear, quite without apparent ancestry,
(1) segmented animals, like the tape-worms, with a coelome, like the
flukes; (2) unsegmented animals, Hke the turbellarians, with a coelome
like the flukes; (3) solitary unsegmented animals without a coelome,
Hke the turbellarians, but with abundant asexual reproduction, hke
Microstomum: and (4) segmented animals without a coelome, like
the tape-worms, but less unified and without the continuous loss of
the units, as in Microstomum.
134 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 13, NO. 7
All four of these intermediate types actually occur. All are entirely
and widely distinct from each other, and show no demonstrable inter-
gradation with any of the remaining animal types, each occupying
a markedly isolated position. Three of the four are extraordinarily
rich in genera and species; the last and least successful is represented
by two closely parallel and non-intergrading forms which are identical
with regard to the features with which we are concerned, but differ
in all others. One of the first three is the only major animal group
which has not persisted to the present time.
The segmented animals with a coelome are the annelids; the un-
segmented animals with a coelome are the priapulids and sipuncuUds;
\
\
V
'"lat resOjuatMBt
2Dd readjuetoent
2
Fig. 1. Showing the first and second readjustments.
the solitary acoelomate animals with abundant asexual reproduction
are the rotifers; and the acoelomate animals forming colonies of sepa-
rate individuals are undoubtedly the graptolites (fig. 3).
Through this readjustment as just described each of the new animal
types would combine the features of two of the original types. But
since in each of these new types two of the four chief features are
absent, there would still exist a condition of unstable equilibrium as
compared with the colonial coelenterate-like ancestor.
A second readjustment of the same nature as the first would be
inevitable by which four animal types would appear in line with the
first, but combining the characters of the intermediates in the second
series (fig. 1).
APR. 4, 1923 CLARK : ORIGIN OF VERTEBRATES 135
Three such intermediates (fig. 3) seem to be clearly indicated in the
polyzoans, colonial and not at all or very imperfectly coelomate,
between the rotifers and the graptolites; the arthropods, with a
segmented body like that of the annelids, but divided into two or three
units showing division of labour (in the insects one controlling and
directing, one locomotor, and one performing the digestive, reproduc-
Itt rcadJuitiMot
Fig. 2. Showing the first to the fourth readjustments.
tive and other vital functions) after the graptolite or polyzoan fashion,
with a poorly developed coelome, with abundant traces of asexual
reproduction (polyembryony, parthenogenesis, fragmentation of
larvae, etc.), with a marked tendency to form (as in the ants) poly-
zoan-like colonies with division of labour among the (dissociated)
units, and sometimes even forming dendritic colonies (as in Thomp-
sonia) ; and the molluscs, always solitary, like the priapulids and sipun-
136 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 13, NO. 7
culids, with a highly developed coelome, and with traces of segmenta-
tion suggesting the annelids. The fourth group should be solitary
with an indication of colonial structure and a coelome, but without
segmentation. It is possible to place the nemerteans here by assum-
ing their imperfect segmentation to be of the Microstomum and not
of the tape-worm type.
Turtaallariuia
snatodee
Fig. 3. Showing the development of the various animal types above the coelenterates.
There is still a condition of unstable equilibrium, for in each of these
four groups one of the original elements is lacking. A third readjust-
ment (fig. 2) would be necessary to recombine all the main features
characteristic of the original four types.
Four animal groups (fig. 3) appear to be the result of such a read-
justment. The echinoderms combine a reduced body consisting of
five half segments of the arthropod type with a highly perfected coe-
APR. 4, 1923 CLARK : ORIGIN OF VERTEBRATES 137
lome ; the chaetognaths suggest a relationship with the molluscs, and
also with the nemerteans; the phoronids suggest a relationship with
the polyzoans, but have a well developed coelome, and the colonial
habit is reduced to the budding off of new individuals ; and the brachi-
opods suggest both the polyzoans and the barnacle-like arthropods.
While by this third readjustment all the four original features are
recombined in each animal type, the balance between them is imper-
fect, for the influence of one of these features in each case is greatly
overshadowed by the influence of the other three.
A fourth readjustment (fig. 2) would correct this imperfect balance
and result in the appearance of four animal types all very much alike.
There are four types which appear to belong here (fig. 3) , the tuni-
cates, the cephalochordates, the balanoglossids and the pterobran-
chiates. The tunicates seem to be in line with the polyzoans, while
they also suggest both the brachiopods and the phoronids; the cephalo-
chordates clearly stand in the cestode-arthropod line, and at the same
time show indubitable affinities with the echinoderms; the balano-
glossids, with no trace of asexual reproduction, may be considered in
line with the flukes and molluscs and between the chaetognaths and
echinoderms; while the pterobranchiates seem to fall between the
chaetognaths and the phoronids.
These four closely related types resulting from this fourth readjust-
ment are each slightly excentric; but they are so close to each other
that a fifth readjustment would presumably give a final perfected
type in which at last all of the four chief features of the original types
would be reunited in the economically most perfected form.
The vertebrates appear to occupy this central position (fig. 3) . In
them we are able to recognize the segmentation of the cestodes, anne-
lids, and cephalochordates, combined with the coelomic structure
first indicated in the flukes, both enclosed in the undivided body of the
turbellarians. Unless the limbs can be compared to budded units
recalling certain highly reduced and specialized units in tunicate or
polyzoan colonies, the influence of the feature represented by Micro-
stomum seems to have disappeared.
In the course of the various readjustments which culminated in the
formation of the vertebrates numerous secondary features, such as
visual and other sense organs, appendages of different kinds, diver-
ticula and other outgrowths from the enteric canal, chitinous and
calcareous skeletons, etc., all of which exist in the coelenterates and
in one or other of the four types derived immediately from them,
became enormously developed and specialized in correlation with the
138 JOURNAL OP THE WASHINGTON ACADEMY OF SCIENCES VOL. 13, NO. 7
increasing bodily efficiency resulting from the recombinations. But
if the analysis of the origin of the various animal types just given
is an approximately true exposition of the facts, the vertebrates, in
spite of their wonderful complexity of structure and their very high
degree of efficiency, represent nothing more than the final recombina-
tion of characters already occurring in the colonial coelenterates which
were widely dissociated at the inception of bilateral symmetry.
PROCEEDINGS OF THE ACADEMY AND AFFILIATED
SOCIETIES
PHILOSOPHICAL SOCIETY
877th meeting
The 877th meeting was held in the Cosmos Club Auditorium on Saturday,
January 27, 1923. The meeting was called to order at 8:15 P.M. by Presi-
dent White with 29 persons in attendance.
Mr. G, T. Rude presented a paper on Instruments and methods for the
observation of tides. The paper was illustrated with lantern slides, and was
discussed by Messrs. Lambert, Bowie, Pauling, Humphreys, Hawks-
worth, Faris, Littlehales, and Tuckerman.
Author's abstract: A continuous record of the rise and fall of the tide is
necessary in connection with a number of engineering and scientific problems.
The simplest method of tidal observation consists in observing the chang-
ing height of the water as noted on a fixed vertical staff. From this it was
but a step to devising some mechanical means for recording automatically
the rising and falling of the surface of the sea.
The earliest automatic tide gauge of which we have record was devised
by an English civil engineer, Henry R. Palmer, and is described in the Philo-
sophical Transactions of the Royal Society, London, for the j^ear 1831.
Of the automatic tide gauges, two classes may be recognized: (1) those
in which the changes in elevation are shown in the form of a curve; (2) those
in which the height of the water at definite intervals is shown by means of
figures, or the so-called printing gauges. The various forms of the tide
gauges in use were shown by means of slides and attention called to the
distinguishing features.
Special attention was directed to a new type of automatic tide gauge
recently developed in the office of the Coast and Geodetic Survey for use of
hydrographic parties in the field. In designing this instrument the ol)jects
sought were ease of installation and minimum size commensurate with the
desircrl accuracy. The gauge is about 10 inches long and 9 inches high.
The clock is placed inside the cylinder carrying the cross-section paper on
which the curve of the tide is drawn. No counterpoise weight is used, a
coiled spring taking the place of the weight. The float ^yell is ordinary
3^ incli stock iron pipe and in addition to serving as a float, it acts as a sup-
port for the gauge. No platform is necessary for the installation of this gauge,
which may ])e lashed to a pile on a bar or to net stakes in bays or rivers. A
metallic cover furnishes the only shelter necessary.