ETIENNE GEOFFROY SAINT-HILAIRE
E. Geoffrey made an experiment, unsuccessful but instructive. He tried
to found a science of pure morphology; he failed: his failure showed,
once and for all, that a pure morphology of organic forms is
impracticable.
Already, in 1796, in one of his earliest memoirs,[86] Geoffroy was
guided by the idea that Nature has formed all living things upon one
plan. Organs which seem anomalous are merely modifications of the
normal; the trunk of an elephant is formed by the excessively
prolonged nostrils, the horn of a rhinoceros is simply a mass of
adhering hairs. In general, however varied their form, all organs are
simply variations of a common scheme; Nature employs no new organs.
Organs which are rudimentary, such as the clavicles in the ostrich and
the nictitating membrane in man, bear witness to the unity of plan. In
this Geoffroy goes no further than his predecessors. They too had
recognised homologies of organs; they too had interpreted rudimentary
organs as vestiges of an original plan.
In a series of papers published in 1807, Geoffroy took a further step,
and sought to establish homologies which were not obvious--homologies,
too, not so much of organs as of parts.
These memoirs (published in the _Annales du Museum d'Histoire
naturelle_, vols. ix. and x., 1807) dealt with the homology between
the bones of the pectoral fin and girdle in fish and the bones of the
arm and shoulder-girdle in higher Vertebrates, with the homologies of
the bones of the sternum, and with the determination of the pieces of
the skull, particularly in the crocodile. All Geoffroy's morphological
doctrine is found in them, but for the full expression of his views we
must take his chief work, the _Philosophie anatomique_, particularly
the first volume (1818). This volume contains, beside the important
"Discours preliminaire" and "Introduction" which we shall presently
consider in detail, five memoirs, which deal with the various bones
connected with the respiratory organs in fishes (the bones of the
operculum, of the hyoid, of the branchial arches, of the pectoral
girdle), and seek to discover their homologies with corresponding
bones in air-breathing Vertebrates.
"Can the organisation of vertebrated animals be referred to one
uniform type?" This is the question with which the _Philosophie
anatomique_ opens, the question to which the whole book is an answer.
But is it not generally acknowledged by naturalists that Vertebrates
are built upon one uniform plan, that, for instance, the fore limb may
be modified for running, climbing, swimming, or flying, yet the
arrangement of the bones remain the same? How else could there be a
"natural method" of classification?[87]
But the homologies so drawn repose upon a vague and confused feeling for
likenesses; they are not based upon an explicit principle. What general
principle can be applied? "Now it is evident that the sole general
principle one can apply is given by the position, the relations, and the
dependencies of the parts, that is to say, by what I name and include
under the term of _connections_." For instance, the part known as the
hand in man and generally as the fore foot in other Vertebrates, is the
fourth part in order in the anterior member, and its homologue can
always be recognised by this fact of its connections (p. xxvi.). The
principle of connections serves as a guide in tracing an organ through
all its functional transformations, for "an organ can be deteriorated,
atrophied, annihilated, but not transposed" (p. xxx.).
It is this principle which enables one to follow out in detail the
further fundamental conception that in every Vertebrate there are found
the same "organic materials," or units of construction. This conception,
which Geoffroy calls the _Theorie des analogues_ (p. xxxii.), is clearly
one part of the old idea of the unity of type; it teaches the _unity of
composition_ of organic beings, while the _Principe des connexions_ adds
the _unity of plan_.
Both conceptions are logically implicit in the vague notion of unity of
type; Geoffroy disengaged them, and pushed each to its logical extreme.
Most of the ordinary homologies of structure in air-breathing
Vertebrates have already been seized, he continues, for they are more or
less obvious, and many intermediate states exist (p. xxxiv.). But
ordinary methods of comparison fail when the attempt is made to
homologise the structure of fishes with that of air-breathing
Vertebrates, for the homologies are anything but obvious and no
intermediate organs are found.
Most air-breathing Vertebrates have a larynx, a trachea, and bronchi,
which are absent in fish; and fish have many parts which seem to be
absent in higher Vertebrates. But apply the "Theory of Analogues"; it
teaches that there can be no organ peculiar to fish and not found in
other Vertebrates; apply the "Principle of Connections," it will show
which organs are homologous in the two types (p. xxxv.).
Comparative anatomists, with few exceptions, had hitherto taken man as
the type, and referred all structure to his; Geoffroy's principles led
him to give preference to no one animal in particular, but to seize upon
each part in the species in which it reaches the maximum of its
development (p. xxxvi.). He is thus led to refer all structures to a
generalised abstract type. In this abstract type each organ exists at
the maximum of its development, each organ shows all its potentialities
realised. In a way, therefore, this type, this abstraction, gives the
scheme of the possible transformations of each organ.
It is true Geoffroy does not refer to this "Archetype" in so many words,
but it must always have been vaguely present in his mind. He has this
idea in his head when he says in one of his later works, "There is,
philosophically speaking, only a single animal."[88] The "single animal"
is simply the generalised type.
Having laid down his two principles Geoffroy goes on to apply them to
the difficult case of the comparison of the skeleton of fish with the
skeleton of the higher Vertebrates. "My present task is to demonstrate
that there is no part of the bony framework of fishes that cannot find
its analogue in the other vertebrated animals."[89] It seems at first
sight that many bones are peculiar to fish, formed expressly for
performing the functions which fish do not share with higher animals.
These are the bones connected with respiration--the operculum, the
branchiostegal rays, the branchial arches, and others. That the peculiar
bones should be connected with the respiratory functions is only
natural, for the contrast between fish and higher Vertebrates is
essentially a contrast between water-breathing and air-breathing
animals. Considering first the general form of the skeleton in fish, we
are met at once with a difficulty; there is no obvious homologue in
fishes of the neck, the trunk, and the abdomen of higher animals. What
apparently corresponds to the trunk is in fishes crowded close up under
the head. But, after all, it is not of the essence of the vertebrate
type to have the trunk and the abdomen attached at definite and
invariable distances along the vertebral column--that is a notion
surviving from the anatomy which made man its type. The "trunk" differs
in position according to the class, in quadrupeds, birds, and fishes (p.
9). Now, says Geoffroy, allow me this one hypothesis, that the trunk
with its organs can, as it were, move bodily along the vertebral column,
so as to be found in one class near the front end of the vertebral
column, in another about the middle, and in a third near the end, then I
can show you in detail that the constituent parts of this trunk are
found in all classes to be invariably in the same positions relatively
to one another (p. 10). It is important to note this hypothesis of a
"metastasis" which Geoffroy makes, for it is the key to the
understanding of many of the far-fetched homologies which he tries to
establish. It is, of course, clear that this hypothesis is in formal
contradiction with his principal hypothesis of the invariability of
connections, and that he, so to speak, gets a hold on his fish to apply
his principle of connections only by admitting at the very outset an
exception to his primary principle. A further application of the
hypothesis of metastasis will be noticed below in connection with the
determination of the sternum of fishes. We note here an interpretation
of the first metastasis in terms of functional adaptation. "The constant
and violent action of the tail, if it does not go so far as actually to
displace and move forward the internal organs, at least fits in well
with an arrangement in which the organs are so disposed" (p. 99).
The first memoir deals with the homologies of the opercular bones.
Geoffroy considers that the external opening of the ear corresponds to
the external opening of the gill-chamber, which lies between the
operculum and the pectoral girdle. The ear communicates with the buccal
cavity by the Eustachian tube, so does the branchial chamber by means of
the gill-slits. The auditory chamber of higher Vertebrates is,
therefore, the homologue of the branchial chamber in fish; the opercular
bones in fish and the ossicles of the ear in other Vertebrates stand in
close relation to this chamber; therefore the opercular bones are the
homologues of the ossicles of the ear, the interoperculum corresponding
to the malleus, the suboperculum to the lenticular, the minute lower
part of the suboperculum to the incus, the operculum to the stapes, and
the pre-operculum to the tympanic ring. In making these particular
determinations Geoffroy professes to be led by his principle of
connections. The pre-operculum has, he says, the same connections with
neighbouring bones as the tympanic bone in other Vertebrates, and the
other pieces of the gill-cover are homologised with particular
ear-ossicles according to the order in which they stand to one another.
The second memoir in the book deals with the sternum, and affords a very
good example of Geoffroy's method of dealing with the facts of
structure. We shall omit here any detailed reference to the other three
memoirs, which deal with the hyoid, with the branchial arches and the
structures which correspond in air-breathing Vertebrates, and with the
bones of the shoulder-girdle.
In the memoir on the sternum Geoffroy's first care is to arrive at a
definition of what a sternum is. He defines it partly by its functions,
partly by its connections, as the system of bones which covers and
protects the thorax, and gives attachment to certain groups of muscles.
The most highly developed sternum (according to this definition) is the
plastron of the tortoise, whose structure it dominates (p. 103). It is
important, therefore, to determine of how many bones the plastron is
composed, since the full number of elementary parts of which an organ is
composed is best seen when the organ is at the maximum of its
development. There are nine bones in the plastron of the tortoise. "The
conclusion to be drawn from this is that every sternum, provided that it
is not inhibited in its development by some obstacle, is composed of
_nine elementary parts_" (p. 105). These nine bones are in Geoffroy's
nomenclature, the episternals, the hyosternals, the hyposternals, the
xiphisternals, which are all paired bones, and the entosternal, which is
unpaired. The arrangement of them is in the tortoise:--
Episternal---------------------------Episternal
|\__ __/|
| \__ __/ |
| \__ __/ |
| \__ Entosternal __/ |
| __/ \__ |
| __/ \__ |
| __/ \__ |
|/ \|
Hyosternal Hyosternal
| |
| |
| |
| |
Hyposternal-------------------------Hyposternal
| |
| |
| |
| |
Xiphisternal------------------------Xiphisternal.
The articulations in the tortoise are indicated by the connecting
lines. Geoffroy tries to show that the sternum in other animals is
composed of these nine bones, or at least of a certain number of them,
always in the same invariable relative positions. Thus in birds the
sternum consists of five pieces, of a huge keeled entosternal, and of
two "annexes" on either side, which are the hyo-and hyposternals.
These are separate only in young birds. Occasionally, especially in
young birds, rudiments of episternals and xiphisternals also occur.
The minuteness of the episternals and the xiphisternals may be
attributed to the gigantic size of the entosternal, in accordance with
the _Loi de balancement_. In the other air-breathing Vertebrates the
nine sternal elements can according to Geoffroy be discovered without
great difficulty. But when we come to the determination of the sternum
in fishes, difficulties abound, which Geoffroy solves in the following
way. He points out that between the clavicles (_cleithra_) and the
hyoid bone (_basihyal_) in fishes there is a long median bone
(_urohyal_) which is attached in front by two strong tendons to the
horns of the hyoid and is free behind (see Fig. 1). Gouan (1720) had
seen in this bone the homologue of the sternum. Geoffroy adopts this
view, but considers that this bone alone cannot represent the whole
sternum. He finds the representatives of other bones of the sternum in
the large bones (_epihyal_ and _ceratohyal_, or the two pieces of the
_ceratohyal_) which are comprised in the hyoid arch. But he is
immediately met by the difficulty that this complex of bones is
situated in front of the pectoral girdle, whereas the sternum in
higher Vertebrates lies behind the pectoral girdle. He reflects,
however, that the gills of fish, situated in front of the clavicles,
are merely the lungs under another name. The gills have become shifted
forward by a metastasis similar to that which brought the whole
thoracic organs far forward in fish. This being so, their supporting
elements, the sternum and the ribs, must have moved with them, and are
hence to be found in front of the pectoral girdle.
[Illustration: FIG. 1.--Hyoid Arch of the Conger. (Original.)]
Geoffroy's next step is to point out that the only possible homologues
of sternal ribs are the branchiostegal rays, which arise from the large
bones of the hyoid arch. If these are sternal ribs, the bones to which
they are attached must be the hyo- and hyposternals or "annexes," the
bones from which in birds the ribs take their origin.
The unpaired sternal bone (_urohyal_) cannot be homologous with the
entosternal, for it has no connections with the annexes. He decides that
it must represent the episternals, for in some young birds there is a
two-headed episternal to which two strong tendons are attached, just in
the same way as the unpaired piece in fish is bound to the bones of the
hyoid by two tendons. "Thus it is not the sternum as a whole that has
shifted in front of the clavicles and covered with its side pieces the
gills placed there; it is a piece exclusively piscine, in the sense that
it is only in the class of fishes that it reaches the _maximum_ of its
development" (p. 83).
To sum up, the sternum in all four vertebrate classes is composed of the
same elements, arranged always in the same way. "One is ... led to the
conception of an ideal type of sternum for all Vertebrates, which then,
considered from a lower standpoint, resolves itself into several
secondary forms according as the whole or the majority of the
constituent materials are employed, or even as these elements come to
change their respective dimensions or proportions" (p. 134). As to the
elementary constituents, "they give proof of individuality, and
sometimes even, in certain abnormalities, of independence, and rise to
the level of primary organisatory materials" (p. 132). What holds good
for the sternum holds good for other organs--and accordingly the unity
of plan and composition can be demonstrated for all the organs of
Vertebrates.
Soon after the publication of the _Philosophie anatomique_ (1818)
Geoffroy went further in his search for unity, and maintained that the
structure of insects and Crustacea could be reduced to the vertebrate
type.
He proposed to replace Cuvier's classification of the animal kingdom
into the four large groups, Vertebrata, Mollusca, Articulata, and
Radiata by the following classification:--[90]
Hauts-Vertebres (Vertebrata, Cuv.).
Vertebres /
\
Dermo-Vertebres (Articulata, Cuv.).
Mollusques (Mollusca, Cuv.).
Invertebres /
\
Rayonnes (Radiata, Cuv.).
The idea upon which is based the comparison of Articulates with
Vertebrates is that each skeletal segment of Articulates is a vertebra.
In the Hauts-vertebres the vertebrae are internal; in the
Dermo-vertebres they are external. "_Every animal lives either outside
or inside its vertebral column_."[91] The essence of a vertebra is not
its form, nor its function, but its composition from four elementary
pieces which unite round a central space (_Isis, loc. cit._, p. 532).
Serres had shown that in the higher animals every vertebra is formed
from four centres of ossification, that the body of the vertebra is at
first tubular, and that afterwards it becomes filled up. In lobsters and
crabs each segment is composed of four elementary pieces, as may be seen
most easily in young ones. "Accordingly each segment corresponds to a
true vertebra in composition: there is the same number of 'materials,'
the same order in the course of ossification, the same kind of
articulation, the same annular arrangement, the same empty space in the
middle" (p. 534). The only difference is that in Articulates the central
space is very great and contains all the organs of the body, whereas in
the higher Vertebrates the body of the vertebra becomes completely
filled up. In the thoracic region of Crustacea it is not the whole
segment with part of the carapace which corresponds to a vertebra, but
merely the part round the ventral nerve-cord (endophragmal skeleton).
If the skeleton of the segment in Articulates corresponds to the body of
a vertebra and is here external, then the appendages of the Articulate
must correspond to ribs (p. 538). The full development of this thought
is found in a Memoir of 1822, "Sur la vertebre."[92] He takes as the
typical vertebra that of a Pleuronectid, probably the turbot. His
original figure is reproduced (Fig. 2).
[Illustration: FIG. 2.--"Vertebra" of a Pleuronectid. (After Geoffroy.)]
He includes as part of the vertebra not only the neural (e', e'') and
haemal (o', o'') arches, but also, above and below these, the radialia
(a'', u') and the fin-rays (a', u''). (Neither the radialia nor the
fin-rays are, by the way, in the same transverse plane as the body of
the vertebra). Every vertebra, he considers, contains these nine
pieces--the cycleal (or body), the two perials (e', e'') and the two
epials (a', a'') above, the two paraals (o', o'') and the two cataals (u',
u'') below. The epials and the cataals are in reality paired bones which
in fish mount one on top of the other to support the median fins. In the
cranial region--the skull is formed of modified vertebrae--the epials
and perials open out so as to form the walls and roof of the brain; in
the thoracic region the paraals and cataals reach their maximum of
development and perform the same service for the thoracic organs, the
paraals becoming vertebral, and the cataals sternal, ribs.
We have seen that in Arthropods the body of the vertebra (cycleal) forms
the open ring of the segment, which lies immediately under the skin, the
vertebral tube coinciding with the epidermal tube. The homologues of the
other eight pieces of the vertebra must accordingly be sought in the
external appendages. At first sight there seems here a contradiction of
the principle of connections, for the appendages in Arthropods are
lateral, whereas the paired bones of the vertebra are dorsal and
ventral. But there is in reality no contradiction, for "what our law of
connections absolutely requires is that all organs, whether internal or
external, should stand to one another in the same relations; but it is
all one whether the box (_coffre_) that encloses them lies with this or
that side on the ground. What similarities in the organisation of man
and the digitate mammals, and yet what differences between their
attitudes when standing! The same holds true as regards the normal
attitudes of the pleuronectids and the other fishes" (p. 107).
The exact way in which Geoffroy homologised the parts of the appendages
in Arthropods with the paired pieces of the typical vertebra is best
shown by the reproduction of his figure of an abdominal segment of the
lobster (Fig. 3), in which the parts homologous with those represented
in the figure of the typical vertebra (Fig. 2) are indicated by the same
letters. The ingenuity of the comparison is astonishing.
[Illustration: FIG. 3.--Abdominal Segment of the Lobster. (After
Geoffroy.)]
The comparison of the Arthropod with the Vertebrate is extended also to
the internal organs. The internal organs of the Arthropod are shown to
stand in the same order to one another as in the Vertebrate, only the
organs are inverted. Thus the nervous system is dorsal in the
Vertebrate, ventral in the Arthropod. Turn the Arthropod on its back and
the relative positions of the systems of organs are the same as in the
Vertebrate. The relation of the organs to the external tube is of course
different in Arthropods and Vertebrates, but this is no contradiction of
the principle of connections. "Such a tube, although it is the organs
essential to life that it contains, can yet behave in different ways
with regard to the mass of these organs: the principle of connections
demands only that all the organs maintain with one another fixed and
definite relations; but the principle would be in no way invalidated if
the whole mass had rotated inside the tube" (p. 112).
Geoffroy pushed the analogy between Arthropods and Vertebrates very far,
for he asserted that every piece in the skeleton of an insect was
homologous with some bone in Vertebrates, that it stood always in its
proper place, and remained faithful to at least one of its
connections.[93] It does not appear that he attempted to prove in detail
this very big assumption, but the beginnings of a detailed comparison
are found in the paper of 1820, _Sur l'organisation des insectes_. Six
segments are distinguished in an insect--the head, the three divisions
of the thorax, the abdomen, and the terminal segment of the abdomen (p.
455).
The skeleton of the insect's head is said to correspond to the bones of
the face, to the bones of the cerebrum and to the hyoid of higher
Vertebrates, the skeleton of the prothorax to the bones of the
cerebellum, of the palate, and the pieces of the larynx, the skeleton of
the mesothorax to the parietals, interparietals, and opercular bones,
and that of the metathorax to the skeleton of the thorax of Vertebrates.
The pieces of the abdomen and of the terminal segment correspond to the
bones of the abdomen and coccyx (p. 458). It does not need the
subsequent likening of the hind wings of insects to the air bladder of
fish, and of the stigmata to the pores of the lateral line, to convince
one finally of the fancifulness of the whole comparison.
In 1830 two young naturalists, Meyranx and Laurencet, presented to the
Academie des Sciences a memoir in which they likened a Cephalopod to a
Vertebrate bent back at the level of the umbilicus, saying that the
Vertebrate in this position had all its organs in the same order as in
the Cephalopod. Geoffroy took up this idea with enthusiasm, seeing in it
a further application of his master-idea of the unity of plan and
composition. By means of this comparison Mollusca definitely took their
place in the _Echelle des etres_, after the Articulata, just as Geoffroy
had maintained in 1820, saying that crabs formed a link between the
other Crustacea and the molluscs.[94] The comparison brought him nearer
to the end he had in view, the reference of all animal structure to one
single type.
But in championing the memoir of Meyranx and Laurencet, Geoffroy found
himself in direct antagonism with Cuvier, who held that his four
"Embranchements" had each a separate and distinct plan of structure. In
a paper read to the Academy in February 1830,[95] Cuvier easily
demolished the crude comparison of the Cephalopod to the Vertebrate. He
gave diagrams of the internal organs of a Cephalopod and of a Vertebrate
bent back in the manner indicated by Meyranx and Laurencet, and he
showed in detail that the arrangement of the main organs was quite
different, that the likeness would have been much greater if the
Cephalopod had been likened to a Vertebrate doubled up the other way,[96]
but that even then the arrangement of the organs would not be the same.
The organs, too, of the Cephalopod are differently constructed. He sums
up his criticism by saying:--"I give true and summary expression to all
these facts when I say that Cephalopods have several organs in common
with Vertebrates, which fulfil in either case similar functions, but
that these organs are differently arranged with respect to one another,
and often constructed in a different way; that they are in Cephalopods
accompanied by several other organs which Vertebrates do not possess,
whilst the latter on their side have many organs which Cephalopods lack"
(p. 257). Geoffroy could not accept this commonsense view of the matter,
but made a fight for his transcendental theories. This was the beginning
of the famous controversy between Geoffroy and Cuvier which so excited
the interest of Goethe. It was a struggle between "comparative anatomy"
and "morphology," between the commonsense teleological view of structure
and the abstract, transcendental. Geoffroy brought forward all his
theories on the homology of the skeleton of fish with the skeleton of
higher Vertebrates, and tried to prove by them his great principle of
the unity of plan and composition; Cuvier took Geoffroy's homologies one
by one, and showed how very slight was their foundation. Cuvier was on
sure ground in insisting upon the observable diversities of structural
type, and his vast knowledge enabled him to score a decisive victory.[97]
The controversy was not, as we are sometimes told, a controversy between
a believer in evolution and an upholder of the fixity of species,
although it raised a question upon which evolution theory was to throw
some light.
In these Darwinian days Geoffroy has reaped a little posthumous glory as
an early believer in evolution. That he did believe in evolution to a
limited extent is certain; that his theory of evolution was, as it were,
a by-product of his life-work, is also certain. Geoffroy was primarily a
morphologist and a seeker after the unity hidden under the diversity of
organic form. His theory of evolution had as good as no influence upon
his morphology, for he did not to any extent interpret unity of plan as
being due to community of descent. His morphological, non-evolutionary
standpoint comes out quite clearly in several places in the _Philosophie
anatomique_. He does not derive the structure of the higher Vertebrates
from the simpler structure of the lower, but when he finds in fish a
part at the maximum of its development, he speaks of the same part,
rudimentary in the higher forms, as being, as it were, held in reserve
for use in the fish. Thus, speaking of the episternal in fish which
forms the central piece of its sternum, he says, "it is a bone that is
rudimentary in birds (one might almost add a bone that is held in
reserve in birds for this fate) which is destined to form in the centre
the principal keel of this new machine" (p. 84). Again, with reference
to the homology of the ossicles of the ear with the opercular bones in
fish, "employing other resources equally hidden and rudimentary, Nature
makes profitable use of the four tiny ossicles lodged in the auditory
passage, and, raising them in fish to the greatest possible dimensions,
forms from them these broad opercula...." (p. 85). Or you may take it
the other way about, and start from the organisation of fishes;
opercular bones are of no use to air-breathing animals, so they dwindle
away, and are pressed into the service of the ear, although they are of
little use in hearing (p. 46).
There is here no thought of evolution; in later years, however, his
researches upon fossil crocodilians led him to consider the possibility
that the living species were descended from the antediluvian. For the
factors of the transformation he refers to Lamarck's hypotheses.[98] In a
memoir of 1828,[99] dealing with the possible genetic relation of living
to fossil species, he still regards the question as more or less open.
Although fossil species are mostly different from living species are we
therefore to conclude, he asks, that they are not the ancestors of the
present day forms? "The contrary idea arises more naturally in the mind;
for otherwise the six-days' creation would have had to be repeated and
new beings produced by a fresh creation. Now this proposition, contrary
as it is to the most ancient historical traditions, is inadmissible" (p.
210). It is sufficiently clear from this quotation that Geoffroy was
thinking only of a transformation of the antediluvian species created by
God, and by no means of an evolution of all species from one primitive
type. In matters of religion Geoffroy was orthodox. He goes on to point
out how great a resemblance there is in essential structure between
fossil and living species. All find their place in one scheme of
classification; does it not seem that all are modifications "of one
single being, of that abstract being or common type, which it is always
possible to denote by the same name?" (p. 211). This type is abstract,
not actual, and it is certainly not conceived as an original ancestor of
all animals.
The fullest development of Geoffroy's views on evolution is found in his
memoir "Le degre d'influence du monde ambiant pour modifier les formes
animales."[100] Here the relation of his evolution-theory to his
morphology is pointed out. The principle of unity of plan and
composition cannot be the final goal of zoology; there must follow on it
a philosophical study of the _differences_ between organic forms. The
causes of these differences are to be found in the environment (pp.
66-7). Geoffroy seems here to be moving from a pure to a causal
morphology. It is probable, he continues, that living species have
descended by uninterrupted generation from the antediluvian species (p.
74), and that they have in the process become modified through external
influences.
Now of all functions respiration is the most important, and upon
respiration everything is regulated. "If it be admitted that the slow
progression of the centuries has brought in its train successive changes
in the proportion of the different elements of the atmosphere, it
follows as a rigorously necessary consequence that the organisation has
been proportionately influenced by them" (p. 76). The respiratory milieu
changes, the species change with it, or are eliminated (p. 79). We may
see, perhaps, in the stress which Geoffroy lays upon respiration and the
respiratory milieu a result of his constant obsession with the
comparison of fish with air-breathing Vertebrates.
In the first geological period, we read in another Memoir of the same
year,[101] when ammonites and _Gryphaea_ flourished, hot-blooded animals
with lungs could not exist. "A lung constructed like that of mammals and
birds would not have been adapted to the essence of the respiratory
element such as in my conception of it the system of the environing air
used to be"[102] (p. 58).
Geoffroy does not tell us exactly how the milieu is to act upon the
organism; the whole theory is little more than a sketch and a pointing
out of the way for future research--and in this prophetic enough. The
action of external agents was apparently considered as physical, and no
power of active adaptation was ascribed to the organism.
From a passage in the memoir "Sur la Vertebre" we may perhaps infer that
he believed increasing complexity of structure to be due to a
realisation of potentialities, to the development of parts present in
the lower animals only in potency--"the organisation ... only awaits
favourable conditions to rise, by addition of parts, from the simplicity
of the first formations to the complication of the creatures at the head
of the scale" (p. 112). Evolution takes place as the environment allows,
and in a sense in opposition to the environment.
He believed in saltatory evolution, for he considered that the lower
oviparous Vertebrates could not be transformed into birds by slow
modification, but only by a sudden transformation of their lungs, which
would bring about the other characteristics of birds (p. 80). He
considered, too, that transformations could arise by means of monstrous
development (p. 86). In this connection the experiments which he made on
the hen's egg[103] in order to produce artificial monstrosities are
significant, though his purpose was rather to obtain proof of the
inadequacy of the preformation hypothesis.[104]
It seems probable enough that if Geoffroy had developed his views on
evolution he would finally have been led to interpret unity of plan in
terms of genetic relationship. But as it was he remained at his
morphological standpoint. He did not interpret rudimentary organs as
useless heritages of the past; he preferred to think that Nature had
prepared double means for the same function, one or other being
predominant according as the animal lived in the water or on the land.
"To the animal that lives exclusively in the air Nature has granted an
organisation suited to this mode of respiration, without however
suppressing the other corresponding means, that is to say, without
depriving it of a second system which is applicable only to the mode of
respiration by the intermediary of water, and _vice versa_."[105]
He seems, in one instance at least, to have hit upon the root-idea of
the biogenetic law, but he was far from appreciating its significance.
He recognised that an amphibian in its development passed through a
stage when it was in all essentials similar to a fish, and he saw in
this visible transformation a picture of the evolutionary
transformation. "An amphibian," he writes,[106] "is at first a fish under
the name of tadpole, and then a reptile [_sic_] under that of frog....
In this observed fact is realised what we have above represented as an
hypothesis, the transformation of one organic stage into the stage
immediately superior." But it is not clear that he considered the
development of the amphibian to be a _repetition_ of its ancestral
history.
He went, however, a certain length towards recognising the main
principle of a law which was a commonplace of German transcendental
thought, and was developed later by his disciple E. Serres, the law that
the higher animals repeat during their development the main features of
the adult organisation of animals lower in the scale. Thus he compared
fish as regards certain parts of their structure with the foetus of
mammals. He compared also Articulates with embryonic Vertebrates in
respect of their vertebrae, for in the higher Vertebrates the body of the
vertebra is tubular at an early stage of development, and in Articulates
the body of the vertebra remains tubular permanently (_supra_, p. 61).
As regards their vertebrae, "insects occupy a place in the series of the
ages and developments of the vertebrate animals, that is to say, they
realise one of the states of their embryo, as fishes do one of the
states of their foetal condition."[107]
This idea was destined to exercise a great influence upon the
development of morphology. A further development of the thought is that
certain abnormalities in the higher animals, resulting from arrest of
development, represent states of organisation which are permanent in the
lower animals.[108]
So far we have considered Geoffroy's theories in their application to
the facts. We go on to discuss the theories themselves, and the general
conception of living things which underlies them.
The principle of unity of plan and composition is the keynote of
Geoffroy's work. It states that the same materials of organisation are
to be found in all animals, and that these materials stand always in the
same general spatial relations to one another. The "materials of
organisation" are not necessarily organs in the physiological sense, and
indeed the principle of the unity of plan cannot be upheld if the unity
has reference to organs only. This became clear to Geoffroy, especially
in his later years. In 1835 he wrote, speaking of the principle of the
unity of plan, "I have, moreover, regenerated this principle, and
obtained for it universality of application, by showing that it is not
always the organs as a whole, but merely the materials composing each
organ, that can be reduced to unity."[109] Even in the _Philosophie
anatomique_ he deals rather with parts than with organs; he deals, for
instance, with the elementary parts of the sternum, not with the organ
"sternum" in its totality. The functions of the sternum vary, and the
primary protective function of the sternum may be assumed by quite other
parts, _e.g._, by the clavicles in fish, which protect the heart.[110]
True homologies can be established between materials of organisation but
not always between organs, which may be composed of different
"materials."
Almost as a corollary to this comes the further view that form is of
little importance in determining homologies. An organ is essentially an
instrument for doing a particular kind of work, and its form is
determined by its function. Organs which perform the same function are
usually similar in form though the elementary materials composing them
may be different. This is seen in many cases of convergence. Organs,
therefore, which perform the same function and are similar in external
form are not necessary homologous. Conversely, the same complex of
materials, say a fore limb, may take on the most varied shapes according
as the function of the organ changes--but homology remains though form
changes. Accordingly, form is one of the least important elements to be
considered in determining a homology. "Nature," he wrote in one of his
early papers, "tends to repeat the same organs in the same number and in
the same relations, and varies to infinity only their form. In
accordance with this principle I shall have to draw my conclusions, in
the determining the bones of the fish's skull, not from a consideration
of their form, but from a consideration of their connections."[111]
Again, after comparing a vertebra of the Aurochs with an abdominal
segment of the crab, he says, "I have insisted upon an identity which
has extended to the least important relation of all, that of form."[112]
Geoffroy's morphological units or materials of organisation were in the
case of the skeleton--with which his researches principally deal--the
single bones. But the interesting point is that he sought his
skeleton-units in the embryo, and considered each separate centre of
ossification as a separate bone. Coalescence of bones originally
separate is one of the most usual events in development, and it is an
occurrence which, more than any other, tends to obscure homologies.
Because of its coalescence with the maxillaries, the intermaxillary in
man was not discovered until Vicq d'Azyr and Goethe found it separate in
the embryo. Apparently quite independently of Goethe, Geoffroy hit upon
this plan of seeking in the embryo the primary elements or materials of
organisation. In an early paper on the skull of Vertebrates,[113] where he
is concerned with showing that each bone of the fish's skull has its
homologue in the skull of higher Vertebrates, he is faced with the
difficulty that the skull of the fish has more bones than the skull of
higher Vertebrates. "Having had the inspiration," he writes, "to reckon
as many bones as there are distinct centres of ossification, and having
made a consistent trial of this method, I have been able to appreciate
the correctness of the idea: fish, in their earliest stages, are in the
same conditions relatively to their development as the foetuses of
mammals, and hence bear out the theory" (p. 344). So, too, in dealing
with the homologies of the sternal elements (_supra_, p. 57) he treats
as separate bones the "annexes" of the sternum in birds, though these
are separate only in the young.
If the same materials of organisation are present in all animals, and if
they are arranged always in the same positions relatively to one
another, how does it come about that animal forms are so varied, what
explanation can be offered of the diversities of organic structure?
Geoffroy's main answer to this question is his _Loi de balancement_. The
law was enunciated by him already in 1807.[114] We take the following
quotation, which represents his thought most nearly, from the _Cours de
l'histoire naturelle des Mammiferes_ (1829). "According to our manner of
regarding the organisation of mammals, there is only a single animal
modified by the inverse reciprocal variation of all or some of its
parts. Now, from the fact that there is only one single general animal,
it follows that for each section of its components or for each of its
organs there is available only a given quantity of formative materials.
Now suppose that the distribution of these materials has not been made
in such a way as to ensure an exact equilibrium between all the parts
concerned, one organ will get more than its share, another less. My law
of the compensation of organs is founded on these principles" (i.,
_Lecon_ 16, p. 12). "The atrophy of one organ turns to the profit of
another; and the reason why this cannot be otherwise is simple, it is
because there is not an unlimited supply of the substance required for
each special purpose."[115] The nutritive material available is limited
for each species; if one part gets more than its share the other parts
must get less--that is all the law means. As an example, take the
minuteness of the episternals and xiphisternals in birds, as contrasted
with the huge size of the entosternal. "The minuteness of the
episternals and xiphisternals might be imputed to this gigantic piece
diverting to its own profit the nutritive fluid, since the bigger it is
the smaller these are."[116]
One has constantly to remember in dealing with Geoffroy's theories that
he was not an evolutionist, but purely a morphologist. It is therefore,
perhaps, to ask too much to require of him an explanation of the causes
of diversity. The morphologist describes, classifies, generalises; he
does not seek for causes. But we must leave this question aside in order
to discuss how far Geoffroy's theory of the unity of plan and
composition fits the facts. As Geoffroy himself admitted on several
occasions, his theory was an _a priori_ one, a theory hit upon by hasty
induction, then erected into a principle and imposed upon the facts. No
more than Goethe did he extract his principle from a sufficient mass of
data.
Now he found his theory to be in its pure form unworkable; he found, for
example, that the skeleton of fishes could not be compared directly,
bone for bone, with the skeleton of higher Vertebrates; he had to admit
differences of position of whole sets of organs in the two groups, he
had to admit various _metastases_, before he could bring the skeleton of
fish into line. And these metastases are due to functional
requirements--for example, the forward position of sternum and thoracic
organs in fish is an adaptation to swimming.
So he does not so much demonstrate the unity of plan of whole organisms
as the unity of plan of particular corresponding parts of them. Thus he
does not prove or attempt to prove that Articulates are in all points
like Vertebrates, but simply that their skeleton is built upon the same
plan as that of Vertebrates. The rest of the organs, while still
comparable with the organs of Vertebrates, stand in different relations
to the skeleton. An Articulate therefore, on his own showing, is not,
_as a whole_, built upon the same general structural plan as a
Vertebrate.
Further, he does not always remain true to his principles, for he does
not establish homologies of parts entirely by their connections but
sometimes by their functions as well. Thus the sternum, or rather the
complex of sternal elements, is defined and discovered in particular
cases not by its connections only but also by its functions. The
framework of the gills is homologised part by part with the framework of
the lungs, not because the relations of the framework to the rest of the
skeleton are the same in fish and air-breathing Vertebrates, but simply
because gills are considered the equivalents of lungs--a comparison
which is purely physiological.
Even with these concessions to the functional view of living things,
Geoffroy was unable to make good his contention that all animals are
built upon the same plan. His arguments failed to carry conviction to
his contemporaries, and Cuvier in particular subjected them to
destructive, and indeed final, criticism.
The paper, already referred to, in which Cuvier disposed of the
transcendentalists' comparison of Cephalopods and Vertebrates is of
great significance, for it states in the clearest way the radical
opposition between the functional and the formal attitudes to living
things.
Cuvier points out that if by unity of composition is meant identity,
then the statement that all animals show the same composition is simply
not true--compare a polyp with a man!--on the other hand, if by unity is
meant simply resemblance or homology, the statement is true within
certain limits, but it has been employed as a principle since the days
of Aristotle, and the theory of unity of composition is original only in
so far as it is false. He admits, however, that Geoffroy has seized upon
many hidden homologies, especially by his valuable discovery of the
importance of foetal structure. In all this Cuvier is undoubtedly right.
Unity of plan and composition, as Geoffroy conceived it, simply does not
exist. Cuvier goes on to say that this principle of Geoffroy's, in the
greatly modified form in which it can be accepted, and has been accepted
from the dawn of zoology, is not the sole and unique principle of the
science. On the contrary, it is merely a subordinate principle,
subordinate to a higher and more fruitful principle, that, namely, of
the conditions of existence, of the adaptation (_convenance_) of the
parts, of the co-ordination of the parts for the role which the animal
is to play in Nature. "That is the true philosophical principle," he
says, "whence may be deduced the possibility of certain resemblances,
the impossibility of certain others; it is the rational principle from
which follows the principle of the unity of plan and composition, and in
which at the same time it finds those limits, which some would like to
disregard" (p. 248).
Geoffroy's position is the direct contrary. He holds that the principle
of the unity of plan and composition is the true base of natural
history,[117] and that this unity limits the possible transformations of
the organism. Thus, speaking of the influence of the respiratory medium,
he says, "All the same this influence of the external world, if it has
ever become a cause which disturbed organisation, must necessarily have
been confined within fairly narrow limits; animals must have opposed to
it certain conditions inherent to their nature, the existence of the
same materials composing them, and a manifest tendency to resemble one
another, and to reproduce invariably the same primordial type."[118] Unity
of plan and composition is, on this view, prior to adaptation and limits
adaptation. Cuvier's view, on the contrary, is that the necessity of
functional and ecological adaptation accounts for the repetition of the
same types of structure. There are, of all the possible combinations of
organs, only a few viable types--those whose structure is adapted to
their life. Therefore it is reasonable that these few types should be
repeated in innumerable exemplars. One must remember, in order to
appreciate Cuvier's view, that he was not obsessed, as we are, by the
idea of evolution.
Cuvier thought in terms of organs, not in terms of "materials of
organisation." He held that the resemblances between the organs of one
class of animals and the organs of another were due to the similarity of
their functions. "Let us conclude, then, that if there are resemblances
between the organs of fish and those of other classes, it is only in the
measure that there is a resemblance between their functions."[119] There
are only a few kinds of organs, each adapted for a particular function,
and these organs are necessarily repeated from class to class.--"As the
animal kingdom has received only a limited number of organs, it is
inevitable that some at least of these organs should be common to
several classes."[120]
Geoffroy thought in terms of "materials," of parts of indefinite
function, parts which might take on any function. He insists upon the
necessity of disregarding function when tracing out the unity of
composition. He considers, in direct opposition to Cuvier's
interpretation of structural resemblance as due to similarity of
function, that unity of composition is the primary fact, and similarity
of function subsidiary. In his reply in the _Mammiferes_ (1829) to
Cuvier's criticisms in the _Histoire naturelle des Poissons_ (1828), he
insists on the necessity of excluding function from consideration in any
truly philosophical treatment of comparative anatomy (Discours prel., p.
25). Cuvier held that function determined structure, or at least that
the necessity of adaptation ruled the transformations of form. Geoffroy
considered that structure determined function, that changes of
structure, however they might arise, caused changes of function.
"Animals," he writes, "have no habits but those that result from the
structure of their organs; if the latter varies, there vary in the same
manner all their springs of action, all their faculties and all their
actions."[121]
Again, "a vegetarian regime is imposed upon the Quadrumana by their
possession of a somewhat ample stomach, and intestines of moderate
length."[122] The hand of the bat has become so modified as to constrain
the bat to live in the air.[123]
The best example of Geoffroy's insistence upon the priority of structure
to function, and so of his purely morphological attitude, is perhaps his
interpretation, already alluded to, of the appendages of Articulates.
The segments of the Articulate are, he says, the equivalents of the
bodies of the vertebrae of higher forms. Now "from the circumstance that
the vertebra is external, it results that the ribs must be so too; and,
as it is impossible that organs of such a size can remain passive and
absolutely functionless, these great arms, hanging there continually at
the disposition of the animal, are pressed into the service of
progression, and become its efficient instruments."[124] The ribs become
locomotory appendages.
We may compare the similar thought that the ear ossicles are simply
opercular bones reduced and turned to other uses.
Geoffroy could not but recognise the correlation of structure to
function, for this is a fact which imposes itself upon every observer.
He recognised also correlation between functions, as when he pointed out
the connection between increased respiration and enhanced muscular
activity in birds.[125] He interpreted structure at times in terms of
function, the short, strong clavicle of the mole as an adaptation to
digging, the keeled sternum of birds as an adaptation to flying, and so
on. But we may say that his whole tendency was to disregard function, to
look upon it as subsidiary. He protests against arguing from function
and habits to structure, as an "abuse of final causes."[126] He was not so
convinced as Cuvier was of the all-importance of functional correlation;
in this view he was probably confirmed by his work on teratology. It did
not surprise him that Insects, in which lungs, heart and circulation
have disappeared(!), should yet have a skeleton built upon the same plan
as the skeleton of Vertebrates, which possess these organs; the
correlation of organ-systems is not so close as to prevent this.[127] So
too, although the other organs of the insect are all inside the body of
the vertebrae, they are yet comparable with the organs of Vertebrates.[128]
The existence of rudimentary organs also seemed to him an argument
against too strict a correlation of parts.
The contrast between the teleological attitude, with its insistence upon
the priority of function to structure, and the morphological attitude,
with its conviction of the priority of structure to function, is one of
the most fundamental in biology.
Cuvier and Geoffroy are the greatest representatives of these opposing
views. Which of them is right? Is there nothing more in the unity and
diversity of organic forms than the results of functional adaptation, or
is Geoffroy right in insisting upon an element of unity which cannot be
explained in terms of adaptation? If there be an irreducible element of
unity, is there any truth in Geoffroy's suggestion that this unity
results from a power which is exercised in the world of atoms where are
elements of inalterable character?[129]
The problem as Geoffroy and Cuvier understood it was not an evolutionary
one. But the problem exists unchanged for the evolutionist, and
evolution-theory is essentially an attempt to solve it in the one
direction or the other. Theories such as Darwin's, which assume a random
variation which is not primarily a response to environmental changes,
answer the problem in Geoffroy's sense. Theories such as Lamarck's,
which postulate an active responsive self-adaptation of the organism,
are essentially a continuation and completing of Cuvier's thought.
[86] "Memoire sur les rapports naturels des makis,"
_Magasin Encyclopedique_, vii.
[87] Discours preliminaire, pp. xv.-xxiv.
[88] _Etudes progressives d'un Naturaliste_, p. 50,
Paris, 1835.
[89] _Philosophie Anatomique_., i., Introduction, p. 1.
[90] "Sur une colonne vertebrale et ses cotes dans les
insectes apiropodes," (_Acad. Sci._, Feb. 12, 1820).
Printed in _Isis_, pp. 527-52, 1820 (2).
[91] "Sur l'organisation des insectes," p. 458. _Isis_,
pp. 452-62, 1820 (2).
[92] _Mem. Mus. d'Hist. nat._, ix., pp. 89-119, Pls.
v-vii.
[93] _Sur l'organisation des insectes_, p. 459.
[94] _Isis_, p. 549.
[95] Published in _Ann. Sci. Nat._, xix., pp. 241-59,
1830.
[96] _Cf._ Aristotle (_supra_, p. 10).
[97] For an account of the controversy reference may be
made to I. Geoffroy St Hilaire, _Vie Travaux et Doctrine
scientifique d'Etienne Geoffroy St Hilaire_, Paris,
1847; also Semper, _Arb. zool. zoot. Instit. Wuerzburg_,
iii., 1876-7, K. E. von Baer, _Lebensgeschichte Cuviers_,
ed. L. Stieda, 1897, and J. Kohlbrugge, in _Zoolog.
Annalen_, v., pp. 143-95. 1913.
[98] "Recherches sur l'organisation des Gavials," _Mem.
Mus. d'Hist. nat._, xii., 1825.
[99] _Mem. Mus. d'Hist. nat._, xvii., pp. 209-29.
[100] _Mem. Acad. Sci._, xii., pp. 63-92, 1833.
[101] _Mem. Acad. Sci._, xii., pp. 43-61, 1833.
[102] Geoffroy's French style is at times incredibly bad,
and more or less literal translations of his sentences
are apt to read queerly!
[103] _Mem. Mus. d'Hist. nat._, xiii., p. 289, 1826.
[104] _Mem. Mus. d'Hist. nat._, xviii., p. 221, 1828. His
teratological work is important, and is chiefly
contained in the second volume of the _Philosophie
anatomique_.
[105] _Phil. anat._, i., p. 449.
[106] _Mem. Acad. Sci._, xii., p. 82, 1833.
[107] _Mem. Mus. d'Hist. nat._, ix., p. 101, 1822.
[108] _Cours de l'histoire naturelle des Mammiferes_, i.,
Lecon 3, p. 13, 1829.
[109] _Etudes progressives d'un Naturaliste_, p. 59, f.n.,
Paris, 1835.
[110] _Phil. Anat._, i., p. 444.
[111] _Ann. Mus. d'Hist. nat._, x., p. 344, 1807.
[112] _Isis_, p. 534, 1820 (2).
[113] _Ann. Mus. d'Hist. nat._, x., pp. 342-65, 1807.
[114] _loc. cit._, x., p. 343.
[115] _Phil. anat._, i., 450, f.n. _Cf._ Aristotle
(_supra_, p. 11).
[116] _Loc. cit._, p. 136.
[117] _Mammiferes_, i., Discours prel., p. 18.
[118] _Phil. anat._, i., p. 208.
[119] Cuvier and Valenciennes, _Hist. nat. Poissons_, i.,
p. 550, 1828.
[120] Cuvier and Valenciennes, _loc. cit._, p. 544.
[121] _Mammiferes_, i., _Lecon_ 4, p. 17.
[122] _Loc. cit._, _Lecon_ 5, p. 8.
[123] _Loc. cit._, _Lecon_ 13, p. 6.
[124] _Isis_, p. 539, 1820 (2).
[125] _Mammiferes_, i., _Lecon_ 4, p. 6.
[126] _Mammiferes_, Discours prel., p. 7.
[127] _Isis_, p. 460, 1820 (2).
[128] _Mem. Mus. d'Hist. nat._, ix., p. 102, 1822.
[129] _Mem. Acad. Sci._., xii., p. 76, 1833.