THE FOLLOWERS OF ETIENNE GEOFFROY SAINT-HILAIRE
Geoffroy's theories were not generally accepted by his contemporaries,
but his methods had considerable influence, especially in France, where
many made essays in pure morphology.
His chief follower was Serres, who is mentioned indeed in the
_Philosophie anatomique_ as a fellow-worker. Serres was primarily a
medical anatomist; his interest lay in human anatomy and embryology,
normal and pathological.
His best early work was an _Anatomie comparee du cerveau_ (1824-26),
which met with a flattering reception from Cuvier.[130] He laid great
stress upon the development of the brain and spinal cord in the
different classes, and was quick to point out analogies not only between
adult but also between embryonic structures. He paid much attention to
cases of correlation, and noted a great many; he observed, for instance,
a constant relation between the development of the spinal cord and of
the corpora quadrigemina, and between the size of the corpora
quadrigemina and the volume of the optic nerves and eyes. In this the
influence of Cuvier is unmistakable.
Serres' early theoretical views are to be found in a series of papers in
the _Annales des Sciences naturelles_,[131] under the general title
_Recherches d'Anatomie transcendante, sur les Lois de l'Organogenie
appliquees a l'anatomie pathologique_, also published separately. We
follow these papers in our expose of Serres' doctrine, reserving for a
future chapter (Chap. XII.) the consideration of his matured views of
thirty years later.
In the first of them he points out how neither position nor function has
proved altogether sufficient to establish homologies. In the early days
anatomists were guided by form; when form failed them, they traced an
organ in its changes throughout the series of animals by considering its
function. This method was satisfactory enough as regards the organs of
the nutritive life. But in the organs of the life of relation, in the
nervous system, the functions of the parts were difficult to discover,
and their form very changeful. Hence a new principle was required, and
Serres found it in the thought which he probably owed to the German
transcendentalists (see Chap. VII.), that the permanent structure of the
lower animals could be compared with phases in the development of the
higher, and particularly of man, or, as he put it, that comparative
anatomy was often only a fixed and permanent anthropogeny, and
anthropogeny a fugitive and transitory comparative anatomy (xi., p.
106).
"In rising towards the first formations," he writes, "transcendental
anatomy recognised that one and the same organ, however complicated its
definitive form might be, repeated in its transitory states the organic
simplicities of the lower classes. Thus the primitive heart of birds was
first of all a canal, then a pocket or single cavity, then finally the
complex organ of the class. Comparative anatomy was thus seen to be
repeated and reproduced by embryogeny" (xii., p. 85).
His explanation of the fact of repetition is that, "in animals belonging
to the lower classes the _formative force_, whatever it may be, has a
less energetic impulsion than in the higher animals, and hence the
organs pass through only a part of the transformations which those of
the higher forms undergo; and it is for this reason that they show
permanently the organic dispositions which are only transitory in the
embryo of man and the higher Vertebrates. Hence these double aortas,
these double venae cavae which one observes more or less constantly among
reptiles" (xxi., p. 48).
The number of stages in embryogeny is proportionate to the complexity of
the adult; the younger the embryo the simpler its organs--such is the
general formula of the relation between the embryo and the adult. But
here in Serres' doctrine of parallelism a complication enters. He
observed that embryonic organs did not always develop in a piece, by
simple growth, but often were formed by the union of separately formed
parts or layers. Thus the kidney in man is formed by the fusion of a
number of "little kidneys," and the spinal cord reaches its full
development by the laying down of successive layers within it. He was
greatly impressed with this fact, which, as a convinced believer in
epigenesis, he used with great effect against the preformistic theories.
"This method of isolated formation," he wrote, "is noticed in early
stages in the thyroid, the liver, the heart, the aorta, the intestinal
canal, the womb, the prostate, the clitoris, and the penis" (xi., p.
69). So, too, in the development of the skeleton, ossification proceeds
from separate centres, foramina are formed by the fusion of separate
bones round them. In his memoir, _Lois d'Osteogenie_ (1819), Serres
established several laws of ossification based upon this principle of
separate formation.[132]
How is the fact of multiple formation to be reconciled with the
principle of repetition, according to which organs are simplest in the
early embryo and in the lower animals? But observation shows that, as a
rule, the further down the scale you go the more divided organs
become--the more numerous the bones of the skull, for example. There is
thus a parallel between multiple formation of organs in the embryos of
the higher Vertebrates and their subdivided state in the lower. Take,
for example, the kidney. In the genus _Felis_, and in birds, each kidney
has two lobes, in the elephant four, in the otter ten, in the ox twelve
to fourteen. The human kidney in its development starts with about a
dozen lobes, and the number diminishes as the kidney grows. Thus the
permanent state of the kidney in the animals mentioned is reproduced by
the stages of its development in man (xii., p. 126).
So, too, at the second or third month the uterus of the human embryo is
bicornuate, and afterwards passes through stages comparable to the adult
and permanent uterus of rodents, ruminants, and carnivores. There is
indeed a time in the development of the human embryo when it resembles
in many of its organs the adult stage of various lower animals. It is
about this time that it possesses a tail.
We note that Serres' theory of parallelism applies, strictly speaking,
only to organs, not to organisms, although he, too, readily fell into
the error of supposing that the organisation of an embryo could be
compared as a whole with the adult organisation of an animal lower in
the scale. Thus he wrote in one of his later papers[133]--"As our
researches have made clear, an animal high in the organic scale only
reaches this rank by passing through all the intermediate states which
separate it from the animals placed below it. Man only becomes man after
traversing transitional organisatory states which assimilate him first
to fish, then to reptiles, then to birds and mammals." Serres was not
altogether free from the besetting sin of the transcendentalists--hasty
generalisation.
The law of parallelism applied not only to Vertebrates but also to
Invertebrates. In a short paper[134] of 1824 Serres attempted an
explanation of the nervous system of Invertebrates. Invertebrates, he
considered, lacked the cerebrospinal axis of Vertebrates, and their
nervous system was the homologue of the sympathetic system of
Vertebrates. The relation of the invertebrate to the vertebrate nervous
system being thus fixed, can the nervous system of Invertebrates be
reduced to one plan? It does not seem possible to establish a common
plan for the adult nervous systems. But apply the principle of
parallelism, which has proved so valuable within the limits of the
vertebrate series. Taking insects as the highest class, we find that
there are three stages in the development of their nervous system; in
the first the nervous system is composed of two separate strands, in the
second the strands unite round the oesophagus, in the third they unite
also behind. Now in _Bulla aperta_, stage (1) is permanent; in _Clio_,
_Doris_, _Aplysia_, _Tritonia_, _Sepia_, _Helix_, stage (2) is
permanent, and in _Unio_ stage (3). In fact, all the varieties of the
nervous system of molluscs fall into one or other of these three
classes. "It follows, then, that as regards their nervous system, the
Mollusca are more or less advanced larvae of insects" (p. 380). The law
of parallelism is here applied to single organ-systems, but in later
years Serres applied it to whole organisations also, saying that the
lower Invertebrates were permanent embryos of the higher.
In the paper of 1834, already referred to, Serres pushed his
speculations further and attempted to establish the unity of type of all
animals, Vertebrates and Invertebrates alike--a favourite pastime of the
transcendentalists. It is incontestable, he admits, that adult
Invertebrates are quite different in structure from adult Vertebrates,
"but if one regards them as what I take them to be, namely, _permanent
embryos_, and if one compares their organisation with the embryogeny of
Vertebrates, one sees the differences disappear, and from their
analogies arise a crowd of unsuspected resemblances" (_loc. cit._, p.
247).
The last point of Serres' doctrine which calls for remark is his
interpretation of abnormalities as being often comparable to grades of
structure permanent in the lower animals. Thus the double aorta which
may occur as an abnormality in man is the normal and permanent state in
reptiles. This idea, of course, he got from Etienne Geoffroy St Hilaire.
It is further developed in his "_Theorie des formations et des
deformations organiques appliquee a l'anatomie comparee des
monstruosites_ (1832), and in his final large memoir of 1860 (see below,
p. 205).
In 1816 appeared a fine piece of work by J. C. Savigny on the homologies
of the appendages in Articulates. The standpoint was that of pure
morphology. "I am convinced," he wrote, "that when a more complete
examination has been made of the mouth of insects, properly so called,
that is to say, having six legs and two antennae, it will be found that
whatever form it affects it is always essentially composed of the same
elements.... The organ remains the same, only the function is modified
or changed--such is Nature's constant plan."[135] In this the influence of
Geoffroy can be traced; but the work was very free from the
exaggerations of the transcendentalists, and many of Savigny's
homologies are accepted even to-day. The first memoir dealt with the
mouth-parts of insects; the second with the anterior appendages of
Articulates generally. Savigny shows that the mouth-parts of insects can
be reduced to the type shown in Orthoptera, where there are clearly two
mandibles, two maxillae, and a lower lip formed by the fusion of two
second maxillae. All other insects have these same mouth-parts, disposed
in the same order, however much their form may have been modified in
response to new functions. He goes on to compare the anterior set of
appendages in a long series of Articulates, in _Julus_, _Scolopendra_,
_Cancer_, _Gammarus_, _Cyamus_, _Nymphon_, _Phalangium_, _Apus_,
_Caligus_, _Limulus_, and a few others. For Crustacea he established the
homologies now accepted, of the mandibles with the mandibles of insects,
of the first and second pairs of maxillae with the parts so named in
insects, and so on. He is quite clear that the maxillipedes of Crustacea
are the homologues of the feet of Hexapoda. "Their disposition must lead
one to think that the six anterior feet of _Julus_, that is to say, all
the feet of the Hexapoda, are here transformed into jaws" (_loc. cit._,
p. 48). In _Scolopendra_ also there is a similar transformation of two
pairs of legs into auxiliary jaws. In _Gammarus_, where there is only
the first pair of maxillipedes, the other two pairs have become
"retransformed" into feet. We find him supporting his comparison of the
three anterior pairs of legs in _Julus_ to the three pairs of legs in
insects by an argument drawn from embryology; for only the first three
pairs of feet are present in _Julus_ at birth (Degeer), "an observation,
which, together with their position, should cause them to be considered
as the representatives of the six thoracic feet of Hexapoda" (p. 44).
His comparison of the Arachnid appendages with those of insects and
Crustacea is very curious. As his starting-point he takes _Cyamus_,
which has antennae (two pairs) and mouth parts (four pairs) as in many
Crustacea, and then seven pairs of legs; he compares with it _Nymphon_,
which has in all seven pairs of appendages. These appendages he
homologises with the seven pairs of legs of _Cyamus_, so that the first
appendage in _Nymphon_ corresponds to the seventh appendage of _Cyamus_.
This homology is extended to all Arachnids; their first two pairs of
appendages, however they may be modified as "false" mandibles and
"false" maxillae, really correspond to the second and third maxillipedes
in Crustacea, and to the second and third pairs of feet in insects. It
is interesting to note that he treats _Limulus_ as an Arachnid, pointing
out that there is as much difference between _Apus_ and _Limulus_ as
between _Cancer_ and _Phalangium_. He describes the "gnathobases" in
_Phalangium_ and _Limulus_. We may note that he had just an inkling of
the modern doctrine that all the appendages of Articulates consist of a
basal joint bearing an inner and an outer terminal piece, for he
observes that the "cirri" of the maxillipedes of Crustacea give the
appendage the same bifid appearance as the appendages of the abdomen and
the thoracic legs of _Mysis_ (p. 50).
V. Audouin, in his memoir, _Recherches anatomiques sur le thorax des
animaux articules_,[135] applied the principle of the unity of plan and
composition to the exoskeleton of insects, Crustaceans, and Arachnids.
His guiding ideas were, "(1) that the skeleton of articulated animals is
formed of a definite number of pieces, which are either distinct or
intimately fused with one another; (2) that in many cases, some pieces
diminish or altogether disappear, while others reach an excessive
development; (3) that the increase of one piece seems to exert on the
neighbouring pieces a kind of influence which explains all the
differences one finds between the individuals of each order, family and
genus" (Sep. copy, p. 16). Geoffroy had already stated, without proof,
that the parts of the Arthropod's skeleton, however they might change in
shape and size, remained faithful to the principle of connections, at
least at their points of insertion.[137] Audouin gave the detailed
demonstration of this by his accurate and minute determination of the
pieces of the arthropod skeleton. He recognised that the body of
Arthropods was made up of a series of similar rings, and that even the
compact head of insects consisted of fused segments. In each segment
Audouin distinguished a fixed number of hard chitinous parts, the dorsal
tergum, the ventral sternum, the lateral "flanc" of three pieces, all to
be recognised by their positions relative to one another. Many of the
names which he proposed are still in use; it was he who introduced the
terms prothorax, mesothorax, and metathorax, for the three segments of
the insect's thorax. He used Geoffroy's _Loi de balancement_ to explain
cases of correlative development, such as the relation between the size
of the front wings and the development of the mesothorax. In another
paper Audouin compared the three pieces of the dorsal skeleton of
Trilobites to the tergum and the upper part of the "flanc."[138] In a
third paper of about the same time he tried to establish the homologies
of the segments throughout the Articulate series--with less success than
Savigny.
Later on, in conjunction with Milne-Edwards, he demonstrated the unity
of composition of the nervous system in Crustacea, showing how the
concentrated system of the crab was formed by the same series of ganglia
as in the Macrura.
The entomologist Latreille also tackled the problem of the homologies of
the segments in the different classes of Arthropods (Cuvier, _loc.
cit._, p. cclxxii.). He thought he could find fifteen segments in all
Arthropods. He made the retrograde step of likening the head of insects
to a single segment. But some of his homologies showed morphological
insight, _e.g._, his comparison of the "first jaws" of Arachnids to
antennae, because they were placed above the upper lip. It was he who
first pointed out the resemblance of the leaf-like gills of Ephemerid
larvae to wings, and suggested that wings were "a sort of tracheal feet."
He made also a rather hazy and speculative contribution on Okenian lines
to the problem of the relation of Arthropods to Vertebrates, likening
the carapace of Crustacea to an enormously developed hyoid, the
appendages of the tail to the ventral and anal fins of fish. The
masticatory organs of Arthropods were jaws disjointed at their
symphysis; antennae, nostrils turned outside in.
Duges also made a comparison of Articulates with Vertebrates.[139] He did
not accept Geoffroy's vertebral theory of the Arthropod skeleton, though
he admitted that in Arthropods the dorsal surface was turned towards the
ground, basing this assumption on the position of the nervous system,
and also, curiously enough, on the inverted position of the embryo on
the lower surface of the yolk. He considered that the mandibles and
first maxillae of Arthropods were the homologues of the upper and lower
jaws of Vertebrates, adducing as confirmatory evidence the fact that in
snakes the rami are separate. The labium was the equivalent of the
hyoid, the labial palps and maxillipedes the equivalent of the "hyoid"
elements which form the branchial arches.
But Duges' main contribution to morphological method was his conception
of the living organism as a colony of lesser units, which were
themselves real "organisms." "By _organism_ the author means a complex
of organs which taken together suffice to constitute, ideally or
actually, a complete animal. An 'organism' is, as it were, an elementary
or simple animal; several organisms combined form a complex animal" (p.
255). Duges hit upon this principle, which was first suggested to him by
A. Moquin-Tandon's work on the leech (1827), as a great aid in
demonstrating the unity of plan and composition throughout the animal
kingdom.[140] According to his view there are three main types of
animals--(1) Biserials, including bilaterally symmetrical animals,
composed of two parallel series of "organisms"; (2) Radiates, composed
of "organisms" arranged like the spokes of a wheel; and (3)
Raceme-animals, in which the separate "organisms" were disposed more or
less irregularly, in bunches (p. 257). The unitary "organism" is
supposed to be the same in all, only the arrangement differing. Duges of
course admitted that the centralisation of the complete organism became
greater the higher it stood in the scale, and that this held good also
in individual development. The appendages of Articulates and Vertebrates
were thought of as the members of as many separate organisms. He went so
far as to suggest that the fingers of a man's hand were the free
extremities of as many thoracic members.
Duges' conception of the organism has often been revived since in a
saner form, _e.g._, by E. Perrier, and it has a certain validity. It has
much affinity with the similar conceptions of Goethe and the German
transcendentalists.
[130] _Mem. Acad. Sci._, iv., pp. cclxxxiv.-ccci., 1824.
[131] _Ann. Sci. Nat._, xi., xii., 1827; xvi., 1829; xxi., 1830.
[132] See Radl, _loc. cit._, i., pp. 225-6.
[133] _Ann. Sci. nat._ (2), ii., p. 248, 1834.
[134] _Ann. Sci. nat._, iii., pp. 377-80, 1824.
[135] _Memoires sur les Animaux sans Vertebres_, Part I.,
p. 10, Paris, 1816.
[136] _Ann. Sci. Nat._, (1), i., pp. 97-135, 416-432,
1824.
[137] _Isis_, p. 456, 1820 (2).
[138] Cuvier, _Mem. Acad. Sci._, iv., p. cclxx., 1824.
[139] _Acad. Sci._ 18th Oct. 1831. Extract in _Ann. Sci.
Nat._, xxiv., pp. 254-60, 1831.
[140] His views were more fully elaborated in his _Memoire
sur la conformite organique dans l'echelle animale_,
Montpellier, 1832.