Form and Function

E >> E. S. (Edward Stuart) Russell >> Form and Function

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[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]

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