Form and Function

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Is it really true, he asks, that the cell is the dominant element in all
organisation; is the cell comparable in importance to the atom of the
chemists; or is it not rather the servant of a higher regulatory power?
Johannes Mueller, who was Reichert's master, had in his _Physiology_[297]
argued splendidly for the existence of a creative force which guides and
rules development, and brings to pass that unity and harmony of
composition which distinguish living things from inorganic products.
Reichert sought in vain in the writings of the biological "atomists" for
any smallest recognition of these broader characteristics of living
things upon which Mueller had rightly laid stress. For the atomists the
cell was the only element of form; they ignored the combination of cells
to form tissues, of tissues to form organs, of organs to form an
organism. For the morphologists the cell was one element among many, and
the lowest of all.

The difference of attitude is clearly shown if we consider from the two
points of view a complicated organ-system such as the central nervous
system. The atomist sees in this a mere aggregate of cells or at the
most of groups of cells. "The morphologist," on the other hand, "sees in
the central nervous system a _proximate_ element in the composition of
the body--a primitive organ. From this point of view he apprehends and
judges its morphological relations with, in the first place, the other
co-ordinated primitive organs in the system as a whole; in all this the
cells remain in the background, and have nothing to do directly with the
determination of these morphological relations" (p. 6). Within the
nervous system there are separate organs which stand to one another in a
definite morphological and functional relationship. These organs are, it
is true, composed of cells; but between the form and connections of
these organs and the cells which compose them there is no direct and
necessary relation (p. 6). It is true that the cell is the ultimate
element of organic form, and that all development takes place by
multiplication and form-change of cells. Yet is the cell in all this not
independent of the unity of the developing embryo, and what the cells
produce, they produce, so to speak, not of their own free will, nor by
chance, but under the guiding influence of the unity of the whole, and
in a certain measure as its agents (p. 7). The atomists will not admit
the truth of this; they see in development nothing more than a process
of the form-change and multiplication of cells. The full meaning of
development escapes them, for they take no cognisance of the increasing
complexity of the embryo, of the separating-out of tissues, of the
moulding of organs, of the harmonious adaptation and adjustment of the
parts to form a working whole.

In general, the fault of the atomists is that they do not respect the
limits which Nature herself has prescribed to the process of logical
analysis and disintegration of the organism; they do not recognise the
existence of natural and rational units or unities; they forget the one
great principle of rational analysis, "that, by universally valid,
inductive, logical method, natural objects must in all cases be accepted
and dealt with in the combination and concatenation in which they are
given" (p. 10).

The atomists at least recognised one natural organic element, the cell;
the materialistic physiologists of the time resolved even this unity
into an aggregate of inorganic compounds, and regarded the organism
itself as nothing but a vastly complicated physico-chemical mechanism.
From this point of view morphology had no right of existence, and we
find Ludwig, one of the foremost of the materialistic school,
maintaining that morphology was of no scientific importance, that it was
nothing more than an artistic game, interesting enough, but completely
superseded and robbed of all value by the advance of materialistic
physiology.[298]

Naturally enough, morphologists did not accept this rather contemptuous
estimate of their science, but held firmly to the morphological
attitude. So Leuckart in his reply to Ludwig, so Rathke in a letter to
Leuckart published in that reply, so Reichert in his _Bericht_, so J. V.
Carus in his _System der thierischen Morphologie_,[299] upheld the
validity, the independence, of morphological methods. Leuckart and
Rathke called attention to the absolute impossibility of explaining by
materialistic physiology the unity of plan underlying the diversity of
animal form. J. V. Carus, who was convinced of the validity of
physiological methods within their proper sphere, drew a sharp
distinction between systematics and morphology on the one hand, and
physiology on the other. Physiology had nothing to do with the problems
of form at all; its business was to study the physical and chemical
processes which lay at the base of all vital activities. Morphology, on
its part, had to accept form as something given, and to study the
abstract relations of forms to one another. "On this point," he writes,
"stress is to be laid, that morphology has to do with animal form as
something _given_ by Nature, that though it follows out the changes
taking place during the development of an animal and tries to explain
them, it does not enquire after the conditions whose necessary and
physical consequence this form actually is" (p. 24). He expressed indeed
a pious hope (p. 25) that physiology might one day be so far advanced
that it could attempt with some hope of success to discover the
physico-chemical determinism of form, but this remained with him merely
a pious hope. Reichert, in his _Bericht_, applied to the rather wild
theorisings of the physiologist Ludwig the same clear commonsense
criticism that he bestowed on the other "atomists."

It would take too long to describe the great development that
materialistic physiology took at this time, and to show how the
separation of morphology from physiology, which originally took place
away back in the 17th century, had by this time become almost absolute.
The years towards the end of the first half of the century marked indeed
the beginning of the classical period as well of physiology as of
dogmatic materialism. Moleschott and Buchner popularised materialism in
Germany in the 'fifties, while Ludwig, du Bois Reymond and von Helmholtz
began to apply the methods of physics to physiology. In France, Claude
Bernard was at the height of his activity, rivalled by workers almost as
great. The doctrine of the conservation of energy was established about
this same time.

Between the cell-theory on the one side, and physiology on the other, it
was a wonder that morphology kept alive at all. The only thing that
preserved it was the return to the sound Cuvierian tradition which had
been made by many zoologists in the 'thirties and 'forties. It is a
significant fact that this return to the functional attitude coincided
in the main with the rise of marine zoology, and that the man who most
typically preserved the Cuvierian attitude, H. Milne-Edwards, was also
one of the first and most consistent of marine biologists. Milne-Edwards
describes in his interesting _Rapport sur les Progres recents des
Sciences zoologiques en France_ (Paris) 1867, how "About the year 1826,
two young naturalists, formed in the schools of Cuvier, Geoffroy and
Majendie, considered that zoology, after having been purely descriptive
or systematic and then anatomical, ought to take on a more physiological
character; they considered that it was not enough to observe living
objects in the repose of death, and that it was desirable to get to
understand the organism in action, especially when the structure of
these animals was so different from that of man that the notions
acquired as to the special physiology of man could not properly be
applied to them" (p. 17). The two young naturalists were H.
Milne-Edwards and V. Audouin. In pursuance of these excellent ideas they
set to work to study the animals of the seashore, producing in 1832-4
two volumes of _Recherches pour servir a l'histoire naturelle du
littoral de la France_. After Audouin's early death A. de Quatrefages
was associated with Milne-Edwards in this pioneer work, and their
valiant struggles with insufficient equipment and lack of all laboratory
accommodation, and the rich harvest they reaped, may be read of in
Quatrefage's fascinating account of their journeyings.[300] Note that
though they called themselves physiologists they meant by physiology
something very different from the mere physical and chemical study of
living things. They were interested, as Cuvier was, primarily in the
problems of form; they sought to penetrate the relation between form and
function; their chief aim was, therefore, the study not of physiology[301]
in the restricted sense, but physiological morphology. As a matter of
fact they produced more taxanomic and anatomical work than work on
physiological morphology, but this was only natural, since such a wealth
of new forms was disclosed to their gaze. Milne-Edwards' masterly
_Histoire Naturelle des Crustaces_[302] and A. de Quatrefage's _Histoire
Naturelle des Anneles marins et d'eau douce_[303] were typical products of
their activity.

In the North, men like Sars and Loven were starting to work on the
littoral fauna of the fjords; in Britain, Edward Forbes was opening up
new worlds by the use of the dredge; Johannes Mueller was using the
tow-net to gather material for his masterly papers on the metamorphoses
of Echinoderms.[304] Work on the taxonomy and anatomy of marine animals
was in general in full swing by the 'fifties and 'sixties.

This return to Nature and to the sea had a very beneficial effect upon
morphology, bringing it out from the laboratory to the open air and the
seashore. It saved morphology from formalism and aridity, and in
particular from a certain narrowness of outlook born of too close
attention paid to the details of microscopical anatomy. It brought
morphologists face to face again with the wonderful diversity of organic
forms, with the unity of plan underlying that diversity, with the
admirable adjustment of organ to function and of both to the life of the
whole.

Milne-Edwards' theoretical views, as expounded in his _Introduction a la
zoologie generale_ (1851), well reflect this Cuvierian attitude.[305] He
acknowledges himself the debt he owes to Cuvier; "the further I advance
in the study of the sciences which he cultivated with so sure a hand,"
he writes in 1867, "the more I venerate him."

Milne-Edwards frankly takes up the teleological standpoint, and
interprets organic forms on the assumption that they are purposive and
rationally constructed. "To arrive at an understanding of the harmony of
the organic creation," he writes, "it seemed to me that it would be well
to accept the hypothesis that Nature has gone about her work as we would
do ourselves according to the light of our own intelligence, if it were
given us to produce a similar result. Comparing and studying living
things as if they were machines created by the industry of man, I have
tried to grasp the manner in which they might have been invented, and
the principles whose application would have led to the production of
such an assemblage of diversified instruments" (p. 435). The problem is
to discover the laws which rule the diversity of organic forms. The
first and most obvious of these laws is the "law of economy," or the law
of unity of type. Nature, as Cuvier pointed out, has not had recourse to
all the possible forms and combinations of organs; she appears to work
with a limited number of types and to get the greatest possible
diversity out of these by varying the proportions of the constitutive
materials of structure. Within the limits of each type Nature has
brought about diversity by raising her creatures to different degrees of
perfection. This is the second law of organic form, and it is this law
that Milne-Edwards chiefly elaborates. Degrees of perfection mean for
him, as for Aristotle, primarily degrees of perfection of function, but
since structure is necessarily in close relation with function,
perfection of function brings in its train increased perfection of
organisation. This can only be attained by a division of labour[306] among
the organs and by their consequent differentiation. An animal is like a
workshop where some complicated product is manufactured, and the organs
are like the workmen. Each workman has his own special piece of work to
do, at which he becomes thoroughly expert; and the finished product is
manufactured more rapidly and efficiently by the co-operation of workers
each skilled in one department than it would be if each workman had to
produce the whole. Applied to the organism this principle of the
division of labour means the differentiating out of the separate
functions, their localisation in different parts of the organism, and
their co-ordination to produce a combined result.

This differentiation of functions implies a corresponding
differentiation of organs, but it is functional differentiation which
always takes the lead. "Where division of labour has not been introduced
into the organism there must exist a great simplicity of structure. But
just as uniformity in the functions of the different parts of the body
implies a uniformity in their mode of constitution, so diversity in
function must be accompanied by particularities in structure; and, in
consequence also, the number of dissimilar parts must be augmented and
the complication of the machine increased" (p. 463). Since function
comes before form there is not always a special organ for every
function. "It is a grave error to believe that a particular function can
be performed only by one and the same organ. Nature can arrive at the
desired result by various ways, and when we look down through the animal
kingdom from the highest to the lowest forms we see that the function
does not disappear even when the special instrument provided for the
purpose in the higher types ceases to exist" (p 470).

Nature, holding fast to the law of economy, does not even always create
a new organ for a new function; she may simply adapt an undifferentiated
part to special functions, or she may even convert to other uses an
organ already specialised (p. 464). So, for example, the function of
respiration is in the lowest animals diffused indifferently over the
whole surface of the body, and only as organisation advances is it
localised in special organs, such as gills. Now suppose that Nature
wishes to adapt a fish, which breathes by gills, to life in the air; she
does not create an organ specially for this purpose, but utilises the
moist gill-chamber (_e.g._, in _Anabas scandens_), modifying it in
certain ways so that the fish can take advantage of the oxygen it
contains. But this gill-chamber lung is at best a makeshift, and when
she comes to the more definitely terrestrial Amphibia Nature gives up
the attempt to use the gill-chamber as a lung, and creates a new organ,
the true vertebrate lung, specially adapted for breathing air (p. 475).

But whatever means Nature adopts, her aim is always the same--to
specialise, to differentiate, to produce diversity from uniformity.

Differentiation not only raises the level of organisation; it usually
also takes the direction of adaptation to particular habits of life, and
this is perhaps the most fruitful cause of diversity. Everywhere we find
animals specialised in adaptation to their environment--to life in air
or water, or on land--and many of their most striking differences are
due to this cause. But adaptation may also act in reducing diversity,
for there necessarily occur many instances of parallel adaptation or
convergence. So we get the extraordinary parallelism between the
families of marsupials and the orders of placentals,[307] the remarkable
similarity between the respiratory organs of land-crabs and
air-breathing fish--to mention only two out of an immense range of
analogous facts.

The last cause of diversity that Milne-Edwards adduces is what he calls
a "borrowing" of peculiarities of structure from another systematic
group. Thus, "among reptiles, the tortoises seem to have borrowed from
birds some of their characteristic features of organisation; and among
the sauroid fishes the piscine type seems to have been influenced by the
type from which reptiles are derived" (p. 479). So many riddles that, a
little later on, stimulated the ingenuity of the evolutionists!

Such, then, were the factors which Milne-Edwards considered adequate to
explain the rich variety of animal forms. We cannot do better than quote
his own summary of his doctrine:--"To sum up, then, the great
differences introduced by Nature into the constitution of animals seem
to depend essentially upon the existence of a certain number of general
plans or distinct types, upon the perfecting in various degrees either
of the whole or of parts of each of these structural plans, upon the
adaptation of each type to varied conditions of existence, and upon the
secondary imitation of foreign types by certain derivatives of each
particular type" (p. 480).

We have laid stress on the fact that Milne-Edwards put function before
form, for this is the mark of the true Cuvierian. With it goes the
belief that Nature forms new parts to meet new requirements, that she is
not limited, as Geoffroy thought, to a definite number of "materials of
organisation," but can produce others at need. Cuvier held, for example,
that many of the muscles and even the bones of fish were peculiar to
them, and without homologues in the other Vertebrates, having been
created by Nature for special ends.[308] So, too, Johannes Mueller, who in
many ways and not least in his sane vitalism was a follower of the
Cuvierian tradition, recognised that many of the complicated cartilages
in the skull of Cyclostomes were specially formed for the important
function of sucking, and had no equivalent in other fish.[309]

So, too, the embryologists after Cuvier often came across instances of
the special formation of parts to meet temporary needs. Thus Reichert
interpreted the "palatine" and "pterygoid," which are formed in the
mouth of the newt larva by a fusion of conical teeth, as special
adaptations to enable the little larva to lead a carnivorous life.[310]

Not many years after the publication of Milne-Edwards' _Introduction a
la zoologie generale_ (1851) there appeared a book by H. G. Bronn in
which was offered a very similar analysis of organic diversity. The
curious thing was that Bronn approached the problem from quite a
different standpoint, from the standpoint, indeed, of
_Naturphilosophie_. Of this the title of the book is itself sufficient
proof--_Morphologische Studien ueber die Gestaltungs-gesetze der
Naturkoerper ueberhaupt und der organischen insbesondere_ (Leipzig and
Heidelberg, 1858).[311] The linking up of organic with inorganic form is
characteristic; there is much talk, too, in the book of _Urstoffe_ and
_Urkraefte_, but underlying the _Naturphilosophie_ we can trace the same
Cuvierian treatment of form, and see crystallise out laws of progressive
development that bear no small analogy with the laws established by
Milne-Edwards.

According to Bronn, the ideal fundamental form of the plant is an ovoid
or strobiloid[312] body, for a plant reaches out in two directions in
search of food--towards the sun and towards the earth. Animals differ
from plants in being endowed with sensation and mobility (_cf._
Aristotle and Cuvier), and it is this characteristic that gives them
their distinctive form. The main types of animal form--the Amorphozoa,
Actinozoa, and Hemisphenozoa--are essentially adaptations to particular
modes of locomotion. Animals either are fixed, or they move in all
directions without reference to any definite axis, or they move in one
main direction.

The Amorphozoa or shapeless animals include many of the Protozoa and
sponges; they have no typical form, and most of them are sessile. The
Actinozoa include such animals as the Coelentera, which are fixed, and
the Echinoderms, which have a central point and move indifferently along
any radial axis; their form differs from the strobiloid mainly in having
radiate rather than spiral symmetry. The Hemisphenozoa, or bilaterally
symmetrical animals, include all those that habitually move forward;
they have a front end and a hind end, a dorsal surface and a ventral,
and the mouth, sense-organs and "brain" are concentrated in the front
end to form a head--all in direct adaptation to this forward movement;
they make up the vast majority of animals.

The fundamental forms of living things are, however, merely so many
themes on which a multitude of further variations are woven, through the
action of the laws which rule the detail of organic diversities. These
further laws may be set down under four main heads. Under the first
comes the law of the existence of certain fundamentally distinct
structural types, which are distinguished from one another by their
ground-form, by the number of organ-systems, and by the number of
homotypic organs they possess, but principally by the relative position
of the organs to one another (principle of connections). The form and
connections of the nervous system are of particular importance in
distinguishing the types (_cf._ Cuvier). The second factor in the
diversity of organic form is the action of certain laws of progressive
development[313] (_Entwickelungsgesetze_), which bear the same relation to
the development of the animal kingdom as the laws of individual
development bear to the development of the embryo, for organs appear in
the different animal series in much the same order and manner as they
develop in the individual. These laws are (1) progressive
differentiation of functions and organs; (2) numerical reduction of
serially repeated parts; (3) concentration of functions and their organs
in particular parts of the body; (4) centralisation of organ-systems and
parts of such, so that they come to depend upon one central organ; (5)
internalisation of the "noblest" organs, unless these are necessarily
external, and (6) increase in size of the whole or of parts. Of these
the law of differentiation is by far the most important, and most of the
others are in a sense merely special cases of this fundamental law. To
this law of differentiation is due the increase in complexity or
perfection of organisation which is shown by all the animal series.
Bronn himself recognised the great similarity of this law of progressive
differentiation to Milne-Edwards' principle of the division of labour;
he seems, however, to have arrived at it independently.

Bronn's third factor in the production of variety of form is adaptation
to environment, or better, functional response to environment. Bronn
gives an excellent account of adaptational modifications and calls
attention, just as Milne-Edwards did, to the numerous analogies of
structure which adaptation brings about. He works out the interesting
view that there is some connection between classificatory groups and
adaptational forms, especially such as are connected with the function
of locomotion:--"Based upon a common characteristic method of locomotion
are whole or nearly whole sub-phyla (Hexapoda), classes (mammals and
reptiles, birds, fishes, gastropods, pteropods, brachiopods, Bryozoa,
Rotifera, jelly-fish, polypes, sponges), sub-classes (mobile and
immobile lamellibranchs, echinoderms, walking and swimming Crustacea,
parasitic and free-living worms, and so on), often, however, only orders
and quite small groups (snakes, eels, bats, sepias, medusae, etc.)" (p.
141).

It was characteristic of the 'forties and 'fifties that transcendental
anatomy, along with Nature-philosophy, went rather out of fashion, its
false simplicities and premature generalisations being overwhelmed by
the flood of new discoveries. A few stalwarts indeed upheld
transcendental views. We have already discussed the morphological system
built up by Richard Owen in the late 'forties, a system transcendental
in its main lines. We have seen the vertebral theory of the skull still
maintained in the 'fifties by such men as Reichert and Koelliker, and we
find J. V. Carus in 1853[314] taking it as almost conclusively proved.[315]

We may mention, too, as showing clear marks of the influence of
transcendental ideas, L. Agassiz's work on the principles of
classification.[316] And Serres, who was Geoffroy's chief disciple,
recanted not a whit of his doctrine of recapitulation, but re-affirmed
and expanded it from time to time, and particularly in a lengthy memoir
published in 1860.[317] But in general we may say that pure morphology in
the Geoffroyan or Okenian sense was becoming gradually discredited. A
curious indication of this is seen in the fact that not only the idea
but the very word "Archetype" came to be regarded with suspicion. Thus
even J. V. Carus, who had much affinity with the transcendentalists,
wrote of the vertebrate archetype (which he took over almost bodily from
Owen)--"It may here be observed that this schema may be used as a
methodological help, but it is not to be placed in the foreground"
(_loc. cit._, p. 395). Huxley, who was definitely a follower of von
Baer, was much more outspoken with regard to ideal types. In an
important memoir on the general anatomy of the Gastropoda and
Cephalopoda,[318] he set himself the task of reducing all their complex
forms to one type. In summing up, he writes:--"From all that has been
stated, I think that it is now possible to form a notion of the
archetype of the Cephalous Mollusca, and I beg it to be understood that
in using this term, I make no reference to any real or imaginary 'ideas'
upon which animal forms are modelled. All that I mean is the conception
of a form embodying the most general propositions that can be affirmed
respecting the Cephalous Mollusca, standing in the same relation to them
as the diagram to a geometrical theorem, and like it, at once imaginary
and true" (i., p. 176). Again, in his Croonian lecture on the theory of
the vertebrate skull, he remarks that a general diagram of the skull
could easily be given. "There is no harm," he continues, "in calling
such a convenient diagram the 'Archetype' of the skull, but I prefer to
avoid a word whose connotation is so fundamentally opposed to the spirit
of modern science" (_Sci. Memoirs_, vol. i., p. 571).

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