THE CLOSE OF THE PRE-EVOLUTIONARY PERIOD
The influence of the cell-theory on morphology was not altogether happy.
The cell-theory was from the first physiological; cells were looked upon
as centres of force rather than elements of form, and the explanation of
all the activities of the organism was sought in the action of these
separate dynamic centres. There resulted a certain loss of feeling for
the problems of form. The organism was seen no longer as a cunningly
constructed complex of organs, tissues and cells; it had become a mere
cell-aggregate; the higher elements of form were disregarded and
ignored.
We have seen this physiological attitude expressed with the utmost
clearness by the founder of the cell-theory himself; we shall see the
same attitude taken up by most of his successors. Thus Vogt, who was
later to become one of the protagonists of materialism in Germany,
developed in his memoir on the embryology of _Coregonus_[293] the theory
of the independent or individual life of the cell. "Each cell," he
wrote, "represents in some measure a separate organism, and while their
development necessarily conforms to the general plan and the particular
tendencies of the parent organism, they nevertheless each follow their
own particular tendency and do not lose their independence until, by
reason of the metamorphoses which they undergo, they lose their cellular
nature" (p. 275).
And again, "... we are obliged to admit the existence in the cell of an
independent life, which makes its development self-sufficient.... Each
cell consequently represents a little independent organism, which
assimilates foreign substances, builds them up, and rejects those that
are useless; from this point of view the embryo can be compared up to a
certain point with a zoophyte stock, of which each polyp, while living
its own independent life, is yet incorporated in the common corm, which
impresses its distinctive character upon every polyp" (p. 293).
Classical expression was given to the "colonial theory" of the organism
by Virchow in his lectures on "Cellular Pathology."[294] For Virchow the
organism resolves itself into an assemblage of living centres, the
cells; the organism has no real existence as a unity, for there is no
one single centre from which its activities are ruled. Even the nervous
system, which appears to act as a co-ordinating centre, is itself an
aggregate of discrete cells. "A tree is a body of definite and orderly
composition, the ultimate elements of which, in every part of it, in
leaf and root, in stem and flower, are cellular elements--so also are
animal forms. _Every animal is a sum of vital units_, each of which
possesses the full characteristics of life. The character and the unity
of life cannot be found in one definite point of a higher organisation,
for example in the brain of man, but only in the definite, constantly
recurring disposition shown individually by each single element. It
follows that the composition of the major organism, the so-called
individual, must be likened to a kind of social arrangement or society,
in which a number of separate existences are dependent upon one another,
in such a way, however, that each element possesses its own particular
activity, and, although receiving the stimulus to activity from the
other elements, carries out its own task by its own powers" (2nd ed.,
pp. 12-13).
Analysis, decomposition, or disintegration of the organism is here
pushed to its extreme point, and the problem of recomposition, synthesis
and co-ordination shirked or forgotten.
The harmful influence of the cell-theory upon morphology did not pass
unnoticed by the broader-minded zoologists of the day. Virchow's earlier
paper[295] on the application of the cell-theory to physiology and
pathology called forth a vigorous protest from Reichert,[296] who
discussed in a very instructive way the contrast between the older
"systematic" and the newer "atomistic" attitude to living Nature.
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).
It is instructive to find that between Serres and Milne-Edwards there
existed the same antagonism as between von Baer and the German
transcendentalists. Milne-Edwards was a constant critic of the law of
parallelism which Serres continued to uphold with little modification
for over thirty years, just as von Baer was a critic of that form of the
doctrine which was current in the early part of the century. As early as
1833, Milne-Edwards, through his studies of crustacean development,[319]
had come to the conclusion, independently of von Baer, that development
always proceeded from the general to the special; that class characters
appeared before family characters, generic characters before specific.
In an interesting paper published in 1844,[320] he discussed the relation
of this law of development to the problems of classification, and
arrived at results almost identical with those set forth by von Baer in
his Fifth Scholion.
Like von Baer he rejected completely the theory of parallelism and the
doctrine of the scale of beings; like von Baer he held that the type of
organisation--of which there are several--is manifested in the very
earliest stages and becomes increasingly specialised throughout the
course of further development; like von Baer, too, he sketched a
classification based upon embryological characters.
These views were further developed in his volume of 1851, and also in
his _Rapport_ of 1867.
They brought him into conflict with his confrere in the Academy of
Sciences, Etienne Serres, who in a number of papers published in the
'thirties and 'forties,[321] and particularly in his comprehensive memoir
of 1860, still maintained the theory of parallelism and the doctrine of
the absolute unity of type. His memoir of 1860 shows how completely
Serres was under the domination of transcendental ideas. Much of it
indeed goes back to Oken. "The animal kingdom," he writes, "may be
considered in its entirety as a single ideal and complex being" (p.
141). His views have become a little more complicated since his first
exposition of them in 1827, and he has been forced to modify in some
respects the rigour of his doctrine. But he still holds fast to the main
thesis of transcendentalism--the absolute unity of plan of all animals,
vertebrate and invertebrate alike,[322] the gradual perfecting of
organisation from monad to man, the repetition in the embryogeny of the
higher animals of the "zoogeny" of the lower.
He recognised, however, that the idea of a simple scale of beings is
only an abstraction, and that the true repetition is of organs rather
than of organisms. He was willing even to admit, at least in the later
pages of his memoir, that there might be not one animal series but
several parallel series, as had been suggested by Isidore Geoffroy St
Hilaire (p. 749). In general, his views are now less dogmatic than they
were in his earlier writings, but they are not for all that changed in
any essential. For, in summing up his main results, he writes, "The
whole animal kingdom can in some measure be regarded ideally as a single
animal, which, in the course of formation and metamorphosis in its
diverse manifestations, here and there arrests its own development, and
thus determines at each point of interruption, by the very state it has
reached, the distinctive characters of the phyla, the classes, families,
genera, and species" (p. 833).[323]
To settle the dispute pending between two of its most illustrious
members, the Academy proposed in 1853, as the subject of one of its
prizes, "the positive determination of the resemblances and differences
in the comparative development of Vertebrates and Invertebrates." A
memoir was presented the next year by Lereboullet[324] which met with the
approval of the Academy in so far as its statements of fact were
concerned, but seemed to them to require amplification in its
theoretical part. But even in this memoir Lereboullet was able to show
that the balance of evidence was greatly in favour of Milne-Edwards'
views, and his general conclusions in 1854 were that "in the presence of
such fundamental differences, one is obliged to give up the idea of one
single plan in the formation of animals; while, on the contrary, the
existence of diverse plans or types is clearly demonstrated by all the
facts" (p. 79). To fulfil the Academy's requirements, Lereboullet
continued his work, and in 1861-63 he published a series of elaborate
monographs[325] on the embryology of the trout, the lizard and the
pond-snail _Lymnaea_, and rounded off his work with a full discussion[326]
of the theoretical questions involved. In this considered and
authoritative judgment he completely disposed of Serres' theories of the
unity of plan and the unity of genetic formation. Except in the very
earliest stages of oogenesis there is no real similarity between the
development of a Zoophyte, a Mollusc, an Articulate and a Vertebrate,
but each is stamped from the beginning with the characteristics of its
type. The lower animals are not, and cannot possibly be the permanent
embryos of the higher animals. "The results which I have obtained," he
writes, "are diametrically opposed to the theory of the zoological
series constituted by stages of increasing perfection, a theory which
tries to demonstrate in the embryonic phases of the higher animals a
repetition of the forms which characterise the lower animals, and which
has led to the assertion that the latter are permanent embryos of the
former. The embryo of a Vertebrate shows the vertebrate type from the
very beginning, and retains this type throughout the whole course of its
development; it never is, and never can be, either a Mollusc or an
Articulate" (xx., p. 54).
"We are led to establish ... as the general result of our researches,
the existence of several types, and, consequently, of different plans,
in the development of animals. These different types are manifested from
the very beginning of embryonic life; the characters distinguishing them
are therefore primordial, and we can say with M. Milne-Edwards that
_everything goes to prove that the distinction established by Nature
between animals belonging to different phyla is a primordial
distinction_" (p. 58).
In other directions also von Baer's work was confirmed and extended by
later observers--those parts of it particularly that had reference to
the germ-layer theory, and to the concept of histological
differentiation. His germ-layer theory was accepted in its main lines by
Rathke, Bischoff and Lereboullet, and applied by them to the multitude
of new facts they discovered. Rathke, in particular, was a firm upholder
of the doctrine, and made considerable use of it in his writings.[327]
Even before the publication of von Baer's book he had interpreted in
terms of the germ-layer theory sketched by his friend Pander the
splitting of the blastoderm which occurs in the early development of
_Astacus_, whereby there are formed a serous and a mucous layer, one
inside the other--like the coats of an onion, to use his own expressive
phrase.[328]
An ingenious application of the Pander-Baer theory was made by Huxley,
who compared the outer and inner cell-layers which form the groundwork
of the Coelentera with the serous and mucous layers of the vertebrate
germ.[329] He laid stress, it is true, rather on the physiological than on
the morphological resemblance. "A complete identity of structure," he
writes, "connects the 'foundation membranes' of the Medusae with the
corresponding organs in the rest of the series; and it is curious to
remark, that throughout, the outer and inner membranes appear to bear
the same physiological relation to one another as do the serous and
mucous layers of the germ; the outer becoming developed into the
muscular system, and giving rise to the organs of offence and defence;
the inner, on the other hand, appearing to be more closely subservient
to the purposes of nutrition and generation" (p. 24). Von Baer had
already hinted at this homology in the second volume of his
_Entwickelungsgeschichte_ (1837), where he says with reference to the
separation of the blastoderm of the chick into two layers. "Yet
originally there are not two distinct or even separable layers, it is
rather the two surfaces of the germ which show this differentiation,
just as polyps show the same contrast of an external surface and an
internal digestive surface. In between the two layers there is in our
germ as in the polyp an indifferent mass" (p. 67). The terms ectoderm
and entoderm were introduced by Allman[330] in 1853 for the two
cell-layers in the Hydrozoa.
Remak is the second great name in the history of the germ-layer theory.
He had the great advantage over von Baer of being able to make use of
the cell-theory in interpreting the formation of the germ-layers.
Microscopical technique also had been greatly improved since 1828.[331]
Remak's greatest service was that he put the germ-layer theory in direct
relation with the cell-theory by demonstrating the cellular continuity
from egg-cell to tissue, and by showing that each germ-layer possessed
distinctive histological characteristics. Hardly less important was his
clear marking-off of the "middle layer" as a separate and distinct layer
of the germ. He it was who introduced the modern conception of the
mesoderm, and cleared up the confusion in which Pander and von Baer had
left the organs formed between the serous and the mucous layer. Remak's
middle layer was a different thing from Pander's ill-defined
"vessel-layer"; it included and unified from a new point of view the
"vessel" and "muscle" layers of von Baer.
There are in the unincubated blastoderm of the chick, according to
Remak,[332] two cell-layers, of which the undermost subsequently splits
into two. Three layers are thus formed--the upper, middle and lower. The
upper layer differentiates into a medullary plate and an epidermic plate
(Remak's _Hornblatt_), and gives origin to the medullary tube with all
its evaginations, and to the skin with all its derivatives and pockets.
It forms such diverse structures as the brain, the spinal cord, the eye,
the ear, the mouth, hairs, feathers, nails, sweat-glands, lacrymal
glands, and so forth. All these parts are connected directly or
indirectly with sensation, and the upper germ-layer may accordingly be
called the _sensory_ layer. The lower layer gives rise to the epithelium
and the proper tissue of the alimentary canal and its derivatives, as
the liver, lungs, pancreas, kidneys, thyroid, thymus, etc. These parts
are all concerned in the processes of assimilation and dissimilation,
and the lower layer may accordingly be called the _trophic_ layer. Now
between the upper or sensory layer and the lower or trophic layer there
exists, in spite of their very different functions, a close histological
likeness, for both are essentially epithelial layers. The resemblance is
particularly strong if we compare the lower layer with the _Hornblatt_
of the upper layer--both consist of epithelial tissue, and of its
derivative, glandular tissue, and form neither vessels nor nerves. The
middle layer, on the contrary, forms nerves and muscles, vessels and
connective tissue, and little or no epithelium. It does not form all the
blood-vessels without exception (and so cannot be called the
vessel-layer), for the blood-vessels of the central nervous system are
in all probability formed from the upper layer. So, too, it does not
form all the nerves and muscles--the optic and auditory nerves and the
nerves and muscles of the iris probably arise in the upper layer. But,
in spite of these exceptions, its general histological character is so
well defined that it may be contrasted with the other two as
preeminently the layer that forms muscular, nervous, vascular and
connective tissue. In view of its functional significance, it may be
called the _motory_ layer, or better, since it forms also the sexual
glands, the _motor-germinative_ layer. The middle layer, early in its
history, shows a division into dorsal plates (_Urwirbelplatten_) and
ventral plates (_Seitenplatten_). The former exhibit almost as soon as
they are formed the characteristic proto-vertebral segmentation, the
latter split to form the pleuro-peritoneal or body-cavity. Remak
describes the latter process as follows:--"In the region of the trunk,
where a greater independence of the fate of the alimentary canal and its
annexes becomes necessary for the voluntary executive organs, the
ventral plates undergo a process of splitting, leading to the formation
of the sensitive part of the integument (the _Hautplatten_), the
muscular part of the alimentary tube (the _Darmfaserplatten_), and the
mother-tissue of the generative organs (the _Mittelplatten_). From the
_Hautplatten_ there develops, without the dorsal plates seeming to take
any part in the process, the rudiment of the extremities" (p. 79).
[Illustration: FIG. 12.--Transverse Section of Chick Embryo. (After
Remak.)]
His _Darmfaserplatten_ form the nervous and muscular tissue of the
alimentary canal and its dependencies, and also the heart; the
_Hautplatten_ form the general body-wall (exclusive of the skin) and the
appendages. In the embryo they line the amniotic cavity. The skeleton
and peripheral nerves originate wholly within the middle layer.
Remak's conception of the relations of the three germ-layers to one
another and to the body-cavity is well illustrated in Fig. 12.
In his germ-layer theory Remak's standpoint is histological rather than
morphological. The distinction which he draws between the sensory and
trophic layers on the one hand, and the motor-germinative layer on the
other, is entirely a histological one. The greater part of his book,
indeed, is devoted to a study of the histogenesis of the different
organs of the body; he is bent chiefly upon unravelling the part which
each germ-layer takes in the formation of each tissue and organ.
His generalisation that two of the germ-layers give rise exclusively or
almost exclusively to one kind of tissue excited great interest at the
time, and gave the direction to histogenetic research for quite a number
of years, though in the end it turned out to be insufficiently founded.
Though Remak's germ-layer theory had thus principally a histological
orientation, it laid down the main lines of the modern morphological
treatment of the germ-layers.
[293] _Embryologie des Salmones_, 1842.
[294] _Die Cellularpathologie in ihrer Begruendung auf
physiologische und pathologische Gewebelehre_, Berlin,
2nd ed. 1859; Eng. trans., by Chance, 1860.
[295] _Arch. path. Anat. Phys_., vii., pp. 1-39 (1854).
[296] _Bericht ueber die Fortschritte der mikroskopischen
Anatomie im jahre 1854._ Mueller's _Archiv_, 1855. See
also 1856.
[297] _Hndb. d. Physiol._, i., 1835.
[298] See Leuckart's reply to Ludwig's criticism, in
_Zeit. f. wiss. Zool._, ii., p. 271, 1850.
[299] Leipzig, 1853.
[300] _Souvenirs d'un Naturaliste_, 2 vols., Paris, 1854.
Eng. Trans. as _Rambles of a Naturalist on the Coasts of
France, Spain, and Italy_, 2 vols., 1857.
[301] Milne-Edwards later published a classical textbook
on comparative anatomy and physiology--_Lecons sur la
Physiologie et l'Anatomie comparees_, 14 vols., Paris,
1857-80.
[302] Paris, 1834-40. Three volumes of the _Suites a
Buffon_.
[303] Paris, 1865. Two volumes of the _Suites a Buffon_.
[304] _U. d. Metamorphose der Ophiuren u. Seeigel._,
Berlin, 1848. _U. d. Metamorphose der Holothurien u.
Asterien._, Berlin, 1851.
[305] As I have been unable to obtain a copy of the
_Introduction_, the passages which follow are taken from
the _Rapport_ of 1867, where Milne-Edwards gives a
complete exposition of his doctrine, sometimes in the
words of the original.
[306] This principle was first developed by Milne-Edwards
in 1827, in the _Dictionnaire classique d'Hist.
naturelle_. It was probably suggested to him by his
studies on the Crustacea, among which the principle is
so beautifully exemplified in the concentration and
specialisation of the appendages and the ganglionic
chain.
[307] Studied by Isidore Geoffroy St Hilaire in his paper
_Classification parallelique des Mammiferes, C. R. Acad.
Sci._, xx., 1845. Remarked upon by Cuvier, _Regne
animal_., i., p. 171, 1817, also by de Blainville.
[308] Cuvier et Valenciennes, _Hist. nat. des Poissons_,
i., p. 550, 1828.
[309] _Myxinoiden_, Th. I. _Abh. k. Akad. Wiss. Berlin_
for 1834, pp. 100, 110, 179, etc.
[310] _Vergl. Entw. Kopf. nackt. Amphibien_, p. 101, 1838.
[311] I have not seen the companion volume on
palaeontological progression, _Unters. ue. d.
Entwickelungsgesetze der organischen Welt waehrend der
Bildungszeit unserer Erdoberflaeche_, Stuttgart, 1858.
[312] "Strobiloid" because of its spiral development. The
theory of the spiral growth of plants played an
important part in botanical morphology about this time.
[313] _Cf._ Meckel's Principle of progressive Evolution,
_supra_, p. 93.
[314] _System der thierischen Morphologie_, pp. 33, 457.
Also C. Bruch, _Die Wirbeltheorie des Schaedels, am
Skelette des Lachses geprueft_, Frankfort-on-Main, 1862.
[315] In France the vertebral theory was advocated by
Lavocat in his _Nouvelle Osteologie comparee de la tete
des animaux domestiques_, Toulouse, 1864. It seems also
that Lacaze-Duthiers held fast to it even in
1872--_Arch. zool. exp. gen._, i., p. 51, 1872.
[316] _An Essay on Classification_, Boston, 1857, London,
1859. He considered the classificatory categories to be
the categories of the Creator's thought, and hence
natural, and in no sense mere conventions.
[317] "Principes d'Embryogenie, de Zoogenie et de
Teratogenie," _Mem. Acad. Sci._, xxv., pp. 1-943, pls.
xxv., 1860.
[318] "On the Morphology of the Cephalous Mollusca,"
_Phil. Trans._, 1853, _Sci. Memoirs_, i., pp. 152-92.
[319] "Observations sur les changements de forme que les
divers Crustaces eprouvent," _Ann. Sci. nat._ (1) xxx.,
p. 360, 1833.
[320] "Considerations sur quelques principes relatifs a la
classification naturelle des animaux," _Ann. Sci. nat._
(3) i., p. 65, 1844.
[321] _Supra_, pp. 79-83. Also _Precis d'anatomie
transcendante, principes d'organogenie_, Paris, 1842.
[322] The inversion of the organs shown by Vertebrates as
compared with Invertebrates is due to the reversed
position of the embryo relatively to the yolk! (pp.
821-6).
[323] It is worth while recording that Serres enunciated a
"law of symmetry" according to which the embryo is
formed by the union of its two symmetrical halves--a law
which recalls the "concrescence theory" of His and some
modern embryologists.
[324] "Embryologie comparee du Brochet, de la Perche, et
de l'Ecrevisse," _Ann. Sci. nat._ (4), i., p. 237, 1854;
ii., p. 39, 1854. _Mem. Savans etrangers_, xvii.
[325] _Ann. Sci. nat._ (4) xvi., p. 113, 1861; xvii., p.
88, 1862; xviii., p. 5, 1862; xix., p. 5, 1863.
[326] xx., p. 5, 1863.
[327] Particularly in his _Blennius_ (1833) and _Natter_
(1839).
[328] In the "preliminary notice" of his Crayfish
paper--_Isis_, pp 1093-1100, 1825.
[329] "On the Anatomy and the Affinities of the Family of
the Medusae," _Phil. Trans._, 1849; _Sci. Memoirs_, i.,
pp. 9-32.
[330] _Phil. Trans._, cxliii., p. 368, 1853.
[331] The principle of achromatism was discovered (by
Fraunhofer) and achromatic microscopes introduced in the
early part of the 19th century. The use of chemical
reagents, such as acetic acid, and various hardening
fluids, came into fashion not long after. J. Mueller
seems to have been one of the first to realise their
importance. Remak himself invented one or two fixing and
hardening mixtures (pp. 87, 127, 1855), which enabled
him to cut excellent hand sections. Section-cutting
machines were not invented till later (V. Hensen, 1866,
His, 1870).
[332] _Untersuchungen ueber die Entwickelung der
Wirbelthiere_, folio, pp. xxxvii + 195, 12 plates,
Berlin, 1850-1855.