ERNST HAECKEL AND CARL GEGENBAUR
At the time when Darwin's work appeared there already existed, as we
have seen, a fully formed morphology with set and definite principles.
The aim of this pre-evolutionary morphology had been to discover and
work out in detail the unity of plan underlying the diversity of forms,
to disentangle the constant in animal form and distinguish from it the
accessory and adaptive. The main principle upon which this work was
based was the principle of connections, so clearly stated by Geoffroy.
The principle of connections served as a guide in the search for the
archetype, and this search was prosecuted in two directions--first, by
the comparison of adult structure; and second, by the comparative study
of developing embryos. It was found that the archetype was shown most
clearly by the early embryo, and this embryological archetype came to be
preferred before the archetype of comparative anatomy. It became
apparent also that the parts first formed (germ-layers) were of primary
importance for the establishing of homologies.
While practically all morphologists were agreed as to the main
principles of their science, they yet showed, as regards their general
attitude to the problems of form, a fairly definite division into two
groups, of which one laid stress upon the intimate relation existing
between form and function, while the other disregarded function
completely, and sought to build up a "pure" or abstract morphology. In
opposition to both groups, in opposition really to morphology
altogether, a movement had gained strength which tended towards the
analysis and disintegration of the organism. This movement took its
origin in the current materialism of the day, and found expression
particularly in the cell-theory and in materialistic physiology.
The separation between morphology as the science of form and physiology
as the science of the physics and chemistry of the living body had by
Darwin's day become well-nigh absolute.
The morphology of the 'fifties lent itself readily to evolutionary
interpretation. Darwin found it easy to give a formal solution of all
the main problems which pre-evolutionary morphology had set--he was able
to interpret the natural system of classification as being in reality
genealogical, systematic relationship as being really
blood-relationship; he was able to interpret homology and analogy in
terms of heredity and adaptation; he was able to explain the unity of
plan by descent from a common ancestor, and for the concept of
"archetype" to substitute that of "ancestral form."
The current morphology, Darwin found, could be taken over, lock, stock
and barrel, to the evolutionary camp.
In what follows we shall see that the coming of evolution made
surprisingly little difference to morphology, that the same methods were
consciously or unconsciously followed, the same mental attitudes taken
up, after as before the publication of the _Origin of Species_.
Darwin himself was not a professional morphologist; the conversion of
morphology to evolutionary ideas was carried out principally by his
followers, Ernst Haeckel and Carl Gegenbaur in Germany, Huxley,
Lankester, and F. M. Balfour in England.
It was in 1866 that Haeckel's chief work appeared, a _General Morphology
of Organisms_,[366] which was intended by its author to bring all
morphology under the sway and domination of evolution.
It was a curious production, this first book of Haeckel's, and
representative not so much of Darwinian as of pre-Darwinian thought. It
was a medley of dogmatic materialism, idealistic morphology, and
evolution theory; its sources were, approximately, Buechner, Theodor
Schwann, Virchow, H. G. Bronn, and, of course, Charles Darwin.
It was scarcely modern even on its first appearance, and many regarded
it, not without reason, as a belated offshoot of _Naturphilosophie_.
Its materialism is of the most intransigent character. The form and
activities of living things are held to be merely the mechanical result
of the physical and chemical composition of their bodies. The simplest
living things, the Monera, are nothing more than homogeneous masses of
protein substance. "They live, but without organs of life; all the
phenomena of their life, nutrition and reproduction, movement and
irritability, appear here as merely the immediate outcome of formless
organic matter, itself an albumen compound" (p. 63, 1906).
Teleology, the Achilles' heel of Kant's (otherwise sound!) philosophy,
is to be regarded as a totally refuted and antiquated doctrine,
definitely put out of court by Darwinism.
Haeckel works out his materialistic philosophy of living things very
much after the fashion of Schwann. There is the same talk of cells as
organic crystals, of crystal trees, of the analogy between assimilation
by the cell and the growth of crystals in a mother liquid. Heredity and
adaptation are shown equally as well by crystals as by organisms; for
heredity, or the internal _Bildungstrieb_ (!), is the mechanical effect
of the material structure of the crystal or the germ, and adaptation, or
the external _Bildungstrieb_, is a name for the modifications induced by
the environment. Adaptation so defined comes to be synonymous with the
fortuitous variation which plays so great a part in Darwin's theory of
natural selection.
It goes without saying that Haeckel allowed to the organism no other nor
higher individuality than belongs to the crystal, and took no account at
all of that harmonious interaction of the organs which Cuvier called the
principle of the "conditions of existence." The concept of correlation
had simply no meaning for Haeckel. The analysis and disintegration of
the organism was pushed by him to its logical extreme, and in this also
he was a child of his time.
A no less important influence clearly visible in the _General
Morphology_ is the idealistic morphology of men like K. G. Carus and H. G.
Bronn. In previous chapters we have seen how K. G. Carus attempted to
work out a geometry of the organism, and how Bronn tried in a modest way
to found a stereometrical morphology, but had the grace not to push his
stereometry _a l'outrance_, recognising very wisely that the greater
part of organic form is functionally determined. Haeckel took over this
idea[367] and pushed it to wild extremes, founding a new science of
"Promorphology" of which he was the greatest--and only--exponent.[368]
This "science" dealt with axes and planes, poles and angles, in a
veritable orgy of barbarous technical terms. It was intended to be a
"crystallography of the organic," and to lay the foundations of a
mechanistic morphology, or morphography at least.
How it was to be linked up with the physics and chemistry of living
matter on the one hand and with the ordinary morphology of real animals
on the other, was never made quite clear.
The science of Promorphology has no historical significance; it is
interesting only because it illustrates Haeckel's close affinity with
the idealistic morphologists.
Another abortive science of Haeckel's, the science of Tectology, was
equally a heritage from idealistic morphology. Tectology is the science
of the composition of organisms from individuals of different orders.
There were six orders of individuals:--(1) Plastids (Cytodes and cells);
(2) Organs (including cell-fusions, tissues, organs, organ-systems); (3)
Antimeres (homotypic parts, _i.e._, halves or rays); (4) Metameres
(homodynamic parts, _i.e._, segments); (5) Persons (individuals in the
ordinary sense); (6) Corms (colonial animals).
The thought is essentially transcendental, and recalls the "theory of
the repetition of parts," of which so much use was made by the German
transcendentalists, such as Goethe,[369] Oken, Meckel and K. G. Carus, as
well as by Duges.
The third, and naturally the most important, ingredient in the _General
Morphology_ was the doctrine of evolution, in the form given to it by
Darwin. We have here no concern with Haeckel's evolutionary philosophy,
with the way in which he combined his evolutionism and his materialism
to form a queer Monism of his own. We are interested only in the way he
applied evolution to morphology, what modifications he introduced into
the principles of the science, and in general in what way he interpreted
the facts and theories of morphology in the light of the new knowledge.
We find that he repeats very much what Darwin said, giving, of course,
more detail to the exposition, and elaborating, particularly in his
recapitulation theory or "biogenetic law," certain doctrines not
explicitly stated by Darwin.
Like Darwin he held that the natural system is in reality genealogical.
"There exists," he writes, "one single connected natural system of
organisms, and this single natural system is the expression of real
relations which actually exist between all organisms, alike those now in
being on the earth and those that have existed there in some past time.
The real relations which unite all living and extinct organisms in one
or other of the principal groups of the natural system, are
genealogical: their relationship in form is blood-relationship; the
natural system is accordingly the genealogical tree of organisms, or
their genealogema.... All organisms are in the last resort descendants
of autogenous Monera, evolved as a consequence of the divergence of
characters through natural selection. The different subordinate groups
of the natural system, the categories of the class, order, family,
genus, etc., are larger or smaller branches of the genealogical tree,
and the degree of their divergence indicates the degree of genealogical
affinity of the related organisms with one another and with the common
ancestral form" (ii., p. 420).
The degree of systematic relationship is thus the degree of genealogical
affinity. It follows that the natural system of classification may be
converted straightway into a genealogical tree, and this is actually
what Haeckel does in the _General Morphology_. The genealogical trees
depicted in the second volume (plates i.-viii.) are nothing more than
graphic representations of the ordinary systematic relationships of
organisms, with a few hypothetical ancestral groups or forms thrown in
to give the whole a genealogical turn.
If the genealogical tree is truly represented by the natural system, it
would seem that for each genus a single ancestral form must be
postulated, for each group of genera a single more primitive form, and
so in general for each of the higher classificatory categories, right up
to the phylum. Species of one genus must be descended from a generic
ancestral form, genera of one family from a single family _Urform_, and
so on for the higher categories.
This consequence was explicitly recognised by Haeckel. "Genera and
families," he writes, "as the next highest systematic grades, are
extinct species which have resolved themselves into a divergent bunch of
forms (_Formenbueschel_)" (ii., p. 420).
The archetype of the genus, family, order, class and phylum was thus
conceived to have had at some past time a real existence.
The natural system of classification is based upon a proper appreciation
of the distinction between homological and analogical characters.
Haeckel, following Darwin, naturally interprets the former as due to
inheritance, the latter as due to adaptation, using these words, we may
note, in their accepted meaning and not in the abstract empty sense he
had previously attributed to them.[370] Similarly the "type of
organisation," in von Baer's sense, was due to heredity, the "grade of
differentiation" to adaptation.
So far Haeckel merely emphasised what Darwin had already said in the
_Origin of Species_. But by his statement of the "biogenetic law," and
particularly by the clever use he made of it, Haeckel went a step beyond
Darwin, and exercised perhaps a more direct influence upon evolutionary
morphology than Darwin himself.
Haeckel was not the original discoverer of the law of recapitulation. It
happened that a few years before the publication of Haeckel's _General
Morphology_, a German doctor, Fritz Mueller by name, stationed in Brazil,
had been working on the development of Crustacea under the direct
inspiration of Darwin's theory, and had published in 1864 a book[371] in
which he showed that individual development gave a clue to ancestral
history.
He conceived that progressive evolution might take place in two
different ways. "Descendants ... reach a new goal, either by deviating
sooner or later whilst still on the way towards the form of their
parents, or by passing along this course without deviation, but then
instead of standing still advancing still farther" (Eng. trans., p.
111). In the former case the developmental history of descendants agrees
with that of the ancestors only up to a certain point and then diverges.
"In the second case the entire development of the progenitors is also
passed through by the descendants, and, therefore, so far as the
production of a species depends upon this second mode of progress, the
historical development of the species will be mirrored in its
developmental history" (p. 112).
Of course the recapitulation of ancestral history will be neither
literal nor extended. "The historical record preserved in developmental
history is gradually _effaced_ as the development strikes into a
constantly straighter course from the egg to the perfect animal, and it
is frequently _sophisticated_ by the struggle for existence which the
free-living larvae have to undergo" (p. 114).
It follows that "the primitive history of a species will be preserved in
its developmental history the more perfectly the longer the series of
young stages through which it passes by uniform steps; and the more
truly, the less the mode of life of the young departs from that of the
adults, and the less the peculiarities of the individual young states
can be conceived as transferred back from later ones in previous periods
of life, or as independently acquired" (p. 121).
Applying these principles to Crustacea, he concluded that the shrimp
_Peneus_ with its long direct development gave the best and truest
picture of the ancestral history of the Malacostraca, and that
accordingly the nauplius and the zoaea larvae represented important
ancestral stages. He conceived it possible so to link up the various
larval forms of Crustacea as to weave a picture of the primeval history
of the class, and he made a plucky attempt to work out the phylogeny of
the various groups.
The thought that development repeats evolution was already implicit in
the first edition of the _Origin_, but the credit for the first clear
and detailed exposition of it belongs to F. Mueller.
In much the same form as it was propounded by Mueller it was adopted by
Haeckel, and made the corner-stone of his evolutionary embryology.
Haeckel gave it more precise and more technical formulation, but added
nothing essentially new to the idea.
It is convenient to use his term for it--the biogenetic law
(_Biogenetische Grundgesetz_)--to distinguish it from the laws of
Meckel-Serres and von Baer, with which it is so often confused.
Haeckel's statement of it may best be summarised in his own words,
"Ontogeny, or the development of the organic individual, being the
series of form-changes which each individual organism traverses during
the whole time of its individual existence, is immediately conditioned
by phylogeny, or the development of the organic stock (phylon) to which
it belongs.
"Ontogeny is the short and rapid recapitulation of phylogeny,
conditioned by the physiological functions of heredity (reproduction)
and adaptation (nutrition). The organic individual (as a morphological
individual of the first to the sixth order) repeats during the rapid and
short course of its individual development the most important of the
form-changes which its ancestors traversed during the long and slow
course of their palaeontological evolution according to the laws of
heredity and adaptation.
"The complete and accurate repetition of phyletic by biontic development
is obliterated and abbreviated by secondary contraction, as ontogeny
strikes out for itself an ever straighter course; accordingly, the
repetition is the more complete the longer the series of young stages
successively passed through.
"The complete and, accurate repetition of phyletic by biontic
development is falsified and altered by secondary adaptation, in that
the bion[372] during its individual development adapts itself to new
conditions: accordingly the repetition is the more accurate the greater
the resemblance between the conditions of existence under which
respectively the bion and its ancestors developed" (ii., p. 300).
The last two propositions, it will be observed, are taken over almost
verbally from F. Mueller.
Now we have seen that the natural system of classification gives a true
picture of the genealogical relationships of organisms, that the smaller
and larger classificatory groups correspond to greater or lesser
branches of the genealogical tree. If ontogeny is a recapitulation of
phylogeny, we must expect to find the embryo repeating the organisation
first of the ancestor of the phylum, then of the ancestor of the class,
the order, the family and the genus to which it belongs. There must be a
threefold parallelism between the natural system, ontogeny and phylogeny
(ii., pp. 421-2).
It will be observed that there is here implied an analogy between the
biogenetic law and the law of von Baer, for both assert that development
proceeds from the general to the special, that the farther back in
development you go the more generalised do you find the structure of the
embryo; both assert, too, that differentiation of structure takes place
not in one progressive or regressive line, but in several diverging
directions.
But the analogy between the biogenetic law and the Meckel-Serres law is
even more obvious, and the resemblance between the two is much more
fundamental. It is a significant fact that in his theory of the
threefold parallelism Haeckel merely resuscitated in an evolutionary
form a doctrine widely discussed in the 'forties and 'fifties,[373] and
championed particularly by L. Agassiz,[374] a doctrine which must be
regarded as a development or expansion of the Meckel-Serres law.[375] It
is the view that a parallelism exists between the natural system,
embryonic development, and palaeontological succession. Actually, as
Agassiz stated it, the doctrine applied neither to types, nor as a
general rule to classes, but merely to orders. It was well exemplified,
he thought, in Crinoids:--"The successive stages of the embryonic growth
of Crinoids typify, as it were, the principal forms of Crinoids which
characterise the successive geological formations. First, it recalls the
Cistoids of the palaeozoic rocks, which are represented in its simple
spheroidal head; next the few-plated Platycrinoids of the Carboniferous
period; next the Pentacrinoids of the Lias and Oolite with their whorls
of cirrhi; and finally, when freed from its stem, it stands as the
highest Crinoid, as the prominent type of the family in the present
period" (p. 171).
The Meckel-Serres law, it will be remembered, expressed the idea that
the higher animals repeat in their ontogeny the adult organisation of
animals lower in the scale. Since Haeckel recognised clearly that a
linear arrangement of the animal kingdom was a mere perversion of
reality, and that a branching arrangement of groups more truly
represented the real relations of animals to one another, he could not
of course entertain the Meckel-Serres theory in its original form. But
he accepted the main tenet of it when he asserted that each stage of
ontogeny had its counterpart in an adult ancestral form. Such ancestral
forms might or might not be in existence as real species at the present
day; they might or might not be discoverable as fossils. That they had
real existence either now or at some past epoch Haeckel never doubted.
In his construction of phylogenetic trees he was so confident in the
truth of his biogenetic law that he largely disregarded and consistently
minimised the importance of the evidence from palaeontology.
The biogenetic law differed from the Meckel-Serres law chiefly in the
circumstance that many of the adult lower forms whose organisation was
supposed to be repeated in the development of the higher animals were
purely hypothetical, being deduced directly from a study of ontogeny and
systematic relationships. The hypothetical ancestral forms which the
theory thus postulated naturally took their place in the natural system,
for they were merely the concrete projections or archetypes of the
classificatory groups.
The transcendentalists, of course, conceived evolution, whether real or
ideal, as a uniserial process, whereas Haeckel conceived it as
multiserial and divergent. It is here that the superficial agreement of
the biogenetic law with the law of von Baer comes in.
We might almost sum up the relation of the biogenetic law to the laws of
von Baer and Meckel-Serres by saying that it was the Meckel-Serres law
applied to the divergent differentiation upheld by von Baer instead of
to the uniserial progression believed in by the transcendentalists.
How near in practice Haeckel's law came to the recapitulation theory of
the transcendentalists may be seen in passages like the following, with
its partial recognition of the _Echelle_ idea:[276]--"As so high and
complicated an organism as that of man ... rises upwards from a simple
cellular state, and as it progresses in its differentiating and
perfecting, it passes through the same series of transformations which
its animal progenitors have passed through, during immense spaces of
time, inconceivable ages ago.... Certain very early and low stages in
the development of man, and other vertebrate animals in general,
correspond completely in many points of structure with conditions which
last for life in the lower fishes. The next phase which follows on this
presents us with a change of the fish-like being into a kind of
amphibious animal. At a later period the mammal, with its special
characteristics, develops out of the amphibian, and we can clearly see,
in the successive stages of its later development, a series of steps of
progressive transformation which evidently correspond with the
differences of different mammalian orders and families."[377]
The biogenetic law went beyond both the Meckel-Serres law and the law of
von Baer in that it recognised that the ancestral history of the species
accounts in part for the course which the development of the individual
takes, that in a certain sense, though not in the crude way supposed by
Haeckel, phylogeny is the cause of ontogeny. This thought, that the
organism is before all an historical being, is of course implied in the
evolution idea, is indeed the essential core of it. Take away this
element from the biogenetic law--not a difficult matter--and it becomes
merely a law of idealistic morphology, applicable to evolution
considered as an ideal process, as the progressive development in the
Divine thought of archetypal models.
As a book, the _General Morphology_ suffers a good deal from the arid,
schematic, almost scholastic manner of exposition adopted. Haeckel's
Prussian mania for organisation, for absolute distinctions, for
iron-bound formalism, is here given full scope. A treatment less
adequate to the variety, fluidity and changeableness of living things
could hardly be imagined.
His doctrine, though it remains essentially unchanged, receives in his
later works a less formal and more concrete expression, and, in
particular, his views on the biogenetic law undergo some small
modification.
Even in the _General Morphology_ Haeckel had recognised that ontogeny is
neither a complete nor an entirely accurate recapitulation of phylogeny;
he had admitted, following F. Mueller, that the true course of
recapitulation was frequently modified by larval and foetal adaptations.
As time went on, he was forced to hedge more and more on this point, and
finally in his _Anthropogenie_ (1874) and his second paper on the
Gastraea theory (1875),[378] he had to work out a distinction between
palingenetic and cenogenetic characters, of which much use was made by
subsequent writers.
The distinction may be given in Haeckel's own words:--"Those ontogenetic
processes," he writes, "which are to be referred immediately, in
accordance with the biogenetic law, to an earlier completely developed
_independent ancestral form_, and are transmitted from this by
_heredity_, obviously possess _primary_ importance for the understanding
of the casual-physiological relations; on the other hand, those
developmental processes which appear subsequently through _adaptation_
to the needs of embryonic or larval life, and accordingly can _not_ be
regarded as repeating the organisation of an earlier independent
ancestral form, can clearly have for the understanding of the ancestral
history only a quite subordinate and _secondary_ importance.
"The first I have named _palingenetic_, the second _cenogenetic_.
Considered from this critical standpoint, the whole of ontogeny falls
into two main parts:--First, _palingenesis_, or 'epitomised history'
(_Auszugsgeschichte_), and second, _cenogenesis_, or 'counterfeit
history' (_Faelschungsgeschichte_). The first is the true ontogenetic
epitome or short recapitulation of past evolutionary history; the second
is the exact contrary, a new foreign ingredient, a falsifying or
concealing of the epitome of phylogeny."[379]
As examples of palingenetic processes in the development of Amniotes,
for instance, may be quoted the separation of two primary germ-layers,
the formation of a simple notochord between medullary tube and
alimentary canal, the appearance of a simple cartilaginous cranium, of
the gill-arches and their vessels, of the primitive kidneys, the
primitive tubular heart, the paired aortae and the cardinal veins, the
hermaphroditic rudiment of the gonads, and so on. Cenogenetic processes,
on the other hand, include such phenomena as the formation of yolk and
the embryonic membranes, the temporary allantoic circulation, the navel,
the curved and contracted shape of the embryo, and the like.
The most important phenomena to be included under the general heading of
cenogenesis are, first, the occurrence of food-yolk, and second, those
anomalies of development which are classed by Haeckel as heterochronies
and heterotopies.
It is to the influence of the different amounts of yolk present in the
egg that are due the great differences in the segmentation and
gastrulation processes, which almost mask their true significance.
Heterochronic processes are such as arise through the dislocation of the
proper phylogenetic order of succession: heterotopic processes in the
same way are caused by a wandering of cells from one germ-layer to
another. The two classes of phenomena are disturbances either of the
proper spatial or of the proper temporal relation of the parts during
development.
Heterochrony shows itself, as a rule, either as an acceleration or as a
retardation of developmental events, as compared with their relative
time of occurrence during phylogeny. Thus the notochord, the brain, the
eyes, the heart, appear earlier in the ontogenetic than in the
phylogenetic series, while, on the other hand, the septum of the
auricles appears in the development of the higher Vertebrates before the
ventricular septum, which is undoubtedly a reversal of the phylogenetic
order.
Cases of heterotopy, or of organs being developed in a position or a
germ-layer other than that in which they originally arose in phylogeny,
are not so easy to find. According to Haeckel, the origin of the
generative products in the mesoderm is a heterotopic phenomenon, for he
considers that they must have originated phylogenetically in one of the
two primary layers, ectoderm or endoderm.
It is worthy of note that the help of comparative anatomy is admittedly
required in deciding what processes are palingenetic and what
cenogenetic (p. 412).
Haeckel's morphological notions, and particularly his biogenetic law,
excited a good deal of adverse criticism from men like His, Claus,
Salensky, Semper and Goette. Nor was his principal work, the _General
Morphology_, received with much favour. Nevertheless, since he did
express, though in a crude, dogmatic and extreme manner, the main
hypotheses upon which evolutionary morphology is founded, his historical
importance is considerable. He cannot perhaps be regarded as typical of
the morphologists of his time--he was too trenchantly materialistic, too
much the populariser of a crude and commonplace philosophy of Nature. In
point of concrete achievement in the field of pure research he fell
notably behind many of his contemporaries.
His friend, Carl Gegenbaur, who gained a great and well-deserved
reputation by his masterly studies on vertebrate morphology,[380] was a
sounder man, and probably exercised a wider and certainly a more
wholesome influence upon the younger generation of professional
morphologists than the more brilliant Haeckel. It is true that in his
famous _Grundzuege der vergleichenden Anatomie_, the second edition of
which, published in 1870, soon came to be regarded as the classical
text-book of evolutionary morphology, Gegenbaur enunciated very much the
same general principles as Haeckel, and referred to the _Generelle
Morphologie_ as the chief and fundamental work on animal morphology. But
in Gegenbaur's pages the Haeckelian doctrines are modified and subdued
by the strong commonsense and thorough appreciation of the older
classical or Cuvierian morphology that characterise Gegenbaur's work.
According to Haeckel,[381] Gegenbaur was greatly influenced by J. Mueller,
who, as we know, laid as much stress on function as on form.
The "General Part" of Gegenbaur's text-book is in many ways a
significant document and deserves close attention.
We note first of all that physiology and morphology are considered by
Gegenbaur to be entirely distinct sciences, with different
subject-matter and different methods. "The task of physiology is the
investigation of the functions of the animal body or of its parts, the
referring back of these functions to elementary processes and their
explanation by general laws. The investigation of the material
substratum of these functions, of the form of the body and its parts,
and the explanation of this form, constitute the task of Morphology"
(2nd ed., p. 3).
Morphology falls naturally into two divisions--comparative anatomy and
embryology. The method of comparative anatomy is _comparison_ (p. 6),
and in employing this method account is to be taken of "the spatial
relations of the parts to one another, their number, extent, structure,
and texture." Through comparison one is enabled to arrange organs in
continuous series, and it comes out very clearly during this proceeding
"that the physiological value of an organ is by no means constant
throughout the different form-states of the organ, that an organ,
through the mere modification of its anatomical relations, can subserve
very different functions. Exclusive regard for their physiological
functions would place morphologically related organs in different
categories. From this it follows that in comparative anatomy we should
never in the first place consider the function of an organ. The
physiological value comes only in the second place into consideration,
when we have to reconstruct the relations to the organism as a whole of
the modification which an organ has undergone as compared with another
state of it. In this way comparative anatomy shows us how to arrange
organs in series; within these series we meet with variations which
sometimes are insignificant and sometimes greater in extent; they affect
the extent, number, shape, and texture of the parts of an organ, and can
even, though only in a slight degree, lead to alterations of position"
(p. 6).
Geoffroy St Hilaire would have subscribed to every word of this
vindication of his "principle of connections."
Between comparative anatomy and embryology there exists a close
connection, for the one throws light on the other. "While in some cases
the same organ shows only slight modifications in its development from
its early beginnings to its perfect state, in other cases the organ is
subjected to manifold modifications before it reaches its definitive
form; we see parts appear in it which later disappear, we observe
alterations in it in all its anatomical relations, alterations which may
even affect its texture. This fact is of great importance, for those
changes which an organ undergoes during its individual development lead
through states which the organ in other cases permanently shows, or at
the least the first appearance of the organ is the equivalent of a
permanent state in another organism. If then the fully developed organ
is in any special case so greatly modified that its proper relation to
some organ-series is obscured, this relation may be cleared up by a
knowledge of the organ's development. The earlier state indicated in
this way enables one to find with ease the proper place for the organ
and so insert it into an already known series. The relations which we
observe in an organ-seriation are then the equivalent of processes which
in certain cases take place in a similar manner during the individual
development of an organ. Embryology enters therefore into the closest
connection with comparative anatomy.... It teaches us to know organs in
their earliest states, and connects them up with the permanent states of
others, whereby they fill up the gaps which we meet with in the various
series formed by the fully developed organs of the body" (pp. 6-7).
This recognition of the parallelism between comparative anatomy and
embryology is, of course, the kernel of the Meckel-Serres law. For
Gegenbaur it had a very definite evolutionary meaning--he subscribed to
the evolutionary form of it, the biogenetic law. How near his conception
of the relation between ontogeny and phylogeny came to the old
Meckel-Serres law may be gauged from the following passage, taken from a
later work:--"Ontogeny thus represents, to a certain degree,
palaeontological development abbreviated or epitomised. The stages which
are passed through by higher organisms in their ontogeny correspond to
stages which are maintained in others as the definitive organisation.
These embryonic stages may accordingly be explained by comparing them
with the mature stages of lower organisms, since we regard them as forms
inherited from ancestors belonging to such lower stages"[382] (p. 6).
It is worth noting that in Gegenbaur's opinion comparative anatomy was
prior in importance to embryology, that embryology could hardly exist as
an independent science, since it must seek the interpretation of its
facts always in the facts of comparative anatomy (_Grundzuege_, pp. 7-8).
While Gegenbaur was at one with all "pure" morphologists, whether
evolutionary or pre-evolutionary, in minimising as far as possible the
importance of function in the study of form, he was too cautious and
sober a thinker not to recognise the immense part which function really
plays. Thus he classified organs, according to their function, into
those that established relations with the external world and those that
had to do with nutrition and reproduction, very much as Bichat had done
before him.
Like Darwin, Haeckel and most evolutionists, he interpreted the
homological resemblances of animals as being due to heredity, their
differences as due to adaptation,[383] but he did not adopt Haeckel's
crude and shallow definition of these terms. For Gegenbaur heredity was
a convenient expression for the fact of transmission, and was not
explained offhand as the mere mechanical result of a certain material
structure handed down from germ to germ. Adaptation he defined in a way
which took the fullest account of function, and was as far as possible
removed from Haeckel's definition of it as the direct mechanical effect
of the environment upon the organism. "The organism is altered," writes
Gegenbaur, "according to the conditions which influence it. The
consequent _Adaptations_ are to be regarded as gradual, but steadily
progressive, changes in the organisation, which are striven after during
the individual life of the organism, preserved by transmission in a
series of generations, and further developed by means of natural
selection. What has been gained by the ancestor becomes the heritage of
the descendant. Adaptation and Transmission are thus alternately
effective, the former representing the modifying, the latter the
conservative principle.... Adaptation is commenced by a change in the
function of organs, so that the _physiological relations_ of organs play
the most important part in it. Since adaptation is merely the material
expression of this change of function, the modification of the function
as much as its expression is to be regarded as a gradual process. In
Adaptation, the closest connection between the function and the
structure of an organ is thus indicated. Physiological functions govern,
in a certain sense, structure; and so far what is morphological is
subordinated to what is physiological" (_Elements_, pp. 8-9). Gegenbaur
recognised also that morphological differentiation depended largely on
the physiological division of labour (_Grundzuege_, p. 49).
It is clear that Gegenbaur realised vividly the importance of function,
and in this respect, as in others, he is far beyond Haeckel. The same
thing comes out markedly in his treatment of correlation. Haeckel had no
slightest feeling for the true meaning of correlation. For him, as for
Darwin, it reduced itself to a law of correlative variation, according
to which "actual adaptation not only changes those parts of the organism
which are directly affected by its influence, but other parts also, not
directly affected by it."[384] Such "correlative adaptation" was due to
nutrition being a "connected, centralised activity."
Gegenbaur, on the contrary, had a firm grasp of the Cuvierian
conception, and expressed it in unmistakable terms. "As indeed follows
from the conception of life as the harmonious expression of a sum of
phenomena rigorously determining one another, no activity of an organ
can in reality be thought of as existing for itself. Each kind of
function (_Verrichtung_) presupposes a series of other functions, and
accordingly every organ must possess close relations with, and be
dependent on, all the others" (_Grundzuege_, p. 71). The organism must be
regarded as an individual whole which is as much conditioned by its
parts as one part is conditioned by the others. For an understanding of
correlation a knowledge of functions, and of the functional relations of
the organism to its environment, is clearly indispensable.
Gegenbaur's morphological system was out-and-out evolutionary. "The most
important part of the business of comparative anatomy," in Gegenbaur's
eyes, "is to find indications of genetic connection in the organisation
of the animal body" (_Elements_, p. 67).
The most important clue to discovering this genetic connection is of
course that given by homology; it is indeed the main principle of
evolutionary morphology that what is common in organisation is due to
common descent, what is divergent is due to adaptation. "Homology ...
corresponds to the hypothetical genetic relationship. In the more or the
less clear homology, we have the expression of the more or less intimate
degree of relationship. Blood-relationship becomes dubious exactly in
proportion as the proof of homologies is uncertain" (_Elements_, p. 63).
It is worth noting that while Gegenbaur agrees with Haeckel generally
that morphological relationships are really genealogical, that, for
instance, each phylum has its ancestral form, he enters a caution
against too hastily assuming the existence of a genetic relation between
two forms on the basis of the comparison of one or two organs. "In
treating comparative anatomy from the genealogical standpoint required
by the evolution-theory," he writes, "we have to take into consideration
the fact that the connections can almost never be discovered in the real
genealogically related objects, for we have almost always to do with the
divergent members of an evolutionary series. We derive, for instance,
the circulatory system of insects from that of Crustacea ... but there
exists neither a form that leads directly from Crustacea to insects nor
any organisatory state (_Organisationszustand_), which as such shows the
transition. Even when one point of organisation can be denoted as
transitional, numerous other points prevent us from regarding the whole
organism strictly in the same light" (_Grundzuege_, p. 75). The real
ancestral forms cannot, as a rule, be discovered among living species,
nor often as extinct. "When we arrange allied forms in series by means
of comparison, and seek to derive the more complex from the simpler, we
recognise in the lower and simpler forms only similarities with the
ancestral form, which remains essentially hypothetical" (p. 75).
The facts of development, Gegenbaur goes on to say, help us out greatly
in our search for ancestral forms, for the early stages in the ontogeny
of a highly organised animal give us some idea of the organisation of
its original ancestor. Characters common to the early ontogeny of all
the members of a large group are particularly important in this respect
(_cf._ von Baer's law).
Gegenbaur distinguishes homologous or morphologically equivalent
structures from such as are analogous or physiologically equivalent,
just as did Owen and the older anatomists. Like von Baer he recognises
homologies, as a rule, only within the type.
He contributed, however, to the common stock a useful analysis of the
concept of homology, and established certain classes and degrees of it.
He distinguished first between general and special homology, in quite a
different sense from Owen.
General homology, in Gegenbaur's sense, relates to resemblances of
organs within the organism, and includes four kinds of resemblance,
homotypy, homodynamy, homonomy and homonymy. Right and left organs are
homotypic, metameric organs are homodynamic; homonomy is the relation
exemplified by fin-rays or fingers, which are arranged with reference to
a transverse axis of the body; homonymy is a sort of metamerism in
secondary parts (not the main axis) of the body, and is shown by the
various divisions of the appendages (_Grundzuege_, p. 80).
Special homology, on the other hand, relates to resemblances between
organs in different animals. The interesting thing is that Gegenbaur
defines it genetically. Special homology is the name we give "to the
relations which obtain between two organs which have had a common
origin, and which have also a common embryonic history" (_Elements_, p.
64). This is his definition; but, in practice, Gegenbaur establishes
homologies by comparison just as the older anatomists did, and infers
common descent from homology, not homology from common descent.
"Special homology," he continues, "must be again separated into
sub-divisions, according as the organs dealt with are essentially
unchanged in their morphological characters, or are altered by the
addition or removal of parts" (p. 65). In the former case the homology
is said to be "complete," in the latter "incomplete." Thus the bones of
the upper arm are completely homologous throughout all vertebrate
classes from Amphibia upwards, while the heart of a fish is incompletely
homologous with the heart of a mammal.
Independently of Gegenbaur, Sir E. Ray Lankester proposed in 1870 a
genetic definition of homology.[385] He proposed, indeed, to do away with
the term homology altogether, on the ground that it included many
resemblances which were obviously not due to common descent--as, for
instance, the resemblance of metameres. So, too, organs which were
homologous in the ordinary sense, as the heart of birds and mammals,
might have arisen separately in evolution. He proposed, therefore, that
"structures which are genetically related, in so far as they have a
single representative in a common ancestor," should be called
_homogenous_(p. 36). All other resemblances were to be called
_homoplastic_. "Homoplasy includes all cases of close resemblance of
form which are not traceable to homogeny, all details of agreement not
homogenous, in structures which are broadly homogenous, as well as in
structures having no genetic affinity" (p. 41). Serial homology, for
instance, was a case of homoplasy.
The term "analogy" was to be retained for cases of functional
resemblance, whether homogenetic or not.
The attempt was an interesting one, but most morphologists wisely
adhered to the old concept of homology, in spite of Lankester's
declaration that this belonged to an older "Platonic" philosophy, and
ought to be superseded by a term more consonant with the new philosophy
of evolution.
[366] _Generelle Morphologie der Organismen. Allgemeine
Grundzuege der organischen Formenwissenschaft, mechanisch
begruendet durch die von Ch. Darwin reformierte
Descendenztheorie_. Berlin, 1866. Reprinted in part as
_Prinzipien der generellen Morphologie der Organismen_.
Berlin, 1906.
[367] He mentions as his predecessors in this field,
Bronn, J. Mueller, Burmeister, and G. Jaeger.
[368] In _Grundriss einer Allgemeinen Naturgeschichte der
Radiolarien_, Berlin, 1887, and _Kunstformen der Natur_,
Suppl. Heft, Leipzig.
[369] Haeckel had an intense admiration for Goethe's
morphological work. It is a curious coincidence that the
work of Goethe, Oken and Haeckel was closely associated
with the town of Jena.
[370] But he himself would not admit this! See _Gen.
Morph._, ii., p. 11.
[371] _Fuer Darwin_, 1864. Eng. trans, by Dallas as _Facts
and Arguments for Darwin_, London, 1869.
[372] The bion is the physiological, as the morphon is the
morphological, individual.
[373] See Vogt, _Embryologie des Salmones_, p. 259, 1842,
and _supra_, p. 230.
[374] _An Essay on Classification_, London, 1859.
[375] It was hinted at by Tiedemann. "It is clear that,
proceeding from the earlier to the more recent strata, a
gradation in fossil forms can be established from the
simplest organised animals, the polyps, up to the most
complex, the mammals, and that accordingly the animal
kingdom as a whole has its developmental periods just
like the single individual organism. The species and
genera which have become extinct during the evolutionary
process may be compared with the organs which disappear
during the development of the individual animal" (p. 73,
1808).
[376] _The History of Creation_, vol. i., p. 310, 1876.
Translation of the _Natuerliche
Schoepfungsgeschichte_, 1868.
[377] _Cf._ a parallel passage from Serres, _supra_, p.
82.
[378] _Jenaische Zeitschrift_, ix., pp. 402-508, 1875.
[379] _Loc. cit._, ix., p. 409.
[380] _Untersuchungen zur vergl. Anatomie d.
Wirbelthiere_, Leipzig, i., 1864; ii., 1865; and iii.,
1872.
[381] "U. d. Biologie in Jena waehrend des 19
Jahrhunderts," _Jenaische Zeitschrift_, xxxix., pp.
713-26, 1905.
[382] _Grundriss der vergl. Anatomie_, 1874, 2nd ed.,
1878. Trans. by F. Jeffrey Bell, revised by E. Ray
Lankester, as _Elements of Comparative Anatomy_, London,
1878.
[383] "This theory (evolution) shows that what was
formerly called 'structural plan' or 'type' is the sum
of the dispositions (_Einrichtungen_) of the animal
organisation which are perpetuated by heredity, while it
explains the modifications of these dispositions as
adaptive states. Heredity and adaptation are thus the
two important factors through which both the unity and
the variety of organisation can be understood"
(_Grundzuege_, p. 19).
[384] _History of Creation_, i., pp. 241-2.
[385] "On the use of the term Homology in Modern Zoology,
and the distinction between Homogenetic and Homoplastic
agreements," _Ann. Mag. Nat. Hist._ (4), vi., pp. 35-43,
1870.