Form and Function

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

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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).

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