Form and Function

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Development can thus be regarded as consisting of a number of stages, at
each of which new specific elements enter automatically into play and
lead the embryo from that stage to the stage succeeding. The germinal
substance on this theory of Rignano's is to be regarded as being
composed of a large number of specific elements, originally formed as a
result of each new functional adaptation, but now forming part of the
hereditary equipment.

The theory represents an advance upon the more static conceptions of
Semon. It owes much to Roux's influence.

In this country, the mnemic theories have been championed particularly
by M. Hartog[517] and Sir Francis Darwin.[518]

[508] The quotations are taken from the 1910 reprint,
London, Fifield.

[509] _Ueber das Gedaechtnis als eine allgemeine Funktion
der organisierten Materie_, Wien, 1870.

[510] Eng. trans, in E. Hering, _Memory_, p. 9, Chicago
and London, 1913.

[511] _Die Perigenesis der Plastidule_, Jena, 1875.

[512] _A Theory of Development and Heredity_, New York,
1893.

[513] _The Primary Factors of Organic Evolution_, Chicago,
1896.

[514] _Die Mneme als erhaltendes Prinzip im Wechsel des
organischen Geschehens_, Leipzig, 1904; 2nd ed., 1908.

[515] _Heredity and Memory_, Cambridge, 1913.

[516] Paris, 1906. Also in Italian and German. Eng. trans.
by B. C. ,H. Harvey, Chicago, 1911.

[517] See _Problems of Life and Reproduction_, London,
1913.

[518] _Presidential Address to the British Association_,
1908.

CHAPTER XX

THE CLASSICAL TRADITION IN MODERN MORPHOLOGY

To write a history of contemporary movements from a purely objective
standpoint is well recognised to be an impossible task. It is difficult
for those in the stream to see where the current is carrying them: the
tendencies of the present will only become clear some twenty years in
the future.

I propose, therefore, in this concluding chapter to deal only with
certain characteristics of modern work on the problems of form which
seem to me to be derived directly from the older classical tradition of
Cuvier and von Baer.

The present time is essentially one of transition. Complete uncertainty
reigns as to the main principles of biology. Many of us think that the
materialistic and simplicist method has proved a complete failure, and
that the time has come to strike out on entirely different lines. Just
in what direction the new biology will grow out is hard to see at
present, so many divergent beginnings have been made--the materialistic
vitalism of Driesch, the profound intuitionalism of Bergson, the
psychological biology of Delpino, France, Pauly, A. Wagner and W.
Mackenzie. But if any of these are destined to give the future direction
to biology, they will in a measure only be bringing biology back to its
pre-materialistic tradition, the tradition of Aristotle, Cuvier, von
Baer and J. Mueller. It may well be that the intransigent materialism of
the 19th century is merely an episode, an aberration rather, in the
history of biology--an aberration brought about by the over-rapid
development of a materialistic and luxurious civilisation, in which
man's material means have outrun his mental and moral growth.

Two movements seem significant in the morphology of the last decade or
so of the 19th century--first, the experimental study of form, and
second, the criticism of the concepts or prejudices of evolutionary
morphology.

The period was characterised also by the great interest taken in
cytology, following upon the pioneer work of Hertwig, van Beneden and
others on the behaviour of the nuclei in fertilisation and
maturation.[519] This line of work gained added importance in connection
with contemporary research and speculation on the nature of hereditary
transmission, and it has in quite recent years received an additional
stimulus from the re-discovery of Mendelian inheritance. Its importance,
however, seems to lie rather in its possible relation to the problems of
heredity than in any meaning it may have for the problems of form. More
significant is the revolt against the cell-theory started by Sedgwick[520]
and Whitman,[521] on the ground that the organism is something more than
an aggregation of discrete, self-centred cells.

The experimental work on the causes of the production and restoration of
form infused new life into morphology. It opened men's eyes to the fact
that the developing organism is very much a living, active, responsive
thing, quite capable of relinquishing at need the beaten track of normal
development which its ancestors have followed for countless generations,
in order to meet emergencies with an immediate and purposive reaction.
It was cases of this kind, cases of active regulation in development and
regeneration, that led men like G. Wolff and H. Driesch to cast off the
bonds of dogmatic Darwinism and declare boldly for vitalism and
teleology.

There was the famous case of the regeneration of the lens in Amphibia
from the edge of the iris--an entirely novel mode of origin, not
occurring in ontogeny. The fact seems to have been discovered first by
Colucci in 1891, and independently by G. Wolff in 1895.[522] The
experiment was later repeated and confirmed by Fischel and other
workers. Wolff drew from this and other facts the conclusion that the
organism possesses a faculty of "primary purposiveness" which cannot
have arisen through natural selection.[523] And, as is well known, Driesch
derived one of his most powerful arguments in favour of vitalism from
the extraordinary regenerative processes shown by _Tubularia_ and
_Clavellina_ in the course of which the organism actually demolishes and
rebuilds a part or the whole of its structure. But under the influence
of physiologists like Loeb many workers held fast to materialistic
methods and conceptions.

The great variety of regulative response of which the organism showed
itself capable made it very difficult for the morphologist to uphold the
generalisations which he had drawn from the facts of normal undisturbed
development. The germ-layer theory was found inadequate to the new
facts, and many reverted to the older criterion of homology based on
destiny rather than origin. The trend of opinion was to reject the
ontogenetic criterion of homology, and to refuse any morphological or
phylogenetic value to the germ-layers.[524]

The biogenetic law came more and more into disfavour, as the developing
organism more and more showed itself to be capable of throwing off the
dead-weight of the past, and working out its own salvation upon original
and individual lines.[525] A. Giard in particular called attention to a
remarkable group of facts which went to show that embryos or larvae of
the same or closely allied species might develop in most dissimilar ways
according to the conditions in which they found themselves.[526] His
classical case of "poecilogeny" was that of the shrimp _Palaemonetes
varians_, the fresh-water form of which develops in an entirely
different way from the salt-water form.

Experimental workers indeed were inclined to rule the law out of
account, to disregard completely the historical element in development,
and this was perhaps the chief weakness of the neo-vitalist systems
which took their origin in this experimental work.

From the side also of descriptive morphology the biogenetic law
underwent a critical revision. It was studied as a fact of embryology
and without phylogenetic bias by men like Oppel, Keibel, Mehnert, O.
Hertwig and Vialleton,[527] and they arrived at a critical estimate of it
very similar to that of von Baer.

Theoretical objections to the biogenetic law had been raised from time
to time by many embryologists, but the positive testing of it by the
comparison of embryos in respect of the degree of development of their
different organs starts with Oppel's work of 1891.[528] He studied a large
number of embryos of different species at different stages of their
development, and determined the relative time of appearance of the
principal organs and their relative size. His results are summarised in
tabular form and have reference to all the more important organs. He was
led to ascribe a certain validity to the biogenetic law, but he drew
particular attention to the very considerable anomalies in the time of
appearance which are shown by many organs, anomalies which had been
classed by Haeckel under the name of heterochronies.

Oppel's main conclusions were as follows:--"There are found in the
developmental stages of different Vertebrates 'similar ontogenetic
series,' that is to say, Vertebrates show at definite stages
similarities with one another in the degree of development of the
different organs. Early stages resemble one another, so also do later
stages; equivalent stages of closely allied species resemble one
another, and older stages of lower animals resemble younger stages of
higher animals; young stages are more alike than old stages.... The
differences which these similar series show (for which reason they
cannot be regarded as identical) may be designated as temporal
disturbances in the degree of development of the separate organs or
organ-systems. Some organs show very considerable temporal dislocations,
others a moderate amount, others again an inconsiderable amount. Among
the developmental stages of various higher animals can be found some
which correspond to the ancestral forms and also to the lower types
which resemble these ancestral forms. On the basis of the tabulated data
here given there can be distinguished with certainty in the ontogeny of
Amniotes a pro-fish stage, a fish-stage, a land-animal stage, a
pro-amniote stage, and following on these a fully developed reptile,
bird or mammal stage."[529]

Oppel's methods were employed by Keibel[530] in his investigations on the
development of the pig, which formed the model for the well-known series
of _Normentafeln_ of the ontogeny of Vertebrates which were issued in
later years under Keibel's editorship. Keibel was more critical of the
biogenetic law than Oppel, and he held that the ancestral stages
distinguished by Oppel could not be satisfactorily established. He
suggested an interesting explanation of heterochrony in development,
according to which the premature or retarded appearance of organs in
ontogeny stands in close relation with the time of their entering upon
functional activity. Thus in many mammals the mesodermal part of the
allantois often appears long before the endodermal part, though this is
phylogenetically older. This Keibel ascribes to the fact that the
endodermal part is almost functionless. "One can directly affirm," he
writes, "that the time of appearance of an organ depends in an eminent
degree upon the time when it has to enter upon functional activity. This
moment is naturally dependent upon the external conditions. Among the
highest Vertebrates, the mammals, the traces of phylogeny shown in
ontogeny are to a great extent obliterated through the adaptation of
ontogeny to the external conditions, and through the modifications which
the germs of more highly organised animals necessarily exhibit from the
very beginning as compared with germs which do not reach such a high
level of development" (p. 754, 1897).

Study of individual variation in the time of appearance of the organs in
embryos of the same species was prosecuted with interesting results by
Bonnet,[531] Mehnert,[532] and Fischel.[533] Fischel found that variability
was greatest among the younger embryos, and became progressively less in
later stages. Like von Baer (_supra_, p. 114) he inferred that
regulatory processes were at work during development which brought
divergent organs back to the normal and enabled them to play their part
as correlated members of a functional whole.

Important theoretical views were developed by Mehnert[534] in a series of
publications appearing from 1891 to 1898. Like Keibel, Mehnert
emphasised the importance of function in determining the late or early
appearance of organs, but he conceived the influence of function to be
exerted not only in ontogeny, but also throughout the whole course of
phylogeny, by reason of the transmission to descendants of the effects
of functioning in the individual life.

In his paper of 1897 Mehnert details the results of an extensive
examination of the development of the extremities throughout the Amniote
series. He finds that in all cases a pentadactylate rudiment is formed,
even in those forms in which only a few of the elements of the hand or
foot come to full development. But whereas in forms with a normally
developed hand, _e.g._ the tortoise and man, all the digits develop and
differentiate at about the same rate, in forms which have in the adult
reduced digits, _e.g._ the ostrich and the pig, these vestigial digits
undergo a very slow and incomplete differentiation, while the others
develop rapidly and completely. He draws a general distinction between
organs that are phylogenetically progressive and such as are
phylogenetically regressive, and seeks to prove that progressive organs
show an ontogenetic acceleration and regressive organs a retardation.[535]
The acceleration or retardation affects not only the mass-growth of the
organs, but also their histological differentiation.

Now between progression and functioning and between regression and
functional atrophy there is obviously a close connection. Loss of
function is well known to be one of the chief causes of the degeneration
of organs in the individual life, and on the other hand, as Roux has
pointed out, all post-embryonic development is ruled and guided by
functioning. It is thus in the long run functioning that brings about
phylogenetic progression, absence of functional activity that causes
phylogenetic regression. This comes about through the transmission of
acquired functional characters, a transmission which Mehnert conceives
to be extraordinarily accurate and complete.

In general Mehnert adopts the functional standpoint of Cuvier, von Baer,
and Roux. His considered judgment as to the phylogenetic value of the
biogenetic law closely resembles that formed by von Baer, for he admits
recapitulation only as regards the single organs, not as regards the
organism as a whole. He has, however, much more sympathy with the law
than either Keibel or Oppel, though he agrees that it cannot be used for
the construction of ancestral trees. But he ascribes to it as a fact of
development considerable importance. The following passage gives a good
summary of his view as to the scope and validity of the law. "The
biogenetic law has not been shaken by the attacks of its opponents. The
assertion is still true that individual organogenesis is exclusively
dependent on phylogeny. But we must not expect to find that all the
stages in the development of the separate organs, which coexisted in any
member of the phylogenetic series, appear _at the same time_ in the
individual ontogeny of the descendants, because each organ possesses its
own specific rate of development. In this way it comes about naturally
that organs which become differentiated rapidly, as, for example, the
medullary tube, as a rule dominate earlier periods of ontogeny than do
the organs of locomotion. For the same reason the cerebral hemispheres
of man are almost as large in youth as in maturity. The picture which an
embryo gives is not a repetition in detail of one and the same
phylogenetic stage; it consists rather of an assemblage of organs, some
of which are at a phyletically early stage of development, while others
are at a phyletically older stage."[536]

A different line of attack was that adopted by O. Hertwig in a series of
papers, which contain also what is perhaps the best critical estimate of
the present position and value of descriptive morphology.[537]

It had not escaped the notice of many previous observers that quite
early embryos not infrequently show specific characters even before the
characters proper to their class, order and genus are developed--in
direct contradiction of the law of von Baer. Thus L. Agassiz[538] had
remarked in 1859 that specific characteristics were often developed
precociously. "The Snapping Turtle, for instance, exhibits its small
crosslike sternum, its long tail, its ferocious habits, even before it
leaves the egg, before it breathes through lungs, before its derm is
ossified to form a bony shield, etc.; nay, it snaps with its gaping jaws
at anything brought near, when it is still surrounded by its amnion and
allantois, and its yolk still exceeds in bulk its whole body" (p. 269).

Wilhelm His,[539] in the course of an acute and damaging criticism of the
biogenetic law as enunciated by Haeckel, showed clearly that by careful
examination the very earliest embryos of a whole series of Vertebrates
could be distinguished with certainty from one another. "An identity in
external form of different animal embryos, despite the common
affirmation to the contrary, does not exist. Even at early stages in
their development embryos possess the characters of their class and
order, nay, we can hardly doubt, of their species and sex, and even
their individual characteristics" (201).

This specificity of embryos was affirmed with even greater confidence by
Sedgwick in a paper critical of von Baer's law.[540] He wrote:--"If v.
Baer's law has any meaning at all, surely it must imply that animals so
closely allied as the fowl and duck would be indistinguishable in the
early stages of development; and that in two species so closely similar
that I was long in doubt whether they were distinct species, viz.,
_Peripatus capensis_ and _Balfouri_, it would be useless to look for
embryonic differences; yet I can distinguish a fowl and a duck embryo on
the second day by the inspection of a single transverse section through
the trunk, and it was the embryonic differences between the Peripatuses
which led me to establish without hesitation the two separate
species.... I need only say ... that a species is distinct and
distinguishable from its allies from the very earliest stages all
through the development, although these embryonic differences do not
necessarily implicate the same organs as do the adult differences" (p.
39).

Hertwig interprets this fact of the specific distinctness of closely
allied embryos in the light of the preformistic conception of heredity.
According to this view the whole adult organisation is represented in
the structure of the germ-plasm contained in the fertilised ovum, from
which it follows that the ova of two different species, and also their
embryos at every stage of development, must be as distinct from one
another as are the adults themselves, even though the differences may
not be so obvious. If this be the case there can be no real
recapitulation in ontogeny of the phylogeny of the race, for the
egg-cell represents not the first term in phylogeny, but the last. The
egg-cell _is_ the organism in an undeveloped state; it has a vastly more
complicated structure than was possessed by the primordial cell from
which its race has sprung, and it can in no way be considered the
equivalent of this ancestral cell.

Hertwig puts this vividly when he says that "the hen's egg is no more
the equivalent of the first link in the phylogenetic chain than is the
hen itself" (p. 160, 1906, b).

If ontogeny is not a recapitulation of phylogeny, how is it that the
early embryonic stages are so alike, even in animals of widely different
organisation? Hertwig's answer to this is very interesting. He takes the
view that many of the processes characterising early embryonic
development are the means necessarily adopted for attaining certain
ends. Such are the processes of segmentation, the formation of a
blastula, of cell-layers, of medullary folds where the nervous system is
a closed tube, the formation of the notochord as a necessary condition
of the development of the vertebral column, and so on. "Looked at from
this standpoint it cannot surprise us that in all animal phyla the
earliest embryonic processes take place in similar fashion, so that we
observe the occurrence both in Vertebrates and Invertebrates of a
segmentation-process, a morula-stage, a blastula and a gastrula. If now
these developmental processes do not depend on chance, but, on the
contrary, are rooted in the nature of the animal cell itself, we have no
reason for inferring from the recurrence of a similar
segmentation-process, morula, blastula, and gastrula in all classes of
the animal kingdom the common descent of all animals from one
blastula-like or gastrula-like ancestral form. We recognise rather in
the successive early stages of animal development only the manifestation
of special laws, by which the shaping of animal forms (as distinct from
plant forms) is brought about" (p. 178, 1906, b).

"The principal reason why certain stages recur in ontogeny with such
constancy and always in essentially the same manner is that they provide
under all circumstances the necessary pre-conditions through which alone
the later and higher stages of ontogeny can be realised. The unicellular
organism can by its very nature transform itself into a multicellular
organism only by the method of cell-division. Hence, in all Metazoa,
ontogeny must start with a segmentation-process, and a similar statement
could be made with regard to all the later stages" (p. 57, 1906, a).

Similarities in early development are therefore no evidence of common
descent, and in the same way the resemblances of adult animals, subsumed
under the concepts of homology and the unity of plan, are not
necessarily due to community of descent, but may also be brought about
by the similarity or identity of the laws which govern the evolution of
these animals. In the absence, therefore, of positive evidence as to the
actual lines of descent (to be obtained only from palaeontology),
homological resemblance cannot be taken as proof of blood relationship,
for homology is a wider concept than homogeny. The only valid definition
of homology is that adopted in pre-evolutionary days, when those organs
were considered homologous "which agree up to a certain point in
structure and composition, in position, arrangement, and relation to the
neighbouring organs, and accordingly possess identical functions and
uses in the organism" (p. 151, 1906, b).

The concept of homology has thus a value quite independent of any
evolutionary interpretation which may be superadded to it. "Homology is
a mental concept obtained by comparison, which under all circumstances
retains its validity, whether the homology finds its explanation in
common descent or in the common laws that rule organic development" (p.
151, 1906, b). As A. Braun long ago pointed out, "It is not descent
which decides in matters of morphology, but, on the contrary, morphology
which has to decide as to the possibility of descent."[541]

Hertwig, in a word, reverts to the pre-evolutionary conception of
homology. "We see in homology," he writes, "only the expression of
regularities (_Gesetzmaessigkeiten_) in the organisation of the animals
showing it, and we regard the question, how far this homology can be
explained by common descent and how far by other principles, as for the
present an open one, requiring for its solution investigations specially
directed towards its elucidation" (p. 179, 1906, b).

Holding, as he does, that no definite conclusions can be drawn from the
facts of comparative anatomy and embryology as to the probable lines of
descent of the animal kingdom, Hertwig accords very little value to
phylogenetic speculation. It is, he admits, quite probable that the
archetype of a class represents in a general sort of way the ancestral
form, but this does not, in his opinion, justify us in assuming that
such generalised types ever existed and gave origin to the present-day
forms. "It is not legitimate to picture to ourselves the ancestral forms
of the more highly organised animals in the guise of the lower animals
of the present day--and that is just what we do when we speak of
Proselachia, Proamphibia and Proreptilia" (p. 155, 1906, b).

He rejects on the same general grounds the evolutionary dogma of
monophyletic or almost monophyletic descent, and admits with Koelliker,
von Baer, Wigand, Naegeli and others that evolution may quite well have
started many times and from many different primordial cells.

There is indeed a great similarity between the views developed by O.
Hertwig and those held by the older critics of Darwinism--von Baer,
Koelliker, Wigand, E. von Hartmann and others. It is true the
philosophical standpoint is on the whole different, for while many of
that older generation were vitalists Hertwig belongs to the mechanistic
school.

But both Hertwig and the older school agree in pointing out the _petitio
principii_ involved in the assumption that the archetype represents the
ancestral form; both reject the simplicist conception of a monophyletic
evolution (which may be likened to the "one animal" idea of the
transcendentalists); both admit the possibility that evolution has taken
place along many separate and parallel lines, and explain the
correspondences shown by these separate lines by the similarity of the
intrinsic laws of evolution; finally, both emphasise the fact that we
know nothing of the actual course of evolution save the few indications
that are furnished by palaeontology, and both insist upon the unique
importance of the palaeontological evidence.[542]

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