Download Atomism, epigenesis, preformation and preexistence: a clarification

Biological Journal of Ihe Linnean Socie!y ( 1986), 28: 33 1-341
Atomism, epigenesis, preformation and
pre-existence: a clarification of terms
and consequences
OLIVIER RIEPPEL F.L.S.
Palaontologisches Institut und Museum der Universitat, Kiinstlergasse 16, CH-8006 Zurich,
Switzerland
Received 16 Juh 1985, accepted for publication 4 February 1986
The meaning of the terms atomism, epigenesis, preformation and pre-existence is clarified by a
historical analysis. Today, two alternative models of organismic change are opposed to each other.
Atomism views the organism as being composed of traits or atoms: it implies the possibility of
gradualistic change and a nominalistic species concept, while an increase in complexity is identified
as an addition of new parts. Epigenesis, in contrast, implies the possibility of saltational change and
an essentialistic species concept, while an increase in complexity is considered to result from an
enhanced compartmentalization and differentiation of the originally homogeneous primordium.
Schwabe & Warr’s (1984) ‘genetic potential hypothesis’ qualifies as pre-existence at the genotypic
level.
KEY WORDS:-Ontogeny
-
phylogeny - preformation - epigenesis.
CONTENTS
Introduction . . . . . . . . .
Aristotle’s critique of atomism . . . . .
William Harvey and his notion of epigenesis .
Pre-existence and the correlation of parts . .
Atomism and epigenesis in the eighteenth century
Darwinism and atomism . . . . . .
The modern critique of Darwinism . . . .
Modern preformationism . . . . . .
Atomism versus epigenesis . . . . . .
References
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INTRODUCTION
All scientific endeavour must be guided by theoretical premises and
methodological rules.. The results of some investigations are determined by
empirical data but even more so by the way these data are interpreted, the
interpretation following theoretical premises inherent in the vocabulary used to
communicate about research programmes. A critical evaluation of the meaning
of certain notions, their theoretical content and consequences is therefore
important in all fields of natural science.
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The present paper deals with critical terms derived from the vocabulary of
modern embryology. The study of mechanisms of ontogenesis plays a n
increasingly important role in the study of tempo and mode of evolution (e.g.
Bonner, 1982; Raff & Kaufman, 1983; Arthur, 1984). I n this context, notions
like preformation or epigenesis make their frequent reappearance (e.g. Katz &
Goffman, 1981; Schwabe & Warr, 1984), and the relative merits of these two
concepts are discussed in the light of modern understanding of embryogenesis
(e.g. Mayr, 1982); however, at the same time the theoretical content of these
notions is by no means clear. The reasons for this are twofold. I n the field of the
history of science a controversy has been triggered by Jacques Roger (1971; first
edition 1963), who distinguished between preformation and pre-existence (see
Wilkie, 1967; Bowler, 1971; Hofieimer, 1982), while such notions were not
unequivocally defined in the great debate which dominated biology during the
eighteenth century. The purpose of this paper is to clarify the meaning of these
models of ontogeny in their historical context, and to investigate the theoretical
implications they carry into present-day discussions.
ARISTOTLE’S CRITIQUE OF ATOMISM
The atomistic philosophy of Democritus, revised by Epicurus and popularized
by Lucretius, considered the organism to be composed of parts or atoms. Each
part of the adult body of both sexes was thought to be represented by
equivalent, i.e. preformed, but miniaturized, atoms in the male and female
seminal fluids. Generation would consist of a mixture of the two seminal fluids
within the mother’s womb stirred by the warmth of the maternal body, and it
would result in the aggregation of the atoms to form the fetus. T h e system
appeared to explain the similarity of the offspring with both parents, while
‘errors’ in the combination of atoms would result in malformations. The
embryo, according to atomists, was a formation de novo, resulting from the
combination of preformed parts derived from the parental bodies (RegnC11, 1967;
Kullmann, 1979; see also Lucretius Carus, D e rerum natura, liber I. 169-170).
This is the original theory of preformation which Aristotle set about to
criticize. How do the parts forming a larva derive from the sexually mature
imago? Why does the newborn boy lack a beard? Why are mutilations not
heritable? Why does usually only one child result from the combination of two
seminal fluids?-These were some of the questions he asked (Kullman, 1979).
Based on the observation of the development of the chick (Kullman, 1979: 43),
Aristotle devised his own theory of generation which was later-through
the
work of W. Harvey (1651)-to become known as epigenesis. The hallmark of this
theory is that the parts of the embryo, forming de novo, are not all present and
preformed at the beginning of its development, but arise one after the other.
The male semen is derived from the blood which itself is prepared by the
heart. It is endowed with the soul which represents the principle of form and of
movement. The ‘knowledge of form’ and the potential movement are
transmitted via the aura seminalis to the receptive female substance (the
menstrual blood) within which the form of the species becomes actualized by the
effect of the actual movement of the heart, the first part of the embryo to be
formed (Balss, 1943: 213). The species-specific form is eternal and hence
ATOMISM AND EPIGENESIS
333
immutable; its continuous actualization by generation provides a means of the
partaking of ephemeral individuals in divine eternity (Balss, 1943: 183). In
contrast to Plato, Aristotle held the causu formalis to be immanent in matter
itself, and so is the telos of the developmental process, the actualization of this
form (Kullman, 1979: 19). Nevertheless, the critique of nominalism hit
Aristotelianism just as severely as Platonism.
WILLIAM HARVEY AND HIS NOTION OF EPIGENESIS
The writings of William Harvey (1578-1657) on matters of generation,
published in 1651, have been analysed by several authors in great detail (e.g.
Adelmann, 1966; Pagel, 1967; Gaskin, 1967; Roger, 1971) which is why only
the most important points pertinent to the understanding of his theory of
epigenesis will be mentioned here.
Harvey closely followed Aristotle, as he himself announced, but not in a
servile manner, deviating from the thoughts of his master when observation of
the developing chick made this necessary. An important point to note is that
during medieval times Aristotelian philosophy had become embedded within
the Christian tradition of thought and thereby had become mixed with Platonic
or Neoplatonic concepts, since Western philosophers took it over from the
Islamic invaders of Spain (Gilson, 1955).
The soul, which according to Aristotle is the principle of knowledge of the
form, immanent in matter (Balss, 1943: 175, 213), is replaced in Harvey’s
system by God. The species-specific form is rooted in the eternal world of Divine
ideas; the Creator’s providence directs the actualization of this form during
embryogenesis. God does so by using the male and female as instrumental causes
for generation (Harvey, 1651, transl. by Whitteridge, 1981: 234) and the soul of
the fertilized egg as a guiding principle (Harvey, 1651, transl. by Whitteridge,
1981: 139, 216-217) residing in the blood (Harvey, 1651, transl. by
Whitteridge, 1981 : 249) which, following Harvey, makes its appearance before
the heart in contra-distinction to what Aristotle had said. With God as principle
of knowledge of the form we encounter a n element of ‘preformation’, not in its
original atomistic meaning but rather in the sense of Divine predetermination of
embryonic development: “. . . I have said that it [the cicutriculu, i.e. the
blastoderm] is the principal part, as being that in which all the other parts exist
potentially, and from whence they later arise, each in its due o r d e r . . .”
(Harvey, 1651, transl. by Whitteridge, 1981: 274; see also pp. 450-452).
According to Harvey, the egg is a formation de novo; the embryo develops
successively, one part after the other, proceeding from the homogeneity of the
primordium to the heterogeneity of the adult, which means that the embryo
differs at first from the adult condition which it approaches successively. Two
mechanisms are invoked by Harvey to explain this developmental process,
budding and subdivision: “. . . because it is certain that the chick is built by
epigenesis, or the addition of parts budding out from one another. . . the first
[part] to exist is the genital part by virtue of which all the remaining parts do
later arise as from their first original. . . at the same time that part divides up and
forms all the other parts in their due order. . ,” (Harvey, 1651, transl. by
Whitteridge, 1981: 240, emphasis added; see also p. 207). The epigenetic process
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of development is one of vegetation and compartmentalization. One organ
formed becomes the material cause of the next one to develop: “. . . the
construction. . . begins from some part as from its original, and by its help the
other members are produced.. .” (Harvey, 1651, transl. by Whitteridge,
1981: 202). This is quite a striking contrast to the atomistic view of generation,
according to which the embryo is built up by the juxtaposition of preformed
parts.
PRE-EXISTENCE AND THE CORRELATION OF PARTS
Charles Bonnet, from Geneva (1720-1 793), had various reasons, predominantly theological and political ones, to defend the hypothesis of preexisting germs (Mam, 1976). Together with Albrecht von Haller (1708-1777)
and Lazzaro Spallanzani (1729-1 799) he fought against the rise of materialistic
philosophy during the Age of Enlightenment and against the theories of
generation which he considered to be derived from that philosophy (Roe, 1981),
thereby identifying two rather different models of embryonic development as
epigenetic with consequences that will be discussed shortly.
One of the many arguments used against epigenesis derived from the
functional correlation of the parts of an organism. When Harvey claimed that
the parts of the fetus develop one after and out of the other, he could obviously
not invoke the functional correlation of these parts as was postulated by Bonnet:
the parts of an organism are “SO manifestly linked together and subordinated to
one another, that the existence of some presupposes the existence of others”
(Bonnet, 1769, Vol. I: 355; see also Bonnet, 1764, Vol. I: 154). Consequently,
the entire embryo must pre-exist from the beginning, i.e. from the time of
creation (Roger, 1971). The egg, which equals the embryo, is no formation de
novo, and therefore the primordium is not homogeneous but heterogeneous, i.e.
organized. Development consists of nothing more than the unfolding (or
“holution”) of pre-existing parts on the basis of mechanistic secondary causes.
Yet Bonnet conceded that the pre-existing parts are not readily visible in the
germ at the beginning of its development, and he knew both from the study of
insect metamorphosis as well as from Harvey’s (1651), Malpighi’s (Adelmann,
1966) and Albrecht von Haller’s (1758) descriptions of the development of the
chick, that the embryo does not resemble the adult but that its parts become
visible one after the other and may undergo changes in shape, position and even
in function before the adult condition is arrived at (Bonnet, 1764, Vol. I: 292;
1768, Vol. I: 122, 253f Vol. 11: 252fl). To reconcile these observational data
with his theory of pre-existence, Bonnet simply avoided the passage from
invisibility to non-existence, following the arguments expounded by the “great
apostle of the pre-existence of the germs” (Bonnet in Savioz, 1948: 93), P. N.
Malebranche, in De la Recherche de la Vkritk. Significantly, when Bonnet turned to
a criticism of Harvey (Bonnet, 1764, Vol. I: 155; 1768, Vol. I: 87), he did not
blame him for his theory so much as for his descriptions. They both adhered to
the concept of an ideal and eternal principle of form, founded in God, who
created all organisms each according to its species. As Harvey described the
successive development of the parts of the chick, however, he had-according to
Bonnet-simply fallen victim to the illusion that what cannot be seen must be
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335
non-existent. If it was not Harvey’s theory, what then was the concept of
epigenesis which Bonnet never became tired of attacking?
ATOMISM AND EPIGENESIS IN THE EIGHTEENTH CENTURY
Bonnet’s Considhations sur les Corps Organist%, first published in 1762, was
largely composed as a critique of the theories of generation put forward by
Georges Buffon ( 1 707-1 788), John Turberville Needham ( 1713-1 78 1 ) and
Pierre-Louis Moreau de Maupertuis (1698-1 759), naturalists whom Bonnet
correctly identified as admirers of Epicurus. Indeed, atomism enjoyed a vigorous
renaissance in the context of enlightened materialism, as is aptly illustrated by
d’Holbach’s Systbme de la Nature. What Bonnet objected to was the exclusion of
final causes from the interpretation of nature by materialists and the threat of
atheism that resulted from such an outlook (letter of Bonnet to Haller dated 11
August 1770; see Sonntag, 1983: 890). No longer was Divine wisdom and
foresight responsible for the formation of a viable organism, but rather the
fortuitous aggregation of atoms, be it by spontaneous or by sexual generation. If
the organism is conceived of as being composed of atoms, the constituent
particles become, in principle, interchangeable. T h e atomistic system thus
seemed to explain rather easily the problems of inheritance, particularly in cases
of hybridization, as well as the origin of malformations. O n the other hand, the
observed constancy of the species-specific form could not be explained if the
aggregation of atoms during embryogenesis were a process governed by chance
alone. This is why most atomists ended up with some kind of vitalism.
The ground was prepared by Pierre Gassendi ( 1592-1655) who resurrected
atomism as a reaction against Cartesian philosophy (Rod, 1978). According to
him, the seminal fluids produced by both sexes consist of atoms derived from the
parental bodies; the atoms would combine to form the embryo under the
guidance of their soul at the moment of conception. To explain the constancy of
the species-specific form, Gassendi was forced to endow the atoms with a soul
which would remember their position in the parent body (Adelmann, 1966, Vol.
11: 776-815). H e thus defended some type of vitalistic atomism (Adam, 1955:
16 1- 162), the vitalistic component being derived from an Aristotelian
background (Adelmann, 1966, Vol. 11: 802). It is crucial to note that Gassendi
postulated not a successive, but an initial and instantaneous combination of the
atoms in view of the functional correlation of all the parts of the developing
embryo! This is the type of theory which Roger (1971) correctly identified as
preformationism.
In Buffon’s Histoire des Anirnaux, first published in 1749 in the second volume
of his Histoire Naturelle, gktrale et particulilre (Paris, 1749-1 767), the atoms have
become the “mol~cules organiques” while the guiding principle for their
combination is provided by the famous and obscure “moules inthieurs”. The
“molt!cules organiques” were believed to derive from nutritional matter; they
would pass through the organs of the adult body where the appropriate form
would be ‘imprinted’ on them before they are transported by the blood to the
genital organs where they are stored in the seminal fluid. Again, ,the
combination of the atoms stemming from the male and female seminal fluids
was thought to occur instantaneously at the moment of conception to warrant
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the functional correlation of at least the ‘essential’ parts of the fetus. Buffon
explicitly rejected Harvey’s views on this point, and although he conceded that
the embryo has to go through some “dheloppement” before attaining the adult
form, his theory clearly qualifies as preformationist (Roger, 1971: 546; see also
Bowler, 1973) in spite of the fact that it became widely identified as epigenetic.
Maupertuis, on the other hand, admitted the influence of Harvey on his
thoughts in the seventh chapter of the first part of his Vknus physique, first
published in 1745. He consequently postulated the successive juxtaposition of
the (preformed: Hofieimer, 1982: 126) atoms derived from male and female
seminal fluids under the influence of purely mechanical forces. Only under the
pressure of Reni-Antoine-Ferchault de Riaumur’s ( 1683-4 757) critique (Roe,
1981: 15) would he endow the atoms, in his Syst2me de la Nature (Maupertuis,
1974), with some kind of ‘intelligence’, ‘memory’, ‘appetite’ and ‘aversion’ to
explain the constancy of the species-specific form. However, he still considered
errors in the combination of the atoms to be possible, which would result in
malformations that might even become heritable: the inheritance of polydactyly
in the family of Jakob Ruhe, surgeon in Berlin, served as a case in point (Glass,
1968). He finally ended up with the suggestion that new species might have
originated in the past by the inheritance of malformations.
Although Maupertuis considered himself to be a disciple of Harvey and hence
an epigenesist, he grossly misinterpreted his master’s writings, as was in fact
noticed by Charles Bonnet ( 1768, Vol. I: 122- 128). Indeed, Maupertuis-like
all other atomists-claimed that the embryo forms by the juxtaposition of parts,
although he added the qualification that their aggregation occurs successively
and not all at once. This model of embryogenesis, resting on the analogy with
the formation of a crystal, does not imply the development of the embryo from a
homogeneous to a heterogeneous condition, but rather stipulates a mode of
growth that was neither admitted by Harvey, nor by another epigenesist of the
eighteenth century, Kaspar Friedrich Wolff (1 743-1 794) in his Theorie von der
Generation (1764). As compared with Harvey, he qualifies as a vitalist since he
put the causa formalis back into matter where Aristotle had it, calling it the vis
essentialis. Under the influence of this essential force the embryo would develop
from a homogeneous primordium to a heterogeneous condition by a process of
‘vegetation’ (Wolff, 1764: 252): “. . . the different parts develop one after the
other, and they develop in such a way that one part is always secreted from or
deposited by the other” (Wolff, 1764: 210). Just as in Harvey’s system, one
organ formed becomes the material cause of the next one to develop.
During the eighteenth century there existed two different models of
embryogenesis, both identified as epigenetic by Charles Bonnet and his allies
and both, indeed, differing from his own theory of pre-existence by the claim
that the embryo was a formation de novo. I n fact, however, only Kaspar
Friedrich Wolff qualifies as a true epigenesist in Harvey’s sense. Development
starts from a homogeneous primordium, which becomes differentiated by a
predetermined, i.e. programmed, process of vegetation, of budding and
compartmentalization. In contrast, Buffon, Needham and Maupertuis all
qualify as atomists and hence as preformationists in the original sense of the
word. They considered the organism as being composed of atoms, derived from
the parents’ bodies and passed on by the male and female seminal fluids. These
particles were in principle interchangeable, which explained the phenomena of
ATOMISM AND EPIGENESIS
inheritance in hybrids as well as the origin of malformations-and
species.
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even of new
DARWINISM AND ATOMISM
Darwin’s theory of evolution was built on two fundamental aspects of nature:
variation and natural selection. To explain the phenomena of variation, Darwin
developed his theory of pan-genesis which, although construed independently,
comes very close to the views of the atomists of the French Enlightenment: “I
have read Buffon: whole pages are laughably like mine” (Ch. Darwin in Fr.
Darwin, 1887, Vol. 11: 375; see also Bowler, 1974: 175; Mayr, 1982: 694).
Darwin claimed that the cells of the adult body would give off invisibly small
particles or ‘gemmules’ which represent the material basis for the inheritance of
paternal and maternal characteristics.
The atomistic background was retained in early Mendelian genetics, as is well
documented by the analysis accomplished by Mayr (1982: 707-709, 713-714,
720). In modern population genetics it is the alleles that have become identified
as ‘atoms’, and the simple 1 : 1 relationship between hereditary elements and
traits of organisms has had to be replaced by elaborate statistical analyses.
Nevertheless, the Darwinian or Neodarwinian theory of evolution requires
that the organism be viewed in terms of variable traits to which correspond an
equally variable genetic basis. T o talk of traits or characters of a n organism
means to conceive of it as being composed of parts or ‘atoms’. To exemplify
evolution, a canon of comparison of organisms and their traits must be agreed
upon. This canon is the principle of homology, an instrument to decompose an
organic structure into constituent elements and to compare these in terms of
topographical relationships, thus establishing relations of coexistence (topographic homology) and of succession (phylogenetic homology) between these
elements or ‘atoms’ on the basis of their connection (Rieppel, 1980, 1985).
Naturally, Darwinism or Neodarwinism does not correspond to atomism in its
original sense. Traits of an organism may change whereas atoms by themselves
were considered immutable. From the viewpoint of genuine atomism, all
organismic change was to be related to a changing combination of immutable
and undivisible particles. Also, the Neodarwinian view of embryogenesis entails
development proceeding from a more or less rigidly mapped or predetermined
but macroscopically homogeneous primordium to a heterogeneous adult
condition by successive formation of parts. Still, it seems possible to identify an
atomistic background of the Neodarwinian view of evolution, both at the
genetic and phenotypic level, treating genes and characters as atoms
respectively and considering the organism as a particular combination or
juxtaposition of these (Webster, 1984). As the atoms are modifiable and
interchangeable, the organism can, in principle, vary and become modified in
all its details. Gradual, in the sense of continuous (Gingerich, 1984), change, if
not necessary, is at least compatible with the atomistic model of evolution. The
particular combination of traits or atoms is continually evaluated, as are
possible changes in their shape or juxtaposition, by natural selection: every step
in a new direction is immediately tested and either admitted and thus made the
basis for further change, or else rejected. Viewed from the atomistic perspective,
an increase in complexity will be conceived of as an addition of new organs, traits
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or atoms that open up new ‘faculties’ (e.g. new adaptational options). The
paradigm for this concept of increasing complexity is evolution by gene
duplication (Ohno, 1970)) coding for new and additional phenotypic traits.
THE MODERN CRITIQUE OF DARWINISM
One line of argument against the Neodarwinian view of evolution centres on
its atomistic background (Webster, 1984). This critique is coupled with the
field-theory of ontogenetic development as construed by Goodwin (1984a, b).
The development of an organism is no longer viewed as a process of
combination of a set of traits or atoms that have so far stood the test of natural
selection, and that will be tested again should any kind of change obtain.
Rather, embryogenesis is viewed as a creative process which comes close to the
original notion of epigenesis. Development proceeds by growth, compartmentalization and differentiation of the initially patterned but macroscopically
homogeneous primordium under the guiding influence of morphogenetic fields.
The organism resulting from such a developmental process is a “Tout organique”
(to use Bonnet’s terminology), subjected to natural selection as a whole and not
as a composite of atoms which may change independently from one another.
Natural selection loses its function as a guiding force of evolutionary change as
the organism loses the capacity to change atomistically or in small steps.
Evolutionary change is the result of changing morphogenetic fields which affect
orgunique”, that natural selection may either admit or
the organism as a LLTo~t
eliminate as a whole. As a consequence, homology can no longer mean relations
of coexistence and succession of parts or atoms, but in this context must
designate shared developmental pathways (Roth, 1984). Finally, increasing
complexity will no longer be understood as a result of the addition of new traits,
characters or atoms, but rather as an enhanced compartmentalization and
differentiation of the initially and macroscopically homogeneous primordium.
The evolution of the great diversity of arthropods from a homonomously
segmented annelid-like ancestor may serve as a paradigm (Raff & Kaufman,
1983: 25 1-261). There emerges the important theoretical consequence that an
epigenetic increase in complexity results from the creative ontogenetic process
and does not have to build on foregoing evolutionary steps along the ladder of
life.
MODERN PREFORMATIONISM
Katz & Goffman (1981) have criticized epigenesis by proposing a
preformationist model of ontogenetic development. By preformation they
understand the determination of conservative topographical relationships or
patterns during early ontogenetic stages, “regardless of how these patterns have
been generated” (Katz & Goffman, 1981: 443). Their notion of preformation
certainly does not match its original atomistic meaning, or the type of preexistence envisaged by Bonnet in an earlier phase of his theorizing. Rather, the
authors have identified an early predetermination of the ontogenetic process
which proceeds epigenetically. Where Harvey ( 1651) recurred to Divine
providence and Wolff ( 1764) to a vis essentialis, Katz & Goffman ( 1981) invoke
some genetic or cytoplasmic patterning of the zygote. The closeness of this
ATOMISM AND EPIGENESIS
339
system of thought to Bonnet’s theory of pre-existence is once more illustrated by
Bonnet’s designation of the germ as an “organized fluid” at a later stage of the
development of his theory (Bonnet, 1768, Vol. 11: 118).
On the other hand, the ‘genetic potential hypothesis’ proposed by Schwabe &
Warr (1984) qualifies as a theory of pre-existence not at the phenotypic but at
the genotypic level. According to these authors evolution loses its emergent
aspect and consists of nothing more than the successive actualization of genetic
information pre-existent since the beginning of life on this earth. In a similar
vein, Bonnet had envisaged a series of encapsulated “germs of resurrection”
from which new (but pre-existent) phenotypes would develop in the course of a
series of earthly revolutions that would alter the conditions of life on the surface
of the earth (Bonnet, 1769). The only problem was to explain how the
successively actualized organisms (of increasing complexity) would be adapted
to the changing environmental conditions. Bonnet had to recur to Leibnitz’
theory of pre-established harmony and to postulate that Divine providence had
not only fore-ordained the environmental effects of the successive earthly
revolutions but also the sequence of actualization of the encapsulated germs at
the moment of Creation. The adaptation of the pre-existent organisms to
changing environmental conditions was thus pre-established. I t will be
interesting to see how Schwabe & Warr (1984) are going to explain the
adaptation of organisms to changing environmental conditions during evolution
by the successive actualization of pre-existing genetic programmes.
ATOMISM VERSUS EPIGENESIS
Drawing on the analogy to the formation of crystals, atomists had described
the process of embryogenesis as one of juxtaposition and coalescence of
modifiable and interchangeable parts. Epigenesists such as William Harvey or
K. F. Wolff emphasized that ontogenetic development produces complexity and
diversity from a homogeneous primordium by the emergence of one structure from
another. Development entails growth (“budding”, “vegetation”) and compartmentalization or subdivision. Attempting a synthesis one might claim that
growth, compartmentalization, subdivision and coalescence are all to be treated
as attributes of an emergent developmental process, of which the polyp (Hydra)
has been evoked as paradigm by the various schools of thought ever since the
discovery of its reproductive and regenerative capabilities by Abraham
Trembley in the years 1740-1 741.
Such an amended notion of epigenesis still transcends atomism in an
important aspect, however: it entails a principle of individuation!
In the context of atomism, an organism represents nothing more than the
passing aggregation of atoms exposed to the contingencies of fundamental and
random variation accumulating from a historical process-everything is in flux:
“Naitre, vivre et passer, c’est changer de formes . . .”, writes Denis Diderot in his
Le Rtve d‘dlembert (1769). Not only is this view of life compatible with Darwin’s
adherence to “that old canon in natural history of natura non fucit saltum”, but if
extrapolated beyond the taxic level of the organism in the hierarchy of nature, it
must lead to a nominalistic species concept, as indeed is documented in
Darwin’s writings (see Mayr, 1982: 267).
The epigenetic or generative paradigm, on the other hand, emphasizes the
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uniqueness and individuality of each organism, of each ‘‘Tout orgunique”: each
form is created anew during ontogeny, complexity emerging from homogeneity,
the more specialized diverging from the more generalized. Ontogeny
corresponds to a process of individualization: “the ontogenetic development of an
individual corresponds to the growth of individuality in every respect”, wrote
the epigenesist K. E. von Baer (1828: 263). The programme of the
developmental process, genetic and epigenetic factors of canalization versus
developmental plasticity, determines the potentialities of the individual which
become actualized during ontogeny, and hence represents the principle of
individuation: it represents an element of ‘being’ in the process of ‘becoming’.
Gould (1983: 326-363) and Webster (1984: 206) have extrapolated this
principle of individuation beyond the taxic level of the organism, and have
consequently arrived at an essentialistic species concept. Distinct gaps between
classes of essentially similar tissues, organs or organisms are bridged by a
hypothesis of dichotomously organized developmental pathways. (See Raff &
Kaufman (1983) and Arthur (1984) for the role played by homoeotic mutations
in development and evolution, as well as Osborn (1984) for an example
concerning the dentition of reptiles and mammals.) Homologies do not concern
separate traits but developmental pathways (Roth, 1984). Shared developmental pathways are said to characterize species and higher taxa, and are
considered as “real essences”, the characters they determine being “nominal
essences’’ of taxonomies (Webster, 1984: 206). The extrapolation of the
epigenetic or generative paradigm to supraindividual levels of the taxic
hierarchy logically results in a break with the principle of continuity and in a
theory of saltatory evolutionary change corresponding to a pattern of a
dichotomously organized hierarchy of nature.
The current debate on the tempo and mode of evolution can thus be traced
back to contrasting paradigms of ontogenetic development. In view of the
uniqueness and individuality of each “ Tout orgunique” it must be borne in mind,
however, “that the relation of homology appears to be a category of the mind
only . . . to search in nature for homology is as futile as to search for identity”
(Nelson, 1970: 378). Homologies, whether they refer to separate traits or to
developmental pathways, entail the abstraction of universals from particulars in
order to reduce the multiplicity of appearances to a unified hierarchy of nature.
Atomism versus epigenesis, continuity versus discontinuity, gradualism versus
punctualism, process versus pattern, represent different ways of abstraction of
universals from individuals, and represent different ways of looking at nature
(Rieppel, 1985), or, in Gould’s (1982: 137) words: different ways of seeing.
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