A major challenge for every attempt to explain the generation of an organism is the question of form: How can the complex and astonishingly functional form of organisms come into existence, every generation anew? And how is it possible that this form is both constant, in the sense that offspring always resemble both parents, and variable, in the sense that we never find two individuals exactly alike? Not even monozygotic twins or two bacterial cells resulting from the division of the same mother cell are identical. With all our knowledge about the principles of genetics and molecular biology today, we still cannot claim that we have found one satisfactory theory that explains all aspects of the development of a complex body, with its different organs and tissues, its properties and abilities. But maybe the lack of such an all-embracing theory of biological development does not have to be regarded merely as a consequence of the still insufficient knowledge about the factors involved, but as a conceptual necessity resulting from the mere complexity of biological phenomena. As Michel Morange and Evelyn Fox Keller have pointed out recently (Keller Reference Keller2002; Morange Reference Morange2005), this complexity might indicate that biological research has to adopt a pluralistic approach, admitting the co-existence and usage of various, even conflicting theories, to be able to capture the multiplicity of relevant coherences at the different levels of organismic organization. The conceptual hybridity and eclecticism of eighteenth-century theories of generation might in fact be a distinctive trait of any attempt to explain the phenomena of life.

Most of what we now consider to be the major distinct theoretical positions on generation are in fact “patchworks” which comprise key elements of the physiological debates of the eighteenth century. An influential contribution to these debates was made by the German embryologist Caspar Friedrich Wolff (1734–1794), whose theory of generation presented a novel explanatory approach to the scientific treatment of developmental processes. In the last decades of the eighteenth century he attempted to establish a new doctrine in medical science to complement anatomy and physiology: the doctrine of generation.

Born in 1734 in Berlin, the son of a master craftsman, Wolff matriculated in 1753 from the Collegium medico-chirurgicum in Berlin and continued his studies at the University of Halle, where he obtained his doctorate in medicine with his dissertation Theoria generationis in 1759. Here he first presented his views on the science of generation, which he developed further in the following years in Berlin. In 1764 he published the Theorie von der Generation defending his views against the critical objections mainly brought forward by Albrecht von Haller (1708–1777) and Charles Bonnet (1720–1793). Wolff's aim in these publications was twofold. On the one hand he intended to give a conclusive account of generation as a process, in order to refute the idea of preformation. His argument here was that if he could deliver the evidence for the gradual development of organismic structures and organs by presenting the different intermediate stages that the developing organism undergoes, the processuality of generation would become evident, thus excluding the possibility of the existence of already pre-formed structures and organs.Footnote ¹ However, this was not his main intention, but rather a necessary precondition of his main aim, which was not only to prove the reality of epigenesis but to deliver a comprehensive theory of the way in which, and the causes by which, organic bodies are formed (Wolff [Reference Wolff1764] 1966, 36ff.). For Wolff, generation relies on two factors: substance and the force acting on it (vis essentialis).Footnote ²

Wolff encountered difficulties when he tried to establish himself as a scientist in Berlin, perhaps due to his inopportune views concerning generation. Therefore he accepted a professorship for anatomy and physiology and membership in the Academy of Sciences in Saint Petersburg. Here his scientific work mainly focused on comparative studies of the internal organization of monstrosities, but he still published two additional important treatises on the theory of generation. In 1768/69 he corroborated his theory with new observation-based evidence, describing in great detail the development of the intestines in chickens in the treatise De formatione intestinorum. In 1789 he further specified his views on the nature of the vis essentialis, which constitutes a main pillar in his theory of generation. This treatise, Von der eigenthümlichen und wesentlichen Kraft der vegetabilischen sowohl als auch der animalischen Substanz, was written on the occasion of the prize question of the Royal Academy of the Sciences of St. Petersburg in 1782 and is a complementary comment to the two winning essays,Footnote ³ in which he presented his own answer to the question: “Which force causes the movements of the juices in the plants and in the . . . parts of the animal's bodies and what is the reason behind this force?” (Petersburg Reference Petersburg1789).Footnote ⁴

Following the terminology of his contemporaries, Wolff defined physiology as “a science of the functions of the organic body” (Wolff [Reference Wolff1764] 1966, 12). He defined anatomy as the historical science of the body, in which “historical” means a science which merely describes the properties of the body, without scrutinizing their causes and functions. The new science of generation, in contrast, would have to act on the organic body and its parts and demonstrate “how these parts came into existence, along with the interconnections between them” (ibid., 7). His theory aimed at refuting the doctrines of “Prädelineation” (ibid., 27), a term he used to refer to the different theories of preformation, the predominant doctrine of generation since the end of the seventeenth century. This, so he claimed, did not explain the development of organic bodies at all, but instead denied it (Wolff [Reference Wolff1759] 1896a, 4). At the same time he tried to confute the arising vitalism as put forth by Johann Friedrich Blumenbach (1752–1840). This doctrine, which refers to a nisus formativus as a specific irreducible principle acting solely in organic matter and responsible for the specific order and function of organic parts within the body as a whole living organism,Footnote ⁵ rests on assumptions which Wolff considered to be unintelligible.

Even if Wolff's theory in its total conception is, then, broadly located in the traditions of his time, being philosophically indebted to the rationalism of Christian Wolff, and biologically indebted to the physiology of Albrecht von Haller (Roe Reference Roe1979; Roe Reference Roe1981); and even if, conversely, his theory was extremely influential in the development of forms of vitalism in the nineteenth century, we need to see that his approach to the problem of organic form was highly original. It cannot properly be described as either “materialist,” “mechanicist,” or “vitalist,” but rather as a novel, “process-oriented” point of view in the debates on generation, which treats the emergence of the organism's form as a mere epiphenomenon resulting from the correlation of matter and function. I will seek to show that Wolff's “processual” account of generation can be regarded as an early concept of self-organization of matter. By concentrating on the process of generation as cause for the form of the organism, with the latter resulting from the functional interactions of its material constituents, among the defenders of epigenesis his approach adds a new component to the materialist point of view, which concentrates mainly on the properties of organic matter and the forces acting on it, in order to explain the generation of organisms. And it differs from those approaches that refer to a specific organizing principle acting solely in living organisms, as for example Buffon's moule intérieur or Blumenbach's nisus formativus.

From Pre-existence to GenerationFootnote ⁶

The theory of pre-existence is probably the most straightforward solution to the question of the origin of organic form ever given in the history of the life sciences. It was developed as the theory of emboîtement or “evolution”Footnote ⁷ in the late seventeenth century by naturalists such as Reigner de Graaf (1641–1673), Jan Swammerdam (1637–1680), and Nicolas Malebranche (1638–1715) (Jahn Reference Jahn and Jahn2000). Referring to the observation that in plants like the tulip it is possible to find the future plant already formed in all its parts within the seeds, and then extending this finding also to the animal kingdom, Malebranche comes to the conclusion “that all the bodies of men and of animals, which have been born up to the consummation of the century, have perhaps been produced as long ago as the creation of the world” (Malebranche, cited in Roe Reference Roe1981, 5). Just as each organism contains within itself all the seeds of the future generations of his offspring, so each seed contains all the future seeds encased within each other like an endless set of Russian dolls.

The idea of pre-existence can be regarded as a response to Descartes’ mechanistic approach towards epigenesis, according to which the structure of bodies can be traced to the laws of motion. Briefly put, the most subtle parts of the parents’ seminal fluids merge and undergo a process of fermentation. The fermentation causes the movements of the parts, which build the different organs and parts of the body according to their forms and properties. A major weakness of Descartes’ attempt to give a materialistic explanation of generation, which was also pointed out by Malebranche and others, lay in the difficulties he met in explaining the way by which subtle parts could organize themselves to form and link the parts of the animal by no other means than the laws of motion (Malebranche [Reference Malebranche1688] 1965). Even by postulating the existence of a multiplicity of Ur-particles with multiple forms and properties, he did not succeed in giving a convincing explanation, either for the differences in the forms of organisms (such as the origin of the difference between male and female bodies), or for the constancy within or the differences among the forms of species (Roger Reference Roger1962, 87f.). Thus this attempt to explain the form of organisms only by reference to the organism's specific matter, transmitted from generation to generation by the parents’ sperm in their relation to the universal laws of nature, had failed.

As Shirley Roe has pointed out, the subsequent success of pre-existence theories, and thus the concomitant exclusion of developmental processes from biological thinking, was not in fact the result of biological, but of philosophical, imperatives such as the methodological strictures of Wolffian rationalism, that is, the doctrine of Christian Wolff (Roe Reference Roe1981, 7f.). There was, also due to the development of anatomical sciences and the invention of the microscope, a growing awareness of the huge complexity on every level of the organization of organisms. This resulted in the feeling that the mechanical laws, to which Descartes referred, were not sufficient when applied to organisms. The immensity of the explanatory task was still felt by Johann Wolfgang von Goethe, by the end of the eighteenth century, when he exclaimed: “Here it will become necessary to adulate the theory of emboîtement as the human understanding is hardly able to apprehend certain phenomena in another way, although even this emboîtement remains beyond comprehension” (Goethe [Reference Goethe1788–1794] 1987, 94).

In the context of pre-existence, the origin of the form of organisms did not lie within the scope of scientific research. As the germs of every past, present, or future organism were already fixed within the first organisms, and the first organisms were created by God, it was evident that the divine act of creation would have to be regarded as the direct cause for the form of every organic body ever living on earth. The formation of living bodies was thus no longer subject to the laws of nature and was therefore completely independent of the properties of the matter constituting the body, the constitution of the body itself, or any earthly influences affecting it. While there was no longer a process of successive development of organisms and their parts, but only a (mechanical) process of growth to consider, scientific effort in the following years could concentrate on deepening the anatomical and physiological knowledge about living bodies, without having to worry about their origin, neither of the immense complexity of their structures nor of the functional relationships among their parts.

Pre-existence in fact met with many objections, and from the 1740s onwards there were attempts to introduce alternative theories of generation. The approaches that received most attention, initiating controversial debates in their time, were those of Pierre-Louis Moreau de Maupertuis (1698–1759), Georges-Louis Leclerc de Buffon (1707–1788), John Turberville Needham (1713–1781), as well as those of materialists like Julien Offray de La Mettrie (1709–1751), Denis Diderot (1713–1784), and Paul Henri Thiry D'Holbach (1723–1789), and, by the end of the eighteenth century, the vitalism of Blumenbach and of course the theory of generation of Caspar F. Wolff.

Caspar F. Wolff – Vis essentialis, Substance, Matter, and Function

At the turning point between the theory of pre-existent germs and the epigenetic development guided by a nisus formativus, Caspar Friedrich Wolff presented his epigenetic theory as an attempt to completely illuminate the black box of embryonic development and its causes. It was his aim to give a valid explanation for the generation of organisms by showing its general principles, which he claimed only Descartes had tried to achieve before him, in De homine & formato foetu (Descartes Reference Descartes1692), but had failed. Thus he did not recognize any of the alternative contemporary theories of epigenesis even as attempts towards an explanation of generation.

Caspar Wolff was a disciple of the rationalist philosopher Christian Wolff (1679–1754), and he relied on the latter's mathematical, logical, and deductive-demonstrative approach towards the explanation of natural phenomena. But the embryologist Wolff maintained a conception of nature as a dynamic and creative instance against the conception of a passive, already preformed and in all its further developments predetermined nature, which is only acted upon, but has no inner principles of activity:

Within such a “miserable nature” any attempt to find an explanation for the generation of an organism would be vain, as one could merely describe ongoing processes, but could never find any natural causes behind the mechanism. Such a nature would not be accessible for any logical, rational scientific research, as it would almost immediately refer the researcher to God as the creator of that strict natural order.

Regarding nature as essentially explicable in a well-ordered logical and systematic way, Wolff aims to establish clear-cut concepts and to precisely narrow down the phenomenon to be explained before developing his theory. Therefore he starts his dissertation, Theoria generationis (1759), in which he presents for the first time his observations and develops his epigenetic theory of generation, with an introductory chapter to establish the object of his research and to define the main notions that he will refer to in his theory. Also his next publication, the Theorie von der Generation (1764), written at the request of his students as a popular compendium of his theory and enriched by supplementary material to answer the criticisms of Haller and Bonnet, begins with a chapter, “The notion of a theory of generation,” where he once more sets out his expectations about what a theory in general and what a theory of generation in particular should comprehend.

The theory C. F. Wolff presents as a theory of generation in the methodological tradition of German rationalism could be described as a process-oriented theory. He defines generation as “the manner by which an organic body (a plant, an animal) in all its parts is generated with the help of another organic body of the same species” (Wolff [Reference Wolff1764] 1966, 7). To give an explanation, and not a mere description of the ongoing processes, according to him it is requisite to make out the necessary causes that allow the deduction of every step in the formation of the body and thereby its final form. By demonstrating that with his theory, based on natural forces and organic substance as the sole necessary causes, he is able to give a satisfying explanation for every observed step during the development of a living body, he aims to confute the doctrine of pre-existence, to rehabilitate development as a natural process accessible to scientific research and explanation, and to pave a way for further empirical research about generation.

Wolff claims to be able to explain all the necessary causes which allow the deduction of every step in the formation of the body and thereby its final form, by reference to the ongoing processes during the formation of an organism. The chief focus of his theory is the dynamics of matter in motion; he seems to regard organisms and their formation as analogous to geological processes and the formation of landscapes,Footnote ⁸ rather than, as was quite current in the eighteenth century, analogous to a machine. The view of the organism as a machine, which depends in its form and vital functions on the shape and the composition of its parts is for him an “imaginary system, that is to say, one to which nothing in the nature of things corresponds” (Wolff [Reference Wolff1759] 1896b, 72). Still he quite frequently refers to the adult body as a “machine.” But for Wolff the machine is the result of a developmental process of inorganic substance. A developing body is not a machine yet, “and the developing substance is to be distinguished from the machine in which it is enveloped” (ibid., 70). So why does Wolff refer to the body as a machine at all? And how does mere substance, which he describes as undifferentiated, “homogenous” substance,Footnote ⁹ generate the machine?

As the quotation above clearly shows, Wolff does not use the term “machine” when referring to the body to draw an analogy regarding its configuration, organization, or generation. The analogy he draws instead regards the functionality of machines, as compound entities that perform work, and as tools to be used to perform certain tasks. In the first sense each living body, be it the body of a simple plant or any higher animal, can be regarded as a machine, in so far as it is able to transport the nutritive fluids. Besides this, some of the animal's organs contribute by their mechanical mode of function to the general process of nutrition, as e.g. the teeth or the stomach. In the second sense the bodies of higher animals and especially humans might be understood as machines, “utilized” by their soul to perform conscious acts, to enable sentience and thoughts. The soul is regarded as a causal entity exterior to the developing body (ibid., 66). In contrast to Stahlian animism, Wolff holds that the soul acts merely as an accelerator or retardant on the movements of the body liquids, thereby affecting its actions; it does not have any influence on the determination of the structure and its generation. From this point of view the generation of a living body and its parts is not related to its functions. The form does not emerge with regard to any particular performance, as a machine is constructed in order to carry out a specific kind of work.

Thus for Wolff there is no necessary correlation between the form of organisms – whether as a whole, or in their constitutive parts – and their function. Instead he seems, at least in his earlier writings, to regard form and function as two clearly separable phenomena. He makes this point quite clear, when he justifies his classification of the different tissues and organs in the Theorie von der Generation:

The generation of organismic forms and the function of these forms must not necessarily be regarded as being causally linked; they can and for the sake of scientific clarity even must be considered separately. The vegetative movements of the body, including nutrition, generation of form, growth, and reproduction for Wolff belong to one and the same causal chain, where one movement can be deduced from another, with nutrition as the fundamental basis. For the animate movements, including involuntary and voluntary movement of the body, no link to this causal chain is considered to be necessary.Footnote ¹⁰

But how are those vegetative machines generated from homogenous substance? Wolff claims that for the explanation of the generation of organisms two responsible factors have to be considered above all: an essential force, the vis essentialis and the ability of plant and animal fluids to solidify. The essential force, in the case of already fully developed plants, attracts the nutritive fluids from the environment and disperses it through the body, accumulates it at different places, and eventually causes the excretion of excessive fluids. On its way through the body the nutritive fluids solidify, building up thereby the organic structures of the body: in plants the vessels and vesicles leading to the formation of all the different organs as the root, the trunk, the leaves, etc; in animals the arteries, the veins, leading to the muscles, nerves, bones, intestines, etc.Footnote ¹¹ That this process results in well organized and specific but not in contingent or amorphous structures is due to the fact that the vis essentialis is not primarily conceived as a motive force acting haphazardly on the nutritive fluids. It is a force that is inherent to the organism's substance, and as Wolff specifies in his treatise Von der eigenthümlichen und wesentlichen Kraft, it is a force that is characterized by its specific attractive and repellent effect as well as by its property not only to attract similar and repulse unlike substances, but also to integrate the attracted substance within the attracting substance. Those two properties of the vis essentialis enable not only the simple growth of organic substance by attraction and integration of similar parts from the nutritive fluids to the substance of the organism, but are also the cause of an “organizing nutrition,” which is “the effect or performance by which the vessels and vesicles in a part are formed” (Wolff Reference Wolff1789, 61). This force, even if it was considered as specific for organismic substances, was thus not a particular vital force. It is derived from the single attractive and repulsive force, acting in nature, differing only in the degree of its action:

The organism's substance will, according to this theory, attract those constituents of the nutritive fluids it has taken up from the environment and incorporate them. As thereby the fluids have to make their way through the already present substance, gradually channels will arise by the mere pressure of the fluids, as long as the substance is still fresh and not yet solidified. These channels, which will build up the system of vessels, thus will develop in such a manner that they always lead to those points of the organism where nutrients are needed and growth takes place. In every new developing part, they are the first organizing structures to appear. Based on the distribution of the nutrients, the interplay between their incorporation in the existing structures and growth and solidification begins: The slower a substance solidifies, the longer it stays flexible and thus is able to attract and incorporate new substance. This interaction is decisive for the form that any new parts of the organism adopt. The different parts of the body thus arise one after the other, one being excerned or deposed by its precursor beforehand as a mere “lump of matter” (Wolff [Reference Wolff1764] 1966, 162), which later is organized by the formation of the new vessels. Here the above mentioned analogy to a geographic process of the formation of a landscape by erosion and sedimentation is evident. And while the attractive effect of the essential force is responsible for the development of all those parts of the body, which contribute to the distribution of nutrients, its repulsive effect will cause the formation of secretory organs, as e.g. the liver or the kidney in animals.

The growth of the organism will go on until either the organism has reached the limits of its growth, not being able to incorporate new substances above the limit of mere nutrition any longer, or a deprivation of nutritive fluids takes place. The limit of growth is caused by the increasing solidification of the substance, which impedes the transport of the fluids. It usually leads to the formation of imperfect body parts: the fruits in plants, with the semen as the most imperfect form and the egg in the female animals. The pollen of the male plant, or the male sperm on the other hand constitutes the most pure nutrient, the nutrimentum perfectum, as it underwent all stages of purification that a nutrient can possibly undergo. If thus the pollen meets the fruit (or the sperm the egg), nutrition and growth will immediately start anew and a new organism will form.

The main factor in the determination of the form of an organism according to Wolff is its substance. The distinction he draws here between the “substance” of the organism and the “nutrient fluid” is most significant. To fully understand the importance of the concept of substance in Caspar Friedrich Wolff's theory of generation, recall the huge influence of Christian Wolff's rationalism on his conception of a scientific approach and on his logical and empirical methods. “Substance” for Christian Wolff refers to a matter that possesses specific qualities determining its nature (Christian Wolff Reference Wolff1730). As Shirley Roe has pointed out, C. F. Wolff applied Christian Wolff's concept of substance to his view of the organism: “qualified matter determines the specific nature – the species – of the organism, just as the specific natures of all substances are determined, according to the principles of rationalism, by their inherent qualities” (Roe Reference Roe1979, 39). Thus it is the specificity of the organic substance, which for example effects the rate and degree of its ability to solidify as well as the degree of efficacy with which its essential force acts. The essential force of the substance of an animal for example differs in degree from that of a plant, and also differs as well in its ability to solidify, which causes the differences between the two kingdoms. But the substance does not contain any specific information about the form of its “machine” which is going to be built. It is a mere potentiality, determining the conditions for formation, not the form itself.

It is via the processes, initiated by the essential force as the dynamic component of the substance, that the form is finally determined. These processes do not depend on the material properties of specific parts interacting with each other; they are not classical mechanical processes. They could rather be designated as processes of fluid mechanics, as Wolff also classifies the resulting bodies, as “hydraulic machines”:

The cause he refers to is the vis essentialis, which acts in an attractive or repulsive manner on the nutritive fluids, to either become integrated in, or expelled from, the substance. Thus the parts obtain their form while realizing, and at the same time in order to realize, the function of the vis essentialis, which finally is the nutrition and reproduction of the substance. The form of the organism arises as a result of the two main properties of organismic substance, the vis essentialis and the ability to solidify in a dynamical process of self-organization.

How to Relate the Generation of the Body and the Function of the Form

The debate concerning the generation of organisms in the late seventeenth and eighteenth centuries can also be understood as a contribution to the scientific process of formulation of “research strategies” in investigating vital phenomena. So it was not only necessary to distinguish the phenomenon specific to living nature, but also to consider whether and how these phenomena could be related. We are accustomed to regarding the problems of generation, form, and function as interdependent, even though there is no agreement yet about how exactly the variables for this rule of three are to be linked. But most of the eighteenth-century defenders of epigenesis, like Wolff, regarded generation as a phenomenon that can be treated as causally isolated from other properties and features of living bodies. According to the theories of Maupertuis, Needham, and most French materialists, the body is not involved in the process of its own development. The cause for its generation and development to a specific final form is attributed either to the properties of the organic matter, or the forces acting upon it, or both. The developing form itself does not contribute to its further development; the bodies are envisaged as “put together” rather than as developed. Any body with its specific form, its properties, and its faculties, is nothing but the sum of its parts, brought together by specific forces. Even if in those theories there may be a link between the generation of the organism and its parts and their functions, insofar as they may emerge from their organization, there is never a link back from the functions to the generation of the organism and its parts.

The case is a little different for George-Louis Leclerc, Comte de Buffon. Buffon put forth a theory of epigenetic generation, with three main variables: organic matter, a penetrating force that “pushes or attracts the organic parts of the nutrient” (Buffon Reference Buffon1749, 45), and an internal mold (moule intérieur). This mold contains, or rather represents to itself, the information regarding the internal and external form of the organism, and the distribution, organization, and assembly of its parts. The penetrating force passes on to every new organic part penetrating the body as nutrient, the exact information about its position in relation to the whole, so that every part of the organism is defined by the mold, while the mold itself is constituted by the interaction of all the parts of the organism. Thus it seems that for Buffon generation is seen not as an isolated phenomenon that leads to a certain form, but rather generation and form are seen together in a certain reciprocity. Nevertheless, the instance which is responsible for this correlation between the parts and the generation of the whole, the instance that establishes the order which is the basis for the arrangement of the parts in relation to a whole with certain functions, is beyond the range of any further explanation. The internal mold eludes all attempts at explanation, but serves itself as a ground for the explanation of the different processes involved in the formation of the body. So, even if we seem to have a circular relationship between the parts and the body that could constitute the unity of the body, the cause for this unity is still beyond the scope of our understanding.

We find a similar circularity in Blumenbach's vitalist approach towards generation. But here what is emphasized is the relationship between the function and the form of the parts and the whole to be generated. One of the main concerns of the vitalist approach towards generation, as formulated by Blumenbach and then advocated by Kant (Kant [Reference Kant1790] 1997, § 81, A370ff., B374ff.), was to demonstrate that the generation of a body and the function of its parts are simply two facets of one problem. Blumenbach had claimed that each organism is formed by the action of a formative or “building” power, the nisus formativus, from the procreational substance (Blumenbach Reference Blumenbach1791). Each formative process in an organism is regarded as being due to a special kind of force, a vital force, acting in the organism to generate its form, during embryonic development as well as in any later regenerative growth. The organism is considered to be an entity resulting from a teleological process, as a completely functionally organized whole, where every part has its specific function in relation to the whole, nothing being contingent or gratuitous. Thus the form of the whole is just as dependent on its different parts and their interactions as the form and function of the different parts is dependent on the final whole that they constitute. But such an organization cannot arise spontaneously: a guiding instance is needed to ensure the realization of the final entity.

So what constitutes the unity of the organism is the function of the parts in regard to the whole. This unity is brought forward by the relationship of the different discerned phenomena of organisms: their form, function, and generation. But the price to pay for trying to establish this relation is the proclamation of a vital force, an architectural, prearranging force.

C. F. Wolff rejected using such a force to explicate generation. He considered the assumption of a constructive force, as Blumenbach had suggested, as scientifically unsound, for two main reasons: First of all, referring to a force that is not scientifically accounted for as an explanation for any phenomenon does not explain anything, but rather declares the phenomenon to be inexplicable. And furthermore, Blumenbach's constructive force would, in Wolff's view, have to be a multifunctional force, as it is referred to as the single and ultimate explanation for phenomena as diverse as the performances of the vegetative bodies. Here the force would have to act in different places and in different situations in different ways to realize, e.g., the generation of the different parts of the body: “Therefore each formative force . . . which shall cause, merely by itself and according to its nature, the generation of an organic body, not requiring any concurring causes, is indeed impossible and no human understanding will be able to demonstrate in any way its possibility or to deny its impossibility” (Wolff Reference Wolff1789, 66).

As we have seen, Wolff's theory separates the explanation of the generation of an organism from the explanation of the function of the organs that are thereby generated. Even if he considers the possibility of a connection between the mechanisms of generation and the mechanisms of certain functions, he never considers any feedback from function to form and therefore from function to generation. The different parts and organs of the body can not be regarded as being functionally related, as they are exclusively imagined on the basis of a linear causal chain with nutrition as an initial point. Form for Wolff appears as a mere epiphenomenon of the material processes taking place. But nevertheless his theory seeks to articulate an inner correlation of the parts, which defines the body as a unit. He presents generation as a causal chain, where each step in development is the basis for all further steps. The younger parts of the body successively emanate from the older ones; the organism as a whole emerges from a real developmental process, where neither the location nor the formation of any part is already prescribed. Therefore the body here appears as a historically elaborated unity, and not as a mere assembly of parts as we saw in the theories of Maupertuis, Needham, and the French materialists.