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Jenkinson JW. Experimental Embryology. (1909) Claredon Press, Oxford.

Jenkinson (1909): 1 Introductory | 2 Cell-Division and Growth | 3 External Factors | 4 Internal Factors | 5 Driesch’s Theories - General Conclusions | 6 Appendices
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Chapter V

=Driesch’s Theories Of Development - General Reflections And Conclusions

To the inquirer into the causes of development the central difiiculty must always be the problem of difierentiation. Growth and division of the nucleus and the cell, processes which always accompany differentiation, are, as we have seen, side issues ; but the increase of structure, the production of form out of the relatively formless germ, and the gradual passing of this into a new individual which is like the parents that gave it birth, this is the marvel which has always excited the wonder of the observer, and demands all his wit to understand and to explain.

Experimental investigation, as far as experiment has at present gone, has shown, first, that a certain complexity of the physical and chemical environment is a necessary condition of normal development ; that complexity may, it is true, vary within certain limits, but those limits can only be transgressed under pain of abnormality or death. In the second place it has been demonstrated that the initial structure of the germ, and the mutual interactions of its parts as they develop are both indispensable internal factors. It now only remains for us to discuss the value of those theories which are not only attempts to explain, to give the most general account of, the phenomena in causal terms, but also serve to provide a light to guide the investigations of the future. One such hypothesis we have already examined in some detail, Roux’s hypothesis of selfdiffercntiation. According to this belief not only is the development of each part determined by causes which reside in itself alone, but the parts—or rather their determinant representatives, which are all ea: /1._1/pot/Eesi necessarily present ab 2'mYz'o in the undeveloped germ—are located in the nucleus. The qualitative division of the nucleus sunders these units from one another, which then determine the characters assumed by the cytoplasm, and so the whole process happens.

As we have seen, the hypothesis, in this its original form, is untenable. Quite apart from the fact that the complex architecture of the nucleus still demands an explanation—an explanation of the same kind, perhaps, which would at once involve us in an infinite regress—the facts which experiment has brought out show conclusively that nuclear division is never a qualitative process. In the other direction the hypothesis errs in attributing a homogeneity, an isotropy to the cytoplasm, for the same experiments have proved the existence in the ovum of definite substances, necessarily concerned in the production of the primary organs of the embryo.

There is, however, no evidence to show that--as imagined in Weismann’s, and to a certain extent in Roux’s hypothesis—there

is a separate morphological unit for each separately inheritable‘

character of the species; such an idea would indeed seem to be precluded by the ease with which, in some cases at least, the germ may be divided into parts, each of which is endowed with the potentialities of the whole.

And yet development is specific. How, then, is this mechanism of inheritance to be conceived of? It is to Hans Driesch that we are indebted for an exhaustive attempt to think out the whole problem. In his Aualytiec/Le T/teorie tier organise/len Entwic/clzmg Driesch starts with the facts with which we are already acquainted, the similarity of the nuclei, the dissimilarity of the cytoplasm in the several regions of the developing germ.‘

The arrangement of these dissimilar substances determines first of all an axis in the egg, an axis with unlike poles ; around this axis the cytoplasm is radially symmetrical or isotropic, but in the direction of the axis it is not ; or, as Boveri puts it, there is a ‘stratification ’ of the substances of the egg at right angles to the axis, the concentration of the animal substance decreasing towards the vegetative pole, that of the vegetative substance in the contrary direction. There are cases (Coelenterates, Sponges, Ctenophora) in which this radial is the only symmetry; but in other types (Bilatcralia) a third point may be established by the disposition of some special substance (the grey crescent in the Frog"s egg or the yellow pigment in the

1 It may be mentioned that in 1887 Platner had already denied the existence in development of any qualitative nuclear division.

Ascidian Cyntfiia, for example), or by the arrangement of the blastomeres (as by the large posterior cell of Annelids and Mollusca), and this point, together with the axis, determines a plane about which the ovum is bilaterally symmetrical. The axis and the plane of symmetry of the egg are definitely related to the axis and symmetry of the embryo, the substances to its primary organs.

Further, in very many, though not in all, instances the parts of the ovum—-blastomeres or egg-fragments——are totipotent ; and the same is true of the parts of elementary organs like the archenteron of Echinoderms or the optic vesicle of Amphibia. The totipotence is, however, sooner or later lost, and this limita

Fig. 165. — Diagram to illustrate Driesch’s conception of the minute structure of the (Echinid) ovum. It is supposed to be composed of particles all similarly polarized and oriented to the whole. In A onemeridional—half is shown. In B this has become spherical and the parts have been disturbed. In 0 they have regained the original orientation. (From Korschelt and Heider.)

tion is apparently due to the way in which the substances are distributed in the ovum, an explanation which seems to be accepted by Driesch for most cases. But in accounting for the phenomena in the Echinoderm egg, the form with which he himself has chiefly experimented, he urges a different hypothesis. Here he conceives of the egg as composed of like particles, each of which is polarized and oriented in the same manner as the egg itself (Fig. 165), and consequently the only limitation to totipotence is due to size ; any isolated part that is not too small can develop into a whole as soon as its polarized particles have reassumed a similar orientation. Again, it is stated that the blastomeres can be disarranged to any extent without interfering with the normal development of the larva; they are all equivalent, without limit, at least until the ectoderm and endoderm have been difierentiated ; any part can contribute to the formation of any organ: ‘ jeder Theil kann jedes.’

This conception, of the absolute equipotentiality of the parts, as we have already had occasion to remark, is erroneous; but for Driesch it is of the first importance, for it dominates, as we shall see, all his theoretical speculations.

The position of the embryonic axes and primary org-ans being thus determined in the whole egg (or its isolated parts), it is still left to inquire into the causes which decide the destinies of the remainder. Driesch’s answer to this question is twofold: there

FIG. 166.—Diagram to illustrate the possible part played by stimuli (‘inductions’) in ontogeny, and by ‘position '. A, B, and C’ are three larval organs (ectoderm, endoderm, stomodaeum). C may exert a stimulus on that part of B nearest it; this part, reacting to the stimulus, becomes (3. Were the position of C’ altered the position of B in the cquipotential system B might be altered too, so that the fate of any part of B would be a function of its position relative to 0. So, under the influence of B, part of A may become a. (After Driesch, 1894.)

are two possible factors, one is ‘ position ’, the other ‘induction ’. In the case of the first, the destiny of a part is imagined to be determined by its distance from the system of points already established, ‘ its fate,’ so runs the famous formula, ‘ is a function of its position in the whole.’ It would, however, be absurd to suppose that the behaviour of any one of a number of precisely similar bodies could depend upon its mere geometrical position. The points already diifcrentiated——the animal pole, for instance— must be supposed to exert an influence with a force which is some function of the distance upon the parts which are at present equivalent, and so to excite their differentiation 1 Fig. 166).

Properly speaking, therefore, this factor of ‘ position ’ belongs to the second category of ‘ induction ’.

An ‘induction’ is simply an effect produced upon the parts that are developing by other parts, or possibly by some factor in the external environment. These inductions are, according to Driesch, of the nature of those events which are brought about by the addition of a single antecedent condition, an ‘occasion’, to an assemblage of antecedent conditions, as a spark fires a rocket or the movement of a lever sets a piece of machinery in operation; or, in the language of physiology, the causes of these inductions are stimuli, the efiects, reactions or responses,

Fig. 167. — Diagram to illustrate Driesch's hypothesis of the part played by the nucleus in development. A is the cytoplasm, '1‘ its totipotent nucleus, ml, ac, are stimuli.

A is sensitive to a:, and becomes A,, T is sensitive to A, and is set in activity (Ta). A‘ being sensitive to Ta becomes A1, and A, is sensitive to .L‘._., and so on. (After Driesch, 1894.)

the quality and the quantity of which are determined not only by the stimulus but also by the nature of the reacting body itself. The analysis of these ‘ inductions ‘is not, however, yet complete. Occurrences of this kind imply the stimulus, the reception of the stimulus and the response. The first of these is some other organ or some external agent; the second and third are functions of the organ which is to be produced; and Driesch imagines that while it is the cytoplasm which is receptive of the stimulus, it is the nucleus which makes the response. Each totipotent nucleus is supposed to contain ferment-like bodies; under the influence of. some stimulus received through the cytoplasm some one of these is called into action; this works on and alters the cytoplasm; this then becomes receptive to a new and dilferent stimulus, and so on, as shown in the accompanying diagram (Fig. 167). The cytoplasm thus becomes perpetually altered, while the nucleus retains the entirety of the potentialities of the organism, a supposition which is held in reserve to account for the phenomena. of budding and regeneration.

The progressive limitation of potentialities, therefore, which we observe in ontogeny is a limitation of sensibilities to stimuli, equality of potentialities an equality of such sensibilities. In an ‘ equipotential system’ (the blastula or the archenteron of Ec/linus) all (meridional) parts are equally susceptible, and what stimulus each will as a matter of fact receive is a function of its position in the whole, that is to say of its distance from the centres of force situate in the dissimilar points fixed ab initio in the system of symmetry of the egg (Fig. 166).

Development then proceeds from the comparatively simple organization given in the structure of the fertilized ovum by the creation of ever-increasing complexity by the action and reaction of the parts on one another. Each ontogenetic eifect produced becomes in turn the cause of further eifects, the possibility of fresh specific action, for it becomes the seat of a new specific stimulus and response, and so on until the complexity of the ‘ ultimate organs ’ of the adult is achieved.

In order, however, that differentiation may be normal it is clear that these stimuli and these responses must be accurately coordinated: the right stimulus must be ready at the right time and at the right place for the right organ to respond to. This indispensable temporal and spatial co-ordination is the ‘ Causal Harmony ’ of development; and it is given by the initial structure of the egg and the constitution of the external environment. ’

This is, however, not the only kind of harmony involved in development. Potentialities become restricted as differentiation progresses, and the development of the primary, secondary, tertiary and subsequent systems of organs successively produced is a process of ‘ self-diiferentiation ’, depending on factors residingin each system itself. Nevertheless these independent systems frequently unite later on to form complex organs, and the necessary co-ordination of these is a ‘ harmony of composition ’.

Such, in brief, is the idea worked out in the Analytiec/ze Theorie. On this hypothesis development is an epigenesis ; it involves a super-additio partium, an increase of structure, a creation of fresh form out of the simple cytoplasmic organization of the egg, and the causes which operate in this process of morphogeny are just that initial structure and the stimuli which the parts respond to, stimuli which may proceed from the parts themselves or from the world outside.

These responses are primarily physico-chemical and only secondarily structural, and they happen in accordance with a pre-established harmony. Only in this sense can development be described as evolution; but this is not the evolution of the preformationists of the eighteenth century, nor even that of the school of Roux and Weismann, less gross, more subtle, but still morphological; rather it is the realization of a form which is physically and chemically predetermined, not structurally preformed, in the simple organization of the germ.

The hypothesis is worthy of the attention of every serious embryologist. It is scientific in the strict sense of the word, for in employing Herbst’s suggestion to bring the events of ontogeny into the category of physiological responses to stimuli it gives meaning and precision to Hertwig’s somewhat vague idea of the ‘ mutual relations of the parts’, and makes thereby a genuine effort to think the particular under the universal, to bring the facts of embryology under wide general laws of causation.

Its chief Weakness is at present lack of evidence; the development of every organ requires to be examined by the touchstone of experiment before the theory can rise beyond the rank of a working hypothesis. That, however, is the common lot of all working hypotheses, and this one has certainly some counterbalancing advantages. Its conception of the réle of the nucleus has been to some extent borne out by Boveri’s recent researches and adopted by their author, a conception which, as we have observed before, is not irreconcilable with the facts of budding and regeneration. Secondly, only the very simplest original organization of the germ is demanded. This organization is not, however, to be looked for in the similar orientation of similarly polarized particles. On the contrary, it is to be found in the existence of definite cytoplasmic organ-‘forming substances whose arrangement must be simple enough to allow of the divisibility of the whole into totipotent parts, complex enough to account for that limitation of the potentialities of these parts which, sooner or later, inevitably ensues. And lastly, when modified in this important respect, Driesch’s views are no longer hopelessly at variance with Roux’s position, provided that we discard the vicious fallacy of qualitative nuclear and cell—division, and substitute for the idea of complete morphological preformae tion at origine the successive preformation by the action of the parts on one another in the organs of each stage of the structures that are to be developed out of them in the next. To use Roux’s own terms, development, in its early stages at least, is both a process of ‘ self- ’ and of ‘ dependent ’ differentiation ; or, as Nag-eli expressed it, it is by the continued combination and permutation of a few original elements that inheritance is brought about.

Driesch, however, is not merely a scientific thinker who is fully alive to the prime importance of pushing the causal analysis to its extreme limit. He is also a philosopher; and as a philosopher he realizes that when science has said all it has to say the account may still need to be completed from a new and distinct point of view. In the case of living organisms this new standpoint is the teleological. The harmony—causal harmony and harmony of composition—deduced from develop: ment, the functional harmony exhibited by the organs of the adult, all appear to be directed to an end, which is the reproduction or the preservation of the specific form, and it is only when this end is understood that the mere reference to beginnings which a knowledge of the mechanism gives acquires a genuine significance. Purposiveness, in a word, is a characteristic of all organic functions and cannot be ignored.

This principle is borrowed avowedly from Kant’s Kritik of Me Teleological Judgement. Like the scientists of to-day, Kant lays it down as a rule that the mechanical method, by which natural phenomena are brought under general laws of causation and so explained, and without which ‘ there can be no proper knowledge of Nature at all,’ should in all cases be pushed as far as it will go,‘ for this is the principle of the determinant judgement.“

There are cases, however, in which this alone does not sufficef‘ The possibility of the growth and nutrition, above all of the reproduction and regeneration of organisms is only fully intelligible to human reason through another quite distinct kind of causality, their purposiveness. Organisms are not mere machines, for those have merely moving power. Organisms possess in themselves formative power of a self-propagating kind which they communicate to their materials. They are, in fact, natural purposes, both cause and eifect of themselves, in which the parts so combine that they are reciprocally both end and means, existing not only by means of one another but for the sake of one another and the whole. The whole is thus an end which determines the process, a final cause which ‘ brings together the required matter, modifies it, forms it, and puts it in its appropriate

' §§ 70, 78, 79, 80.

’ It should be mentioned, perhaps, that Kant employs the terms ‘determinant’ and ‘reflective’ judgement in two senses. In the Introduction to the Kritik of Judgement judgement is defined as that faculty which thinks the particular as contained under the universal, and is stated to be of two kinds. It is ‘determinant’ when the universal is given (as in the mathematical sciences). When, however, only the particular is given, for which the universal has to be found (as in the inductive sciences), it is ‘ reflective ’, and Kant insists that the ‘ reflective’ judgement requires a principle which it cannot borrow from experience, and this principle is, in brief, the ultimate intelligibility of Nature by us. Only so far as this holds good can we hope to gain a knowledge of general, empirical laws of causation.

The ‘reflective’ judgement is again of two kinds, for which Kant, however unfortunately, employs the same two terms, ‘ determinant‘ and ‘reflective’. The duty of the former is, assuming that all phenomena are explicable in mechanical terms (‘ causal’ terms in the usual sense of the word), to push the analysis of these efficient causes (nexus e;fi'eclit'us) to its extreme limit. The latter, on the other hand, is concerned with a kind of causality after the analogy of our own causality according to purposes, in order that it may have before it a rule according to which certain products of nature, namely organisms, must be investigated. The whole argument of the Kritilc is directed to proving that these two uses of the judgement are mutually supplementary and both indispensable, though in the last resort the first has to be subordinated to the second.

In the text the terms ‘ determinant’ and ‘reflective’ are used in the second sense. The ‘determinant judgement‘ of the deductive sciences does not of course come into this discussion at all.

‘ place ’.1 Such purposiveness is internal, for the organism is at once its own cause and an end to itself, not merely a means to other ends, like a machine whose purposiveness is relative and whose cause is external.

Such is the principle of the teleological judgement. It is a ‘heuristic principle’ 2 rightly brought to hear, at least problematically, upon the investigation of organic nature, ‘ by a distant ‘ analogy with our own causality according to purposes generally,’ 3 and indispensable to us, as anatomists, ‘ as a guiding thread if we ‘ wish to learn how to cognize the constitution of organisms ‘ without aspiring to an investigation into their first origin.’ 4

For ‘ we cannot adequately cognize, much less explain, ‘ organized beings and their internal possibility, according to ‘ mere mechanical principles of nature ’, and it is therefore absurd ‘ to hope that another Newton will arise in the future who shall ‘make comprehensible by us the production of a blade of grass ‘ according to natural laws which no design has ordered ’.5

Could our cognitive faculties rest content in this maxim of the reflective judgement it would be impossible for them to conceive of the production of these things in any other fashion than by attributing them to a cause working by design, to a Being which would be ‘productive in a way analogous to the ‘ causality of an intelligence ’.°

Natural science, however, needs not merely reflective but determinant principles which alone can inform us of the possibility of finding the ultimate explanation of the world of organisms in a causal combination for which an Understanding is not explicitly assumed, since the principle of purposes ‘ does ‘ not make the mode of origination ’ of organic beings ‘ any more ‘comprehensible ’.7 And then, in a passage“ remarkable for its prophetic insight, Kant proceeds to show how this might be. ‘ The agreement of so many genera of animals in a common ‘ scheme . . . allows a ray of hope, however faint, to penetrate into ‘ our minds, that here something may be accomplished by the aid ‘ of the principle of the mechanism of nature (without which ‘there can be no natural science in general). This analogy of

1;8s6. ‘@978. 3§e5. 4§72. v§7.5. °§-15.

‘forms, he says, ‘which with all their differences seem ‘ to have been produced according to a common original type, ‘strengthens our suspicions of an actual relationship between

.‘ them in their production from a common parent, through the

‘ gradual approximation of one animal-genus to another—from ‘ those in which the principle of purposes seems to be best authen‘ ticated, that is from man, down to the polype, and again from ‘this down to mosses and lichens, and finally to the lowest ‘ stage of nature noticeable by us, namely, to crude matter. ‘And so the whole Technic of nature, which is so incompre‘hensible to us in organized beings that we believe ourselves ‘ compelled to think a different principle for it, seems to be ‘derived from matter and its powers according to mechanical ‘ laws (like those by which it works in the formation of crystals).’

A purposiveness, however, must be attributed even to the crude matter, otherwise it would not be possible to think the purposive form of animals and plants.

Although there are doubtless in the Kritil: many obscurities and apparent inconsistencies, to which we cannot allude now, the general meaning of Kant’s reflections upon organisms is perfectly clear. He who would ‘complete the perfect round’ of his knowledge must think not only in beginnings but in ends. The end in the case of a living being is plain-—it is the maintenance and reproduction of its form; the end in the case of the cosmic process, though perhaps not so plain, is to be sought in the ethical, or, in Kantian phraseology, the ‘practical’ concept of the freedom of the moral consciousness of man.

Such a position is quite intelligible, philosophically; and it can only be a matter of surprise that Driesch has not been able to abide by it. In his later writings he has indeed ‘executed acomplete change of front and repudiated the philosophical doctrine laid down in the earlier treatise; and the principal reason for this voZte- ace is that there are cases in which the localization of

ontogenetic effects cannot be explained by any theory of formative stimuli. In the theory we have already considered the causal harmony which secures the due co-ordination in space and

in time of the stimuli and responses into which the process of differentiation is resolved is held to result from the initial structure of the germ and to be maintained by the constitution of the environment. Now, however, it is urged that no materia factor can possibly account either for this harmony or for the secondary harmony of composition or the functional harmony seen in the activities of the adult. When, for example, the gastrula of a sea-urchin is transversely divided into two, each develops into a diminished whole larva in which the gut becomes divided into the characteristic three regions, and all the other organs are formed in correct proportion. For each of these acts in the whole uninjured larva an explanation may conceivably be given in terms of stimuli or forces emanating from the originally distinct parts of the egg and producing effects which vary with the distance upon other parts, as suggested before. A mechanism may be thought of which, when set in motion, will achieve a certain end in accordance with its own pre-established harmony, but a mechanism which can be subdivided ad libitrm, or almost arl libitzcm, and the parts of which will still achieve the same end, will still behave as wholes with their parts co-ordinated in the same ratio, temporally and spatially! Such a mechanism is an inconceivability, for to ensure the result which does happen the working distance of the forces imagined must be altered in each case according to the size of the fragment removed. Something is therefore required to superintend, to co-ordinate, to harmonize the causes of development in the case not only of the part but of the whole egg as well; and this something is not material. A corroborative proof of the inadequacy of the purely material explanation——-the causal explanation in the ordinary sense of the word—may be derived from a consideration of certain other vital processes. The facts of acclimatization and immunity betray an extraordinary adaptability of the organism to a change in its environment; an organ will adapt itself structurally to an alteration, quantitative or qualitative, of function (Roux’s ‘ functional adaptation ’) ; lost parts can be regenerated; and then there is the physiology of the nervous system!

In all these cases of ‘ regulation ’—and indeed'in all other responses to stimuli—-the same element, inexplicable in chemical and physical terms, exists as must exist in development. This entity is not a form of energy but a vital constant, analogous to the constants or ultimate conceptions of mechanics and physics and chemistry and crystallography, but not reducible to these, just as these cannot be translated into one another. Driesch describes it as rudimentary feeling and willing, as a ‘ psychoid ’, as ‘ morphaesthetic ’, or perceptive of that form which is the desired end towards which it controls and directs all the material elements of differentiation. Its activities are thus verae causae— unconditional and invariable antecedents—psychical factors which can intervene in the purely physical series of causes and efiects, and for it he revives the Aristotelian term ‘Entelechy ’. Such is the ‘vitalism’ introduced by Hans Driesch, a teleological theory clearly, but not the ‘static ’ teleology of the Analytiac/ze T/zeorie ; rather it is a ‘ dynamic’ teleology which not only sees an end in every organic process but postulates an immaterial entity to guide the merely mechanical forces towards the realization of that end.

This theory would seem to be open to serious criticism, and from two sides, the scientific and the philosophical.

In the first place, we must remind Driesch that on his own showing a comparatively simple structure is all that is necessary to form the starting-point of a developmental process, however complex that may be, and that there is no reason why such a structure should not be divisible into portions, each of which will possess all the parts of the structure in correct proportions, and be therefore totipotent. But such division cannot continue indefinitely, for as we know, and as Driesch knows too, there is always, sooner or later, a restriction of potentialities, and this is due to the manner of distribution at or2'gz'ne of the constituent parts of the whole. When Driesch asserts that this restriction is due to size alone, to mere lack of material, and not lack of specific material, when he tells us that the blastomeres can be dislocated indefinitely without prejudice to a normal development, when he exclaims that ‘ J edes jedes kann ’, he is manifestly led away by the reaction against the theory of the preformation of as many units as there are inheritable characters on the one hand, and on the othe; by his own erroneous presuppositions as to the construction of the egg out of like particles all similarly polarized and all oriented in the same way.‘

This being so, the first argument based on the ‘ causal harmony ’ will fall to the ground; for this will be given in the initial structure of the egg, and if that may be divided, then the ‘ causal harmony ’ may be divided too. The correct proportionality of the organs of partial larvae, then, offers no peculiar difficulty. The corroborative argument is founded on a consideration of responses to stimuli. This is not a question for the embryologist, but it may be pointed out that there are still physiologists who maintain that even the complex phenomena presented to us in the activities of the nervous system are susceptible of a purely mechanical explanation.

The second series of objections to the new ‘vitalism’ is philosophical. Driesch has quoted the authority of Kant and Aristotle in support of his doctrine. The former is, however, rather a difficult witness, as Driesch is well aware. He complains, indeed, that Kant’s teleology is descriptive or ‘static ’ rather than ‘dynamic’, as is perfectly true, except in the case of man, a point of which Driesch naturally makes the most. There are no doubt passages where Kant speaks of ‘a cause ‘ which brings together the required matter, modifies it, forms it ‘and puts it into its appropriate place ’,2 but against these must be set the explicit statement that if the body has an alien principle (the soul) in communion with it, ‘the body must either ‘ be the instrument of the soul—which does not make the soul ‘a whit more comprehensible’3— or be made by the soul, in which case it would not be corporeal at all. ‘ Vitalism ’ can glean small comfort from this.

Let us turn, then, to the second authority.

Aristotle’s matured reflections on the soul (xpvxfi), its nature, functions, and development, are to be found in the treatises De Anima and De Generatione Animalium.

‘ C. M. Child (Biol. Centralbl. xxviii, 1908) has recently published a similar criticism of Driesch’s absolutely equipotential systems. Child points out in particular that in the _regeneration of the head of Tubularia the pro ortionality of the parts d1fi'ers in different regions of the stem, and un er different conditions, and cannot therefore always be exact. This §eq1éipotentiality osf gh6e5system is therefore not absolute.

Soul is defined in the most general way as an activity of a natural organic living body, life being autonomous nutrition and growth and decay. The activity (évrekéxaa) may, however, be latent or patent, passive or active, sleeping or waking, without losing its peculiar characters. This activity is substance (ofmia), but substance as ‘ form ’, as opposed to the material substance of the body; the living body is therefore also a substance, in a double sense.

Soul is not, however, identical with the body, but as form, proportion ()\o'yos‘), activity (e’ve’p~/eta), essence (16 ref 1312 Juan), it is related to the body, mere matter (z'5)\n) and potentiality (bzfuams), in just the same way as the seal is related to the wax, and the body is the instrument whereby it effects its purposes; though subsequent in time it is prior in thought to the body, as all activities are to the materials with which they operate.

At the same time neither it nor its parts are separable from the body, with the exception, possibly, of mind (vofis); it is indeed the actual or possible functioning of the body, like the seeing of the eye or the cutting of the axe, and with the disappearance of the capacity of this functioning the soul itself also perishes.‘ Lastly, it is a cause (dpxip xal atria) in a triple sense: first, as the source of motion, secondly, as that for the sake of which the body exists, and thirdly, as its essence (ofiafa), or formal cause.”

The soul is of several kinds, which form together an ascending series, each member of which is necessarily involved in those above it.“

The lowest is the nutritive soul (fipsvrrmf), found in all living things, and the only soul possessed by plants. It is defined as motion in respect of nutrition, decay and growth, processes which involve alteration (&Mo[aoa-is‘) in the body, and its functions (épya) are to utilize the food’ for the maintenance and reproduction of the form of the body, and to control and limit growth.

The second is the perceptive soul (aicrflm-uni), the possession of which distinguihes animals from plants. Perception is a kind of alteration (dkkoiwcts 119),‘ and consists in being moved and affected. The fundamental and indispensable perception is touch (écjni), for it is concerned in the acquisition of the food. It is invariably present; the others may or may not, some or all, be present.

1 De Au. II. 1. ’ Ibid. II.4. 3 Ibid. II. 3,4. ‘ Ibid. II. 5.

Some animals are also possessed of a capacity of locomotion, and the performance of this function requires again a special kind of soul.

Lastly, there is the reasoning soul (ecauonnmi), or mind (vofis). This is found in man alone, unless there be other beings similar to him, or even nobler than he. Mind alone is eternal and separable from the body.

In all reproduction (except in generafio eguivoca) the startingpoint of a new individual is what Aristotle calls a a--rréppa. In plants, in which he does not recognize the sexes, this is the seed; in sexually produced animals it is the result (mfimua) of the mingling of the male (yomi, or anépua in a narrower sense) and female elements ; the latter is an egg or, in Mammals, the catamenia.‘

This avréppa, he holds, does not come from all the organs of the body by a kind of pangenesis, but is a tissue, homogeneous like bone and flesh, and separated out from the food in its final stage of digestion, when it is in the form of blood and ready for assimilatiou, and hereditary resemblance is explained by the fact that the food which is about to be assimilated by the organs is naturally like that set aside to form the <r1re'pp.a."

In the matter of the o-n-éppza all the parts of the organism that is to be formed are indeed present potentially, but this means no more than that the material is there.“ Actually (évepyetq), they cannot be present until the soul has been developed, and in particular the soul that is characteristic of animals, the perceptive.‘ Out of this matter the organs are differentiated successively, the heart first, not only as a matter of observation (on the chick), but as a. theoretical necessity, since in it is the principle of growth, then the blood, the blood-vessels, the tissues gathering about these by a process of condensation and coagulation, the foreparts of the body first, and then the hinder.5 The anéppa, then, or xiinua, is a material cause of development, but it is also a cause in other senses ,- it is the eflicient cause, since it must

‘ De Gen. I. 18-20. ’ Ibid. I. 18, 19; IV. 1. 39. I 3 Ibid. II. 4. ‘ Ibid. 1. 19; II. 3, 5. ‘ Ibid. II. 6. ,

contain the source of motion, and it is further the final and the formal cause. These several causes are not, however, all contributed by both parents. The teaching of Aristotle is that the matter is provided by the female, and the female alone.‘ The egg (or catamenia) is described as being matter (z'5)\n), body (a63p.a), potentiality (bfiuams), passive'(1ra0nru<6v), and merely quantitative, although it is true that a sort of soul, the nutritive, is somewhat grudgingly conceded to it, since unfertilized eggs appear in some sense to be alive.“ The male element, on the other hand, provides the principle of motion (dpxi; 1-fir xwfio-ems) and the form (eZ6os) ,- it is qualitative, it is activity, it produces the perceptive soul, if it is not itself that soul, and it is responsible for the ‘ correct proportionality ’ (Myos) of the organization.3

The male element contributes only motion; it acts upon the female element as rennet acts when it coagulates milk, except that the analogy is incomplete, since the yomi brings about a qualitative, and not merely a quantitative, change in the material on which it operates.‘ To this it imparts the same kind of motion which itself possesses, the motion which was present in the particles of the food in its final form from which it was itself derived.5

The communication of this motion is enough to set going the machinery (az’;ro';ua1-ov) ; the rest then follows of itself in proper order.“ To impart this necessary motion is the function of the nutritive soul, which is primarily associated with the male, only somewhat doubtfully with the female, element; the perceptive soul which is, and therefore presumably also imparts, motion of a kind (dmofwms) is found in the former alone.7 As to the third kind of soul, mind, Aristotle says little, but it is not introduced in the male element: it is separable and comes in from outside.

Lastly, the sperm of the male acts like a cunning workman who makes a work of art, using heat and cold as its implements as the workman uses his tools; 3 for this heat and this cold could never of theme1ves—by coagulations and condensations——produce the form of the body, as the older naturalists had supposed,

‘ De Gen. I. 20, 21; II. 1, 4. 3 Ibid. II. 5. 3 Ibid. I. 20, 21; II. 1, 4. ‘ Ibid. I. 20; IV. 4. “ Ibid. II. 3. “ Ibid. II. I, 5. 7 Ibid. II. E’); De An. II. 5. ‘ De Gen. II. 4.

regarding only the material and eflicient, and ignoring the formal and the final cause ; for the organic body is not what it is because it is produced in such and such a fashion, rather it is because it is to be such and such that it must be developed as it is.‘ And here lies the kernel of the whole matter. For while Aristotle has made it perfectly plain that according to his idea, the soul, at least its nutritive and perceptive faculties, is to be regarded as a function of matter and that this function may be ultimately expressed in terms of movement, and further that development is a mechanism which is set going by the communication of motion proceeding from the ‘soul’ of the male element, and derivable in the last resort from the ‘ motions’ into which the ‘ functions ’ or ‘ soul ’ of the parent can be resolved, to the mere matter which the female provides, it is equally evident that he does not regard this mechanical explanation—in terms of material and eflicient causes—as satisfactory or complete. But when we inquire why, he gives us no certain and consistent answer. On the one hand there are passages in which he tells us that there must be something which controls the material forces and imposes upon them a limit and proportionality of growth,” that the soul makes use of these forces as the artist makes use of his implements,“ and such passages are naturally interpreted by Driesch in the sense of a ‘ dynamic ’ teleology ; it is the xpvxrf (not, of course, voiis, but the two lower kinds) which superintends and controls, and the \[tux1f is ‘ Entelechy ’. Elsewhere, however, we are informed that even the proportionality of the developing parts is simply the outcome of the motion imparted by the male, which is actu what the female material only is 12otent2'a”.4 Moreover it may be questioned whether Aristotle ever intended to imply more than an ‘ analogy with the causality of purpose’ when he uses the figure of the workman and his implements to illustrate his meaning of the formal cause. The formal cause of a work of art is an intelligible ‘ vera causa ’, it is the idea in the mind of the artist antecedent to the execution of the work, but the formal or final cause of an organism, the end which it apparently strives to attain, is only metaphorically prior in time to the existence of the organism itself. Prior in thought, however, it certainly is, for it is only the performance of its functions (iv-rehéxeca) by the organism complete in all its parts that makes the mere mechanism of development comprehensible to us; the process, therefore, exists for the sake of the end. Only as eflicient cause is the soul prior in time ; only so far as it is prior in thought can it be said to be a final cause.‘

‘ De Gen. V. 1. 9 De An. II. 4. ’ De Gen. II. 4. ‘ Ibid. II. 1.

Such a teleology is, it is obvious, indistinguishable in principle from the position in which Kant leaves us. It is the position adopted by Driesch, as we have seen, in the Analytisc/le T/zeorie, but abandoned in the Vitalismue in favour of a theory of ‘pyschoids ’.

Now, quite apart from the meaning which Aristotle may or may not have intended to convey, there appear to be grave objections to this belief.

This ‘ psychoid ’, to which the name ‘ Entelechy’ is surely misapplied, this rudimentary feeling and- willing, which is aware of the form it desires to produce, must be, psychically, at least as complex as the phenomena it is designed to account for, and stand, therefore, as much in need of explanation as they, which will involve us at once in an infinite series of such entities. In fact, to borrow the epithet which Driesch himself has bestowed on the nuclear architecture imagined in the Roux-Weismann hypothesis, it is only a photograph of the problem, and not a solution at all. Again, when we ask what the modus opcramli of this cause is we get no reply either from Driesch or from any other neo-vitalist, though this is just the knowledge that we so urgently stand in need of. The objection that the intervention of a psychical cause in a physical process is unintelligible, an objection which would probably appeal to many, may be waived, for in the last resort the connexion between any——even simply mechanical—-causes and efiects is equally hard to understand. It may, however, be seriously doubted whether these entities are not being ‘ multiplied beyond necessity ’, and whether the progress of science would not be better served by an adherence to a simpler philosophy.

‘Vere scire est per causas scire.’ The maxim of the great founder of modern inductive science is the watchword of embryologists to-day. By exact observation and crucial experi ‘ De Part. II. 1. 7.

ment, utilizing every canon of induction, the facts of development are to be brought under wide general laws of causation, which will be in the first instance physiological laws—of response to stimuli, of metabolism, and of growth: by means of these laws we can predict, and our predictions can be verified. The thought process cannot, however, rest here. Ultimately—-—as we believe—it may be possible, no more than that can yet be said, but it may be possible to state the widest generalizations of biology in chemical and physical, and these again in purely mechanical, terms. Thus evolution of form in the individual as well as the larger evolution of form in the race, become but the final terms in a far vaster cosmic progress, from ‘ homogeneity to heterogeneity ’.

The idea, of course, is perfectly familiar ; it is the analysis of purely physical causes carried to its extremest limit. Phenomena are thought out in terms not of origins merely but of one origin, and that one origin is the only mystery that remains. This unification of the sciences always has been, and must still remain, the dream and the faith and the inspiration of the scientific man, and could such an edifice of the intellect ever be realized the task of science would have been completed.

But where science leaves off there philosophy begins, and it is for philosophy to attempt the solution of this last mystery of all.

Philosophy cannot rest content in an endless regress of cause and efiect, and a first supreme cause, first in time that is, is metaphysically out of the question. An original homogeneity is equally unthinkable, for out of a system all whose parts are absolutely alike, by no imaginable process could any heterogeneity ever be evolved.

That first simplicity must have contained potentici all that has since developed out of it, it must have possessed a structure, an arrangement of parts such that the end which it has realized, or is to realize, would be what it is or will be, and to regard the end as well as the beginning is the duty of philosophy, a duty which Aristotle and Kant have both impressed on us. The outlook of pure science, an outlook to which, gm? science, it cannot too rigidly confine itself, is thus supplemented and enlarged. V GENERAL REFLECTIONS AND CONCLUSIONS 299

Knowledge through material and eflicient causes is rounded into a whole through a knowledge of the final cause, which, in the last analysis, is just as much a rem cauea as they are; for in our total ignorance of what constitutes the invariable connexion we observe between antecedent and consequent it is as true that the second causes the first as the converse. Only because of the inherent desire of the human mind to predict from the past, which is known, to the future, which is not, have we come in ordinary usage to restrict the term to the antecedent.

The ultimate end of the human or any other race we cannot tell, the ultimate end of the universe we cannot tell, any more than we can imagine its absolute beginning, unless we find these ends in the freedom of the moral consciousness of man. But the end of an organism, the production of specific form and the maintenance of that form which itself has produced, does seem self-evident and plain, though we must never forget that form is variable and subject to change, that species are not immortal any more than individuals, and that the effort to achieve that form does not invariably succeed.

Here, however, we touch on the fringe of a problem—the problem of evil—too large to be discussed in this place. Putting this aside, a purposiveness is an unmistakable characteristic of the functions of living things, of the production and preservation of form, a characteristic which still remains when those functions have been expressed in terms of the chemistry of the proteids. It is only, however, by a remote analogy with our own ‘causality according to purposes’ that we can speak of organic functions as purposive ,- it is only as they were guided and controlled by an intelligence; their purposiveness is indeed only the expression of our inability to comprehend their beginnings except in terms of their ends; it is relative to us, though not, therefore, any the less real.

Biology, then, although built upon the ultimate conceptions of chemistry and physics, has yet peculiar features of its own. Its relation, indeed, to these lower sciences is just what their relation is to one another. A survey of the whole hierarchy displays to our view a series in ascending order of complexity; each member of this series has its own ultimate conceptions, the most general expression for the facts with which it deals, but the ultimate conceptions of each, as in Aristotle’s series of ‘ souls ’, are necessarily involved in the one next above, while conversely each endeavours to translate its own ultimates into those of the science below: a translation, however, which, be it never forgotten, leaves the reality of the original undestroyed.

Thus, mechanics expresses molar motions in terms of pure numbers, physics explains forms of energy-—heat and light and electricity—as the motions of molecules; chemical affinity is to be reduced to the mutual attractions of intramolecular atoms; purposive responses to stimuli may be stated in terms of chemical reaction, and the psychical phenomena of mental and moral science—understanding and feeling and will—are a form of these.

The cosmic process thus takes place in a succession of stages, and the peculiar features which mark each individual stage are simply the outcome of an increase in complexity of the peculiarities characteristic of the stage below. To establish this is the final achievement of science.

Nevertheless, the facts with which each science starts, the facts which come first in the order in which knowledge is acquired, do not become wholly merged in those simpler facts into which they are at each stage translated ,- when the translation has been accomplished the original still remains.

The ‘secondary qualities’, as well as the other properties of those bodies whose behaviour the physicist investigates, are as real as, no more and no less relative to our intelligence than, those ‘primary qualities ’ of impenetrability and extensionexhibited by those bodies whose behaviour forms the subjectmatter of mechanical science—into which it is his endeavour to translate them, just as the primary qualities themselves obstinately refuse to be reduced to mere number. Chemical affinity remains as a phenomenon sui generis after it has been reduced to the operations of intramolecular forces, and the purposiveness of responses to stimuli is something over and above the chemical reactions to which they are rightly referred. Last of all, the final term in the series, "the mental and moral consciousness, the ‘other side’ of certain purely physical functionings of the organism, is as real as any’ of those qualities of matter which have been step by step involved in its evolution. In a word, the increase in the complexity- of the phenomena which marks the transition from each of these stages to the next is itself a new phenomenon and cannot be ignored.

And herein we may perhaps discover the essence of the relation between ‘mind’ and ‘ matter’, whether in ordinary function or in development. The mind is not matter, not even living matter; rather it is the new quality constituted by an increase in the complexity of living matter, immaterial and as distinct from that matter as is ‘ blueness’ from vibration of a certain wave-length. Dependent on and inseparable from matter, however, it is; when that matter, whether in the individual or in the race, attains a certain degree of complexity, then and then only does mind appear; and with the disappearance of that complexity it perishes.

While, therefore, we have no reason for supposing that the mind ‘ comes in from outside ’, we are at the same time saved from that somewhat extravagant ‘ psycho-physical parallelism ’ which, to explain the evolution of consciousness, postulates a complete psychical, accompanying the complete physical series of causes and eifects, and credits, of necessity, the merest matter with the rudiments of feeling, thought and will. On the other hand, if the mind cannot be said to intervene in the physical series, there seems to be no alternative but to suppose that the operations of the one, when they exist, are parallel to those of the other.

Such a system of philosophy as that which we have here ventured to suggest can give no countenance to a Vitalism which interpolates an unnecessary psychical element into the complete causal chain of physical events. But it is not for that reason to be condemned as materialistic ; for the mind, developed out of and conditioned by matter, the last term and final cause of the whole process, is not itself matter but an accompaniment of certain material complexes, and still remains when they have been resolved into simple mechanical expressions. And in this mind, last in time but first in thought, a larger philosophy will perceive not only the end towards which, in time and space, matter strives, but the Undertanding which, itself eternal, imposes the forms pf space and time upon that Nature which it makes.


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Jenkinson (1909): 1 Introductory | 2 Cell-Division and Growth | 3 External Factors | 4 Internal Factors | 5 Driesch’s Theories - General Conclusions | 6 Appendices

Cite this page: Hill, M.A. (2024, April 17) Embryology Book - Experimental Embryology (1909) 5. Retrieved from

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