The most important point has already been emphasised. Even if the theory of the struggle for existence were correct, it would be possible to subject the world as a whole to a teleological interpretation. But these anti-Darwinian theories now emerging, though they do not directly induce teleological interpretation, suggest it much more strongly than orthodox Darwinism does. A world which in its evolution is not exposed, for good or ill, to the action of chance factors-playing with it and forcing it hither and thither-but which, exposed indeed to the most diverse conditions of existence and their influences, and harmonising with them, nevertheless carries implicitly and infallibly within itself the laws of its own expression, and especially the necessity to develop upwards into higher and higher forms, is expressly suited for teleological consideration, and we can understand how it is that the old physico-teleological evidences of the existence of G.o.d are beginning to hold up their heads again. They are wrong when they try to demonstrate G.o.d, but quite right when they simply seek to show that nature does not contradict-in fact that it allows room and validity to-belief in the Highest Wisdom as the cause and guide of all things natural.

As far as the question of the right to interpret nature teleologically is concerned, it would be entirely indifferent whether what Korschinsky calls "the tendency to progress," and the system of laws in obedience to which evolution brings forth its forms, can be interpreted "mechanically" or not; that is to say, whether or not evolution depends on conditions and potentialities of living matter, which can be demonstrated and made mechanically commensurable or not. It may be that they can neither be demonstrated nor made mechanically commensurable, but lie in the impenetrable mystery inherent in all life. Whether this mystery really exists, and whether religion has any particular interest in it if it does, must be considered in the following chapter.

CHAPTER VIII. THE MECHANICAL THEORY OF LIFE.

What is life-not in the spiritual and transcendental sense, but in its physical and physiological aspects? What is this mysterious complex of processes and phenomena, common to everything animate, from the seaweed to the rose, and from the human body to the bacterium, this ability to "move"

of itself, to change and yet to remain like itself, to take up dead substances into itself, to a.s.similate and to excrete, to initiate and sustain, in respiration, in nutrition, in external and internal movements, the most complex chemical and physical processes, to develop and build up through a long series of stages a complete whole from the primitive beginnings in the germ, to grow, to become mature, and gradually to break up again, and with all this to repeat in itself the type of its parent, and to bring forth others like itself, thus perpetuating its own species, to react effectively to stimuli, to produce protective devices against injury, and to regenerate lost parts? All this is done by living organisms, all this is the expression in them of "Life." What is it?

Whence comes it? And how can it be explained?

The problem of the nature of life, of the principle of vitality, is almost as old as philosophy itself, and from the earliest times in which men began to ponder over the problem, the same ant.i.theses have been apparent which we find to-day. Disguised under various catchwords and with the greatest diversities of expression, the ant.i.theses remain essentially the same through the centuries, competing with one another, often mingling curiously, so that from time to time one or other almost disappears, but always crops up again, so that it seems as if the conflict would be a never-ending one-the ant.i.theses between the mechanical and the "vitalistic" view of life. On the one side there is the conviction that the processes of life may be interpreted in terms of natural processes of a simple and obvious kind, indeed directly in terms of those which are most general and most intelligible-namely, the simplest movements of the smallest particles of matter, which are governed by the same laws as movement in general. And a.s.sociated with this is the attempt to take away any special halo from around the processes of life, to admit even here no other processes but the mechanical ones, and to explain everything as the effect of material causes. On the opposite side is the conviction that vital phenomena occupy a special and peculiar sphere in the world of natural phenomena, a higher platform; that they cannot be explained by merely physical or chemical or mechanical factors, and that, if "explaining" means reducing to terms of such factors, they do in truth include something inexplicable. These opposing conceptions of the living and the organic have been contrasted with one another, in most precise form and exact expression, by Kant in certain chapters of the Kritik der Urteilskraft, which must be regarded as a cla.s.sic for our subject.(56) But as far as their general tendency is concerned, they were already represented in the nature-philosophies of Democritus on the one hand, and of Aristotle on the other.

All the essential const.i.tuents of the modern mechanical theories are really to be found in Democritus, the causal interpretation, the denial of any operative purposes or formative principles, the admission and a.s.sertion of quant.i.tative explanations alone, the denial of qualities, the reduction of all cosmic developments to the "mechanics of the atom" (even to attractions and repulsions, thus setting aside the "energies"), the inevitable necessity of these mechanical sequences, indeed at bottom even the conviction of the "constancy of the sum of matter and energy." (For, as he says, "nothing comes out of nothing.") And although he makes the "soul" the principle of the phenomena of life, that is in no way contradictory to his general mechanical theory, but is quite congruent with it. For the "soul" is to him only an aggregation of thinner, smoother, and rounder atoms, which as such are more mobile, and can, as it were, quarter themselves in the body, but nevertheless stand in a purely mechanical relation to it.

Aristotle, who was well aware of the diametrical opposition, represents, as compared with Democritus, the Socratic-Platonic teleological interpretation of nature, and in regard to the question of living organisms his point of view may quite well be designated by the modern name of "vitalism." Especially in his theory of the vegetable soul, the essence of vitalism is already contained. It is the ????? ?????? (logos enhylos), the idea immanent in the matter, the conceptual essence of the organism, or its ideal whole, which is inherent in it from its beginnings in the germ, and determines, like a directing law, all its vegetative processes, and so raises it from a state of "possibility" to one of "reality." All that we meet with later as "nisus formativus," as "life-force" (vis vitalis), as "endeavour after an end" (Zielstrebigkeit), is included in the scope of Aristotelian thought. And he has the advantage over many of his successors of being very much clearer.(57)

The present state of the problem of life may be regarded as due to a reaction of biological investigation and opinion from the "vitalistic"

theories which prevailed in the first half of last century, and which were in turn at once the root and the fruit of the German Nature-philosophy of that time.

Lotze in his oft-quoted article, "Leben, Lebenskraft" (Life, Vital Force), in Wagner's "Hand-Worterbuch der Physiologie," 1842, gave the signal for this reaction. The change, however, did not take place suddenly. The most important investigators in their special domain, the physiologist Johannes Muller, the chemist Julius Liebig, remained faithful to a modified vitalistic standpoint. But in the following generation the revolution was complete and energetic. With Du Bois-Reymond, Virchow, Haeckel, the anti-vitalistic trend became more definite and more widespread. It had a powerful ally in the Darwinian theory, which had been promulgated meanwhile, and at the same time in the increasingly materialistic tendency of thought, which afforded support to the mechanical system and also sought foundations in it.

The naturalistic, "mechanical" interpretation of life was so much in the tenor of Darwin's doctrine that it would have arisen out of it if it had not existed before. It is so generally regarded as a self-evident and necessary corollary of the strictly Darwinian doctrine, that it is often included with it under the name of Darwinism, although Darwin personally did not devote any attention to the problem of the mechanical interpretation of life. Any estimate of the value of one must be a.s.sociated with an estimate of the other also.

It goes without saying that the theory of life is dependent upon, and in a large measure consists of physico-chemical interpretations, investigations, and methods. For ever since the attention of investigators was directed to the problems of growth, of nutrition, of development and so on, and particularly as knowledge has pa.s.sed from primitive and unmethodical forms to real science, it has been taken as a matter of course that chemical and physical processes play a large part in life, and indeed that everything demonstrable, visible, or a.n.a.lysable, does come about "naturally," as it is said. And from the vitalistic standpoint it has to be asked whether detailed biological investigation and a.n.a.lysis can ever accomplish more than the observation and tracing out of these chemical and physical processes. Anything beyond this will probably be only the defining and formulating of the limits of its own proper sphere of inquiry, and a recognition, though no knowledge, of what lies beyond and of the co-operative factors. The difference between vitalism and the mechanical theory of life is not, that the one regards the processes in the organism as opposed to those in the inorganic world while the other identifies them, but that vitalism regards life as a combination of chemical and physical processes, with the co-operation and under the regulation of other principles, while the mechanical theory leaves these other principles out.

Notwithstanding the many noteworthy reactions, we are bound to regard the present state of the theory of life as on the whole mechanical. The majority of experts-not to speak of the popular materialists, and especially those who, sailing under the flag of materialistic interpretation, have their ships full of vitalistic contraband-regard as the ideal of their science an ultimate a.n.a.lysis of the phenomena of life into mechanical processes, into "mechanics of the atom." They believe in this ideal, and without concealing that it is still very far off, do not doubt its ultimate attainability, and regard vitalistic a.s.sumptions as obstacles to the progress of investigation. Moreover, this aspect of the problem seems likely enough to be permanent with the majority, or, at any rate, with many naturalists, though it is obviously one-sided. For it has always been the task of this line of investigation to extend the sphere within which physical and chemical laws can be validly applied in interpreting vital processes, and the results reached along this line will always be so numerous and important that even on psychological grounds the mechanical point of view has the best chance for the future. Furthermore, the maxim that all the phenomena of nature must be explained by means of the simplest factors and according to the smallest possible number of laws, is usually regarded as one of the most legitimate maxims of science in general, so that the resolute pertinacity with which many investigators maintain the entire sufficiency of the mechanical interpretation, far from being condemned as materialistic fanaticism, must be respected as the expression of scientific conscience. Even when confidence in the one-sided mechanical interpretation of vital processes sometimes fails in face of the great and striking riddles of life, it is to be expected that it will revive again with each new success, great or small.(58)

The mechanical conception of life which now prevails is made up of the following characteristics and component elements. These also indicate the lines along which the arguments are worked out-lines which glimmered faintly through the mechanical theories of ancient times, but which have now been definitely formulated and supported by evidence.

The Conservation of Matter and Energy.

1. The whole mechanical theory is based upon a law which is not strictly biological but belongs to science in general-the law of the conservation of matter and energy. This was first recognised by Kant as a general rational concept in his "Critique" and in the "Grundlegung der Metaphysik der Naturwissenschaft," and was transferred by Robert Mayer and Helmholtz(59) to the domain of natural science. Just as no particle of matter can come from nothing or become nothing, so no quantum of energy can come from nothing or become nothing. It must come from somewhere and must remain somewhere. The form of energy is continually changing, but the sum of energy in the universe remains invariable and constant. Therefore, it seems to follow, there can be no specific vital phenomena. The energies concerned in the up-building, growth, and decay of the organism, and the sum of the functions performed by it, must be the exact resultant and equivalent of the potential energies stored in its material substance and the co-operative energies of its environment. The particular course of transformations they follow must have its sufficient reason in the configuration of the parts of the organism, in its relations to the environment, and the like. An intervention of "vitalistic" principles, directions and so forth, would, we are told, involve a sudden obtrusion and disappearance again of energy-effects which had no efficient cause in the previous phenomena. From any point of view it would be a miracle, and in particular it would be doing violence to the law of the constancy of the sum of energy.

Apart from the inherent general "instinct"-_sit venia verbo_, for no more definite word is available-which is the quiet Socius, the concealed but powerful spring of the mechanistic convictions, as of most others, this law of the conservation of energy is probably the really central argument, and it meets us again more or less disguised in what follows.

The Organic and the Inorganic.

2. What is on _a priori_ grounds demanded as a necessity, or set aside as impossible, on the strength of the axiom of the conservation of energy, must be proved _a posteriori_ by investigation. It must be shown in detail that the difference between the organic and the inorganic is only apparent. And it is here that the mechanical view of life celebrates its greatest triumph.

For a long time it seemed as though there were an absolute difference between "inorganic" and "organic" chemistry, between the chemical processes and products found in free nature, and those within the "living"

body. The same elements were indeed found in both, but it seemed as if they were subject in the living body to other and higher laws than those observed in inanimate nature. Out of these elements the organism builds up, by unexplained processes, peculiar chemical individualities, highly organised and complex combinations which are never attained in inorganic nature. This seems to afford indubitable evidence of a vital force with mysterious super-chemical capacities.

But modern chemical science has succeeded in doing away with this absolute difference between the two departments of chemistry, for it has achieved, in retorts, in the laboratory, and with "natural" chemical means, what had hitherto only been accomplished by "organic" chemistry. Since Wohler's discovery that urea could be built up by artificial combination, more and more of the carbon-compounds which were previously regarded as specialities of the vital force have been produced by artificial syntheses. The highest synthesis, that of proteids, has not yet been discovered, but perhaps that, too, may yet be achieved.

And further: intensive observation through the microscope and in the laboratory increases the knowledge of processes which can be a.n.a.lysed into simple chemical processes, both in the plant and the animal body. These are astonishing in their diversity and complexity, but nevertheless they fulfil themselves according to known chemical laws, and they can be imitated apart from the living substance. The "breaking up" of the molecules of nutritive material,-that is to say, the preparation of them as building material for the body,-does not take place magically and automatically, but is a.s.sociated with definitely demonstrable chemical stuffs, which produce their effect even outside of the organism. The fundamental function of living matter-"metabolism,"-that is, the constant disruption and reconstruction of its own substance, has, it seems, been brought at least nearer to a possible future explanation by the recognition of a series of phenomena of a purely chemical nature, the catalytic phenomena (the effects of ferments or "enzymes"). Ingenious hypotheses are already being constructed, if not to explain, at least to give a general formulation of these facts, which will serve as a framework and guiding clue, as a "working hypothesis" for the further progress of investigation.

The most recent of these hypotheses is that set forth by Verworn in his book "Die Biogenhypothese."(60) He a.s.sumes, as the central vehicle of the vital functions, a unified living substance, the "biogen," nearly related to the proteids which form the fundamental substance of protoplasm and of the cell-nucleus, and in contrast to which the other substances found in the living body are in part raw materials and reserves, and in part of a derivative nature, or the results of disruptive metabolism. Very complex chemically, "biogen" is able to operate upon the circulating or reserve "nutritive" materials in a way comparable, for instance, to the action of "nitric acid in the production of English sulphuric acid." That is to say, it is able to set up processes of disruption and of recombination, apparently by its mere presence, but, in reality, by its own continual breaking down and building up again. At the same time it has the power, a.n.a.logous to that of polymerisation in molecules, of increasing, of "growing."

The case is the same in regard to physical laws. They are identical in the living and the non-living. And many of the processes of life have already been a.n.a.lysed into a complex of simpler physical processes. The circulation of the blood is subject to the same laws of hydrostatics as are ill.u.s.trated in all other fluids. Mechanical, static, and osmotic processes occur in the organism and const.i.tute its vital phenomena. The eye is a _camera obscura_, an optical apparatus; the ear an acoustic instrument; the skeleton an ingenious system of levers, which obey the same laws as all other levers. E. du Bois-Reymond, in his lectures on "The Physics of Organic Metabolism" ("Physik des organischen Stoffwechsels"),(61) compiles a long and detailed list of the physical factors a.s.sociated and intertwined in the most diverse ways with the fundamental phenomenon of life, namely, metabolism:-the capacities and effects of solution, diffusion of liquids, capillarity, surface tension, coagulation, transfusion with filtration, the capacities and effects of gases, aero-diffusion through porous walls, the absorption of gases through solid bodies and through fluids, and so on.

Very impressive, too, are the manifold "mechanical" interpretations of intimate vital characteristics, such as the infinitely fine structure of protoplasm. For protoplasm does not fill the cell as a compact ma.s.s, but spreads itself out and builds itself up in the most delicate network or meshwork, of which it forms the threads and walls, enclosing innumerable vacuoles and alveoli, and Butschli succeeded in making a surprisingly good imitation of this "structure" by mechanical means. Drops of oil intimately mixed with potash and placed between gla.s.s plates formed a very similar emulsion-like or foam-like structure with a visible network and with enclosed alveoli.(62)

Rhumbler, too, succeeded in explaining by "developmental mechanics" some of the apparently extremely subtle processes at the beginning of embryonic development (the inv.a.g.i.n.ation of the blastula to form the gastrula); by imitating the sphere of cells which compose the blastula with elastic steel bands he deduced the inv.a.g.i.n.ation mechanically from the model.(63)

Here, too, must be mentioned Verworn's attempts to explain "the movements of the living substance."(64) "Kinesis," the power to move, has since the time of Aristotle been regarded as one of the peculiar characteristics of life. From the gliding "amboid" movements of the moneron, with its mysterious power of shifting its position, spreading itself out, and spinning out long threads ("pseudopodia"), up to the contractility of the muscle-fibre, the same riddle reappears in many different forms. Verworn attacks it at the lowest level, and attempts to solve it by reference to the surface tension to which all fluid bodies are subject, and to the partial relaxation of this, which forces the ma.s.s to give off radiating processes or "pseudopodia." The mechanical causes of the suspension of the surface tension are inquired into, and striking examples of pseudopod-like rays are found in the inorganic world, for instance, in a drop of oil.

Thus a starting-point is discovered for mechanical interpretations at a higher level.(65)

Irritability.

3. A property which seems to be quite peculiar to living matter is irritability, or the power of responding to "stimuli," that is to say, of reacting to some influence from without in such a manner that _the reaction_ is not the mere equivalent of the action, but that the stimulus is to the organism as a contingent cause or impulse setting up a new process or a new series of processes, which seem as though they occurred spontaneously and freely. Thus the sensitive plant _Mimosa pudica_ droops its feathery leaves when touched. Here, too, must be cla.s.sed also all the innumerable phenomena of Heliotropism, Geotropism, Rheotropism, Chemotropism, and other tropisms, in which the sun, or the earth, or currents, or chemical stimuli so affect a form of life-plant, alga, or spore-that it disposes its own movements or the arrangements of its parts accordingly, turning towards, or away from, or in an oblique direction to the source of stimulus, or otherwise behaving in some definite manner which could not have been deduced or predicted from the direct effects of the stimulating factors. The upholders of the mechanical theory have attempted to conquer this vast and mysterious domain of facts by seeking to do away with the appearance of spontaneity and freedom, by demonstrating in suitable cases that these phenomena of spontaneity and the like would be impossible were it not that the potential energies previously stored up within the organism are liberated by the stimulus.

Thus the effect caused is not equivalent to the stimulus alone, but is rather the resultant of the conditions given in the chemo-physical predispositions of the organism itself, and in the architecture of its parts, _plus_ the stimulus.

Directly a.s.sociated with this property of irritability is another form of spontaneity and freedom in living beings-the power of adapting themselves to changed conditions of existence. Some do not show this at all, while others show it in an astonishing degree, helping themselves out by new contrivances, so to speak. Thus the organism may protect itself against temperature and other influences, against injury, making damages good again by self-repairing processes, "regenerating" lost organs, and sometimes even building up the whole organism anew from amputated parts.

The mechanical interpretation must here proceed in the same way as in dealing with the question of stimuli, applying to the development of form the same explanations as are there employed. And just because this domain does not lend itself readily to mechanical explanation, we can understand that confidence in the sufficiency of this mode of interpretation grows rapidly with each fresh conquest, when this or that particular process is shown to be actually explicable on mechanical principles. Processes of development or morphogenesis-which are among the most intricate and difficult-are attacked in various ways. The processes of regeneration, for instance, are compared with the similar tendencies observed in crystals, which when they are injured have the capacity of restoring their normal form. This capacity therefore obtains in the realm of the inorganic as well as among organisms, and is referred to the tendency of all substances to maintain a definite state of equilibrium, conditioned by their form, and, if that is disturbed, to return to a similar or a new state of equilibrium. Or, the procedure may be to reduce the processes of a developmental or morphogenetic category to processes of stimulation in general, and then it is believed, or even demonstrated, that chemo-physical a.n.a.logies or explanations can be found for them.

Thus, for instance, it is shown that the egg of the sea-urchin may be "stimulated" to development, not exclusively by the fertilising sperm, but even by a simple chemical agent, or that spermatozoids which are seeking the ovum to be fertilised may be attracted by malic acid. These are "reductions" of the higher phenomena of life to the terms of a lower and simpler process of "stimulus," that is to say, to chemotropism in the second case and something a.n.a.logous in the first. A further reduction would be to show that the movement of the spermatozoids towards the malic acid is not a "vitalistic" act, much less a psychically conditioned one, (that is, conditioned by "taste," "sensation," and the voluntary or instinctive impulse liberated thereby), but is a chemo-physical process, although perhaps an exceedingly complex one. It would be another "reduction" of this second kind, if, for instance, the well-known effect of light on plants, which makes them turn their leaves towards it (heliotropism), could be shown to be due to more rapid growth of the leaf on the shaded side, which would lift up the leaf and cause it to turn, or to an increase of turgescence on the shaded side, and if it could be shown that the increase in either case was a simple and obvious physical process, the necessary consequence of the decreased amount of light.

It is obvious, and it is also thoroughly justifiable, that all attempts along these lines of interpretation should be undertaken in the first place in connection with the simplest and lowest forms of life. It is in the investigation of the "Protists," the study of the vital phenomena of the microscopically minute unicellular organisms, that attempts of this kind have been most frequently made. And they follow the course we have just indicated; the "apparently" vitalistic and psychical behaviour of unicellulars (impulse, will, spontaneous movement, selecting and experimenting) is interpreted in terms of reflex processes and the "irritability" of the cell, and these again are traced back, like all stimulus-processes, to the subtle mechanics of the atoms.

Spontaneous Generation.

4. This reduction of known biological phenomena to simpler terms, the lessening of the gap between inorganic and organic chemistry, and the formulation of the doctrine of the conservation of energy, have all prepared the way for a fourth step, the establishment of the inevitable theory of _generatio spontanea sive equivoca_, the spontaneous generation of the living, that is to say, the gradual evolution of the living from the not living. Since the earth, and with it the conditions under which alone life is possible, have had a beginning in time, life upon the earth must also have had a beginning. The a.s.sumption that the first living organisms may have come to the earth on meteorites simply shifts the problem a step farther back, for according to all current theories of the universe, if there are in any of the heavenly bodies conditions admitting of the presence of life, these conditions have arisen from others in which life was impossible. Therefore, since this suggestion is on the face of it a mere evasion of the difficulty, the theory of spontaneous generation naturally arose. There is something almost comical in the change in the att.i.tude of the natural sciences to this theory. For centuries it was one of the beliefs of popular superst.i.tion, with its nave way of regarding nature, that earthworms "developed" from damp soil, and vermin from shavings, and in general that the living arose from the non-living. On the other hand it was one of the characteristics and axioms of scientific thought to reject this nave _generatio equivoca_, and to hold fast to the proposition, _omne vivum ex ovo_, or, at least, _omne vivum ex vivo_. And it was regarded as one of the triumphs of modern science when, about the middle of the last century, Pasteur gave definiteness to this doctrine, and when through him, through Virchow, and indeed the whole younger generation of naturalists, the proposition was modified, on the basis of the newly discovered cell-theory, to _omnis cellula ex cellula_. But a short time after Pasteur's discoveries, the ideas of Darwinism and the theory of evolution gained widespread acceptance. And now it appeared that, in rejecting the theory of _generatio equivoca_, naturalists had, so to speak, sawn off the branch on which they desired to sit, and thus many, like Haeckel, became enthusiastic converts to the theory which natural science had previously rejected.

Constructing theories and speculations as to the possibilities of spontaneous generation is regarded by some naturalists as somewhat gratuitous (_cf._ Du Bois-Reymond). In general, it is regarded as sufficient to point out that the reduction of the phenomena of life as we know them to those of a simpler order, and the unification of organic and inorganic chemistry, have made the problem of the first origin of life essentially simpler, and that the law of the constancy and ident.i.ty of energy throughout the universe permits no other theory. But others go more determinedly to work, and attempt to give concrete ill.u.s.trations of the problem. The most elementary form of life known to us is the cell. From cells and their combinations, their products and secretions, all organisms, plant and animal alike, are built up. If we succeed in deriving the cell, the derivation of the whole world of life seems, with the help of the doctrine of descent, a comparatively simple matter. The cell itself seems to stand nearer to the inorganic, and to be less absolutely apart from the inanimate world than a highly organised body, differentiated as to its functions and organs, such as a mammal. It almost seems as if we might regard the lowest forms of life known to us, which seem little more than aggregated h.o.m.ogeneous ma.s.ses of flowing rather than creeping protoplasm, as an intermediate link between the higher forms of life and the non-living. But the theory does not begin with the cell; it a.s.sumes a series of connecting-links (which may of course be as long and as complicated as the series from the cell upwards to man) between the cell and matter which is still quite "inorganic" and which is capable only of the everyday chemical and physical phenomena, and not of the higher syntheses of these, which in their increasing complexity and diversity ultimately come to represent "life" in its most primitive forms. As proteid is the chief const.i.tuent of protoplasm, it is regarded as the specific physical basis of life, and life is looked upon as the sum of its functions. And it is not doubted that, if the conditions of the universe brought about a natural combination of carbon, hydrogen, nitrogen and oxygen in certain proportions, so that proteid resulted, the transition to proteid which forms itself and renews itself from the surrounding elements, to a.s.similating, growing, dividing proteid, and ultimately to the most primitive plasmic structure, to non-nucleated, nucleated, and finally fully formed cells, could also come about.

Haeckel's demonstration of the possibility of spontaneous generation is along these lines. He refers to the cytodes, the blood corpuscles, to alleged or actual non-nucleated cells, to bacteria, to the simplest forms of cell-structure, as proofs of the possibility of a descending series of connecting-links. He (and with him Nageli) calls these links, below the level of the cell, Probia or Probions, and for a time he believed that he had discovered in _Bathybius Haeckeli_ presently existing h.o.m.ogeneous living ma.s.ses, without cell division, nucleus or structure, the "primitive slime" which apparently existed in the abysmal depths of the ocean to this day. Unfortunately, this primitive slime soon proved itself an illusion.

Opinions differ as to whether spontaneous generation took place only in the beginning of evolution, or whether it occurred repeatedly and is still going on. Most naturalists incline to the former idea; Nageli champions the latter. There are also differences of opinion as to whether the origin of life from the non-living was manifold, and took place at many different places on the earth, or whether all the forms of life now in existence have arisen from a common source (monophyletic and polyphyletic theories).