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 assimilate 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 antitheses have been apparent
which we find to-day. Disguised under various catchwords and with the
greatest diversities of expression, the antitheses 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 antitheses 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 associated 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 classic 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 constituents 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
assertion of quantitative 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 λόγος ἐνυλος (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-Wörterbuch 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
Müller, 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
associated 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 passed 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 analysable, does come
about “naturally,” as it is said. And from the vitalistic standpoint it
has to be asked whether detailed biological investigation and analysis 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 analysis 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 assumptions 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 _à 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 _à 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 Wöhler’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 analysed 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 associated 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 assumes, 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,
analogous 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 analysed into a complex of simpler physical processes. The
circulation of the blood is subject to the same laws of hydrostatics as
are illustrated in all other fluids. Mechanical, static, and osmotic
processes occur in the organism and constitute 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 associated 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 mass, 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 Bütschli succeeded in making a surprisingly good
imitation of this “structure” by mechanical means. Drops of oil intimately
mixed with potash and placed between glass 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 invagination of the blastula to form the gastrula); by
imitating the sphere of cells which compose the blastula with elastic
steel bands he deduced the invagination 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 “amœboid” 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 mass 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 classed 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 associated 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 analogies 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 analogous 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 assumption 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
attitude of the natural sciences to this theory. For centuries it was one
of the beliefs of popular superstition, with its naïve 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 naïve _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 identity of
energy throughout the universe permits no other theory. But others go more
determinedly to work, and attempt to give concrete illustrations 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 homogeneous masses 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 assumes 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 constituent 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 assimilating, 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 Nägeli) 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 homogeneous
living masses, 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; Nägeli 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).
The Mechanics of Development.
5. The minds of the supporters of the mechanical theory had still to move
along a fifth line in order to solve the riddle of the development of the
living individual from the egg, or of the germ to its finished form, the
riddle of morphogenesis. They cannot assume the existence of “the whole”
before the part, or equip it with the idea of the thing as a _spiritus
rector_, playing the part of a metaphysical controlling agency. Here as
elsewhere they must demonstrate the existence of purely mechanical
principles. It is simply from the potential energies inherent in its
constituent parts that the supply of energy must flow, by means of which
the germ is able to make use of inorganic material from without, to
assimilate it and increase its own substance, and, by using it up, to
maintain and increase its power of work, to break up the carbonic acid of
the atmosphere and to gain the carbon which is so important for its vital
functions, to institute and organise the innumerable chemico-physical
processes by means of which its form is built up. Purely as a consequence
of the chemico-physical nature of the germ, of the properties of the
substances included in it on the one hand, and of the implicit structure
and configuration of its parts, down to the intrinsic specific undulatory
rhythm of its molecules, it must follow that its mass grows exactly as it
does, and not otherwise, that it behaves as it does and not otherwise,
duplicating itself by division after division, and by intricate changes
arranging and rearranging the results of division until the embryo or
larva, and finally the complete organism, is formed.
An extraordinary amount of ingenuity has been expended in this connection,
in order to avoid here, where perhaps it is most difficult of all, the use
of “teleological” principles, and to remain faithful to the orthodox,
exclusively mechanical mode of interpretation. To this category belong
Darwin’s gemmules, Haeckel’s plastidules, Nägeli’s micellæ, Weismann’s
labyrinth of ids, determinants, and biophors within the germ-plasm, and
Roux’s ingenious hypothesis of the struggle of parts, which is an attempt
to apply the Darwinian principle within the organism in order here also to
rebut the teleological interpretation by giving a scientific one.(66)
Heredity.
6. With this fifth line of thought a sixth is associated and intertwined.
The problem of development is closely bound up with that of “heredity.” A
developing organism follows the parental type. The acorn in its growth
follows the type of the parent oak, repeating all its morphological and
physiological characters down to the most intimate detail. And the animal
organism adds to this also the whole psychical equipment, the instincts,
the capacities of will and consciousness which distinguish its parents.
The problems of the fifth and sixth order are closely inter-related, the
sixth problem being in reality the same as the fifth, only in greater
complexity.
A step towards the mechanical solution of this problem was indicated in
the “preformation theory” advanced by Leibnitz, and elaborated by Bonnet.
According to this theory the developing organism is enclosed in the
minutest possible form within the egg, and is thus included in the
parental organism, in miniature indeed, but quite complete. Thus the
problem of the “development of form” or of “heredity” was, so to speak,
ruled out of court; all that was assumed was continuous growth and
self-unfolding.
Opposed to this theory was one of later growth, the theory of epigenesis,
which maintained that the organism developed without preformation from the
still undifferentiated and homogeneous substance of the egg. The
supporters of the first theory considered themselves much more scientific
and exact than those of the second. And not without reason. For the theory
of epigenesis obviously required mysterious formative principles, and
equally mysterious powers of recollection and recapitulation, which
impelled the undifferentiated ovum substance into the final form,
precisely like that of its ancestors. Nor need the preformationists have
greatly feared the reproach, that the parental organism must have been
included within the grand-parental, and so on backwards to the first
parents in Paradise. For this “Chinese box” encapsulement theory only
requires that we should grant the idea of the infinitely little, and that
idea is already an integral part of our thinking.
Modern biologists ridicule the preformation hypothesis as altogether too
artificial. And undoubtedly it founders on the facts of embryology, which
disclose nothing to suggest the unfolding of a pre-existent miniature
model, but show us how the egg-cell divides into two, into four, and so
on, with continued multiplication followed by varied arrangements and
rearrangements of cells—in short, all the complex changes which constitute
development. But a preformation in some sense or other there must be;—some
peculiar material predisposition of the germ, which, as such, supplies the
directing principle for the development, and the sufficient reason for the
repetition of the parental form. This is of such obvious importance from
the mechanical point of view that the speculations of to-day tend to move
along the old preformationist lines. To these modern preformationists are
opposed the modern upholders of epigenesis or gradual differentiation, who
attempt to elaborate a mechanical theory of development. And with the
contrast between these two schools there is necessarily associated the
discussion as to the inheritance or non-inheritance of acquired
characters.
Darwin’s contribution to the problem of the sixth order was his rather
vague theory of “Pangenesis.” The living organism, according to him, forms
in its various organs, parts, and cells exceedingly minute particles of
living matter (gemmules), which, “in some way or other,” bear within them
the special characteristics of the part in which they are produced. These
may wander through the organism and meet in the germ-plasm, and then, when
a child-organism is produced, they “swarm,” so to speak, in it again “in
some way or other,” and in some fashion control the development. This
gemmule-theory was too obviously a _quid pro quo_ to hold its ground for
long. Various theories were elaborated, and the world of the invisibly
minute was flooded with speculations.
The most subtle of these, on the side of consistent Darwinism, is that of
Weismann, a pronounced preformation theory which has been increasingly
refined and elaborated in the course of years of reflection. According to
Weismann, the individual parts and characteristics of the organism are
represented in the germ-plasm, not in finished form, but as “determinants”
in a definite system which is itself the directing principle in the
building up of the bodily system, and with definite characteristics, which
determine the peculiarities of the individual organs and parts, down to
scales, hairs, skin-spots, and birth-marks. As the germ-cells have the
power of growth, and can increase endlessly by dividing and re-dividing,
and as each process of division takes place in such a way that each half
(each product of division) maintains the previous system, there arise
innumerable germ-cells corresponding to one another, from which,
therefore, corresponding bodies must arise (inheritance). It is not in
reality the newly developed bodies which give rise to new germ-cells and
transfer to them something of their own characters; the germ-cells of the
child-organism develop from that of the parent (“immortality” of the
germ-cells). Therefore there can be no inheritance of acquired characters,
and no modifications of type through external causes; and all variations
which appear in a series of generations are due solely to internal
variations in the germ-cells, whether brought about by the complication of
their system through the fusion of the male and female germ-cells, or
through differences in the growth of the individual determinants
themselves. The numerous subsidiary theses interwoven in Weismann’s theory
are entirely coherent, and have been thought out to their conclusions with
praiseworthy determination.(67) To the theory as a whole, because of its
fundamental conception of preformation, and to its subsidiary hypotheses,
piece by piece, there has been energetic opposition on the part of the
upholders of the modern mechanical theory of epigenesis. This opposition
is most concretely and comprehensively expressed in Haacke’s “Gestaltung
und Vererbung.” The infinitely complex intricacy of Weismann’s minute
microcosm within the germ-cell, indeed within every id in it, is justly
described as a mere duplication, a repetition in the infinitely little of
the essential difficulties to be explained. The complicated processes of
developing in the growing and inheriting organism cannot be explained,
they say, in terms of processes of the equally complex and likewise
developing germ-plasm. The complex, if it is to be explained at all, must
be explained by the simple—in this case by the functions of a homogeneous
uniform plasm.
At an earlier date Haeckel had made an attempt in this direction in his
theory of the “perigenesis of the plastidules.” Peculiar states of
oscillation and rhythm in the molecules of the germ-substance, handed on
to it from the parent organism and transferable to all the assimilated
matter of the offspring, represent, according to this theory, the
principle which impels development to follow a particular course
corresponding to the type of the parents. This was a _physical_ way of
interpreting the matter. Other investigators have given a _chemical_
expression to their theoretical schemes for explaining heredity.
Haacke declares both these to be unsatisfactory, and replaces them by
morphological formative principles. It is the _structure_ of the otherwise
homogeneous living matter that explains morphogenesis and inheritance.
Minute “gemmæ,” homogeneous fundamental particles of living substance, not
to be compared to or confused with Darwin’s “gemmules,” are aggregated in
“Gemmaria,” whose configuration, stability, symmetrical or asymmetrical
structure, and so on, are determined by the relative positions of the
gemmæ to each other, and these in their turn control the organism and give
it a corresponding symmetrical or asymmetrical, a firmly or loosely
aggregated structure. The completed organism then forms a system in
organic equilibrium, which is constantly exposed to variations and
influences due to external causes (St. Hilaire), and to use and disuse of
organs (Lamarck). These influences affect the structure of the gemmaria,
and as the germ-cells consist of gemmaria, like those of the rest of the
organism, the possibility of the transmission of acquired new characters
is self-evident. The importance of correlated growth and orthogenesis is
explained on a similar basis, and the Darwinian conceptions of the
independent variation of individual parts, of the exclusive dominance of
utility, of the influence of the struggle for existence in regard to
individual selection, and of the omnipotence of natural selection, are
energetically denied.
Oscar Hertwig,(68) de Vries, Driesch(69) and others attempt to reconcile
the preformationist and the epigenetic standpoints, and “to extract what
is good and usable out of both.” Hertwig and Driesch, however, can only be
mentioned with reservations in this connection.
We cannot better sum up the whole tendency of the construction of
mechanical theories on these last lines than in the words of Schwann:
“There is within the organism no fundamental force working according to a
definite idea; it arises in obedience to the blind laws of necessity.”
So much for the different lines followed by the mechanical theories of
to-day. An idea of their general tenor can be gained from a series of much
quoted general treatises, of which we must mention at least the
“classics.” In Wagner’s “Handwörterbuch der Physiologie,” 1842, Vol. I.,
Lotze wrote a long introductory article to the whole work, on “Life and
Vital Force.” It was the challenge of the newer views to the previously
vitalistic standpoint, and at the same time it was based on Lotze’s
general principles and interspersed with philosophical criticism of the
concepts of force, cause, effect, law, &c.(70) A similar train of ideas to
Lotze’s is followed to-day by O. Hertwig, especially in his “Mechanismus
und Biologie.”(71) Lighter and more elegant was the polemic against vital
force, and the outline of a mechanical theory which Du Bois-Reymond
prefaced to his great work, “Untersuchungen über die tierische
Electricität” (1849). It did not go nearly so deep as Lotze’s essay, but
perhaps for that very reason its phrases and epigrams soon became common
property. We may recall how he speaks of vital force as a “general servant
for everybody,” of the iron atom which remains the same whether it be in
the meteorite in cosmic space, in the wheel of the railway carriage, or in
the blood of the thinker, and of analytic mechanics which may be applied
even to the problem of personal freedom.
The most comprehensive and detailed elaboration of the mechanical theory
of life is to be found in Herbert Spencer’s “Principles of Biology.”(72)
Friedrich Albert Lange’s “History of Materialism” is a brilliant plea for
mechanical theories,(73) which he afterwards surpassed and neutralised by
his Kantian Criticism. Verworn, too, in his “Physiology”(74) gives a clear
example of the way in which the mechanical theory in its most consistent
form is sublimed, apparently in the idealism of Kant and Fichte, but in
reality in its opposite—the Berkeleyan psychology. A similar outcome is in
various ways indicated in the modern trend of things.