THE VITAL IMPETUS
Two main conclusions emerge from the discussions of the last three
chapters: (1) that physiology encourages no notions as to a “vital
principle” or force, or form of energy peculiar to the organism; and
(2) that although physiological analysis resolves the metabolism of
the plant and animal body into physico-chemical reactions, yet the
direction taken by these is not that taken by corresponding reactions
occurring in inorganic materials. From these two main conclusions we
have, therefore, to construct a conception of the organism which shall
be other than that of a physico-chemical mechanism.
The ordinary person, unacquainted with the results of physiological
analysis, and knowing only the general modes of functioning of the
human organism, has, probably, no doubt at all that it is “animated”
by a principle or agency which has no counterpart in the inorganic
world. This is the “natural” conclusion, and the other one, that life
is only an affair of physics and chemistry, must appear altogether
fanciful to anyone who knows no more than that the heart propels the
blood, that the latter is “purified” in the lungs, that the stomach
and liver secrete substances which digest the food, and so on. It is
difficult for the modern student of biology, saturated with notions of
bio-chemical activities, gels and sols and colloids and reversible
enzymes and kinases and the like, to realise that the belief in a vital
agency is an intuitive one, and that the mechanistic conception of
life is only the result of the extension to biology of _methods_ of
investigation, and not a legitimate conclusion from their _results_.
To the anatomist, the embryologist, and the naturalist, as well as to
the physicist unacquainted with the details of physiology, no less
than to the ordinary person this is perhaps by far the most general
attitude of mind. It would probably be impossible for anyone to study
only organic form and habits and come to any other conclusion than that
there was something immanent in the organism entirely different from
the agencies which, for instance, shape continents, or deltas, or river
valleys. And this conclusion would probably come with still greater
force to the embryologist, even though he still possessed a general
knowledge of physiological science.
The mechanistic conception of life has, without doubt, been the result
of the success of a method of analysis. One sees clearly that just in
proportion as physical and chemical sciences have been most prolific
of discovery, so physiology, leaning upon them and borrowing their
methods, has been most progressive and mechanistic.
Mechanistic hypotheses of the organism may all be traced back to
Descartes, who built upon the work of Galileo and Harvey. The anatomy
of Vesalius and his successors would have led to no such notions,
had not the discoveries of Copernicus, Tycho, and Kepler shown men
an universe actuated by mechanical law. To a thinker like Descartes,
at once the very type of philosopher and man of science, Harvey’s
discovery of the circulation of the blood must have suggested
irresistibly the extension of mechanical law to the functioning of
the human organism, and it is significant that he made this extension
without including a single chemical idea, and yet produced a logical
hypothesis of life as satisfactory and complete in its day as, for
instance, the Weismannian hypothesis of heredity has been in ours.
His hypothesis of the organism was purely mechanical. It has been
said that his organism was an automaton, like the mechanical Diana of
the palace gardens which hid among the rose-bushes when the foot of
a prying stranger pressed upon the springs hidden in the ground. Its
functions were matters of hydraulics: of heat, and fluids, and valves.
His physiology was Galenic, apart from Harvey’s discovery of the motion
of the blood in a circuit, for he did not accept the notion of the
heart as a propulsive apparatus. The food of the intestine was absorbed
as chyle by the blood and carried to the liver, where it became endued
with the “natural spirits,” and then passing to the heart it became
charged with the “vital spirits” by virtue of the flame, or innate
heat, of the heart, and the action of the lungs. This flame of the
heart, fed by the natural spirits, expanded and rarefied the blood, and
the expansion of the fluid produced a motion, which, directed by the
valves of the heart and great vessels, became the circulation. The more
rarefied parts of the blood ascended to the brain, and there, in the
ventricles, became the “animal spirits.”
Subtle and rarefied though they were, these animal spirits were a
fluid, amenable to all the laws of hydro-dynamics. This was contained
in the cerebral ventricles, and its flow was regulated just like the
water in the pipes and fountains of the garden mechanisms. From the
brain it flowed through the nerves, which were delicate tubes in
communication with the ventricles, and which were provided with valves;
and this outward flow corresponds to our modern efferent nervous
impulse. The afferent impulse was represented by the action of the
axial threads contained in the nerve tubuli. When a sensory surface
was stimulated, these threads became pulled, and the pull, acting on
the wall of the cerebral ventricle, caused a valve to open and allowed
animal spirits to flow along the nerve to all the parts of the body
supplied by the latter. In the effector organs, muscles or glands, this
influx of animal spirits produced motion or other effects. This, in
brief, was the physiology of Descartes.
He spoiled it, says Huxley, by his conception of the “rational soul.”
Fearing the fate of Galileo, he introduced the soul into his philosophy
of the organism as a sop to the Cerberus of the Church. It was
unworthy: a sacrifice of the truth which he saw clearly. Is it likely
that Descartes deliberately made part of his philosophy antagonistic to
the rest with the object of averting the censure of the Church? He was
not a man likely to rush upon disaster, but the conviction that what he
wrote had in it something great and lasting must have made it hardly
possible that he should traffic with what he held to be the truth.
The rational soul was something superadded to the bodily mechanism.
It was not a part of the body though it was placed in the pineal
gland; a part of the brain, which by its sequestered situation and
rich blood supply suggested itself as the seat of some important and
mysterious function. Its existence was bound up with the integrity of
the body, and on the death of the latter the soul departed. But the
body did not die because the soul quitted it, it had rather become an
unfit habitation for the soul. Without the latter the functions of
the healthy body might still proceed automatically, and if the soul
influenced action it actuated an existing mechanism, and without that
mechanism it could not act, though the mechanism might act without the
soul. Thought, understanding, feeling, will, imagination, memory, these
were the prerogatives of the soul, and not those of the automatic body.
But the movements of the latter, even voluntary movements, depended on
a proper disposition of organs, and without this they were wanting or
imperfect.
Thus to a thoroughgoing mechanism Descartes joined a spiritualistic
and immortal entity; and this, to the materialism of the middle of
the nineteenth century, was the blemish on his philosophy. Now of
all men who have ever lived he is probably the one who has most
profoundly influenced modern thought and investigation: to us what
he wrote seems strangely modern, and this apparently arbitrary
association of spiritualistic and materialistic elements in life seems
almost the most modern thing in his writings. Being, he said, was
indeed thought, but how could he derive thought from his clockwork
body, with its valves and conduits and wires? No more can we derive
consciousness from the wave of molecular disturbance passing through
afferent nerve and cerebral tracts. We must account for all the energy
of this disturbance, from its origin in the receptor organ to its
transformation into the wave of chemical reaction in the muscle, and we
must regard its transmission as a conservative process. But how does
the state of consciousness accompanying the passage through the cortex
of this molecular disturbance come into existence? None of the energy
of the nerve disturbance has been transformed into consciousness:
the latter is not energy nor anything physical. It is something
concomitant with the physico-chemical events involved in a nervous
process, an “epiphenomenon.” We have to imagine a “parallelism” between
the mechanistic body and the mind. But if we admit that consciousness
may be an effective agency in our behaviour, what is the difference
between modern theories of physico-psychic parallelism and the
Cartesian theory of a rational soul in association with an automatic
body? Descartes denied the existence in animals other than man of the
rational soul; the latter was not necessary. But he, like us, must have
been familiar with reflex actions and must have seen that consciousness
was not invariably associated, even in himself, with bodily activity.
And he must have recognised the great distinction between the
intelligent acting of man and the instinctive behaviour of the lower
animals. There was something in man that was not in the brute.
Thus the first physiology, borrowing its ideas and methods from the
first physics, was, like the latter, a mechanical science. After
Galileo and Torricelli came Borelli with his purely mechanical
conceptions of animal movement, and of the blood circulation,
introducing even then mathematics into biology. There was no chemistry
in these speculations, though Basil Valentine and Paracelsus and
Van Helmont had preceded Descartes and Borelli. This chemistry was
mystical, and though chemical reactions had been studied in the
organism, they were supposed to be controlled by spiritual agencies,
the “archei” of the first bio-chemists. But that notion was to
disappear, and with Sylvius the conception of the animal body as a
chemical mechanism arose. All that was valuable in Van Helmont’s
chemistry was taken up by Sylvius, but in his mind the fermentations of
the older chemists were sufficient in themselves without the mystical
“sensitive soul” and “archei.” With Sylvius and Mayow physiology
became based upon chemical discovery and again became mechanistic, and
remained so until the time of Stahl, when chemical discovery attained
for the time its greatest development.
The seventeenth century ended with the work of Stahl. It is well
known to students of science how the views of this great chemist
sterilised chemical investigation almost until the time of Lavoisier.
The notion of phlogiston as an active constituent of material bodies
entering and leaving them in their reactions with each other was a
clear and simple one, and it served as a working hypothesis for the
chemists who immediately followed Stahl. It was, of course, a false
hypothesis, and retarded discovery to the extent that the greater part
of the eighteenth century is a blank for chemistry, when compared with
the seventeenth and nineteenth centuries. Deprived therefore of the
stimulus afforded by new physico-chemical methods of investigation,
physiology ceased to maintain the progress it had made during the
previous century, and the only great name of this period is that of
von Haller. Comparative anatomy, and zoological exploration, on the
other hand, made enormous advances, and for these branches of biology
the eighteenth century was the great period. It was the period of the
historic vitalistic views--vital principles, and vital and formative
forces. Stahl’s teaching dominated physiology just as it did chemistry.
Chemical and physical reactions occurred in the living body just as
they did in non-living matter, but they were controlled and modified by
the soul, or vital principle. It has been said that Stahl’s vitalistic
teaching retarded the progress of physiology, but it does not seem
clear that this was the case. What did retard physiological discovery
was the lack of progress made by chemistry and physics, and this may
have been the result of the Stahlian phlogistic hypothesis.
However this may be, it seems clear that it was the discoveries of
the great chemists of the close of the eighteenth century that again
introduced mechanistic views into physiology. With the discoveries of
Lavoisier and his successors the latter science acquired new methods
of research and the older working hypotheses were re-introduced.
There has been no recession from this position during the nineteenth
century. Mechanistic biology culminated in the writings of Huxley and
Max Verworn and received a new accession of strength almost in our
own day in the modern discoveries of physical chemistry; and when
physiology became truly a comparative science, and embraced the lower
invertebrates, it became perhaps most mechanistic--witness the writings
of Jacques Loeb.
Of far greater philosophical importance than the physico-chemical
investigation of the functioning of individual organisms has been the
essentially modern experimental study of embryological processes. The
former deals essentially with the _means_ of growth, reproduction, and
so on. We can no longer doubt that the changes which we can observe
taking place in the organism, either the developing embryo or the fully
formed animal, are, in the long run, physico-chemical changes; and in
ultimate analysis we cannot expect to find anything else than processes
of this nature.
But physiological investigation has failed to provide anything more
than this analysis. If we watch the development of the egg of an animal
into a larval form, and continue to trace the metamorphosis of the
larva into the perfect animal, we cannot fail to conclude that, beside
the individual physico-chemical reactions which proceed, there is also
_organisation_. The elementary processes must be _integrated_. There
must be a due order and succession in them. In studying developmental
processes, in considering the developing organism _as a whole_,
we are impressed above all else with the notion that not only do
physico-chemical reactions occur, but that these are _marshalled_
into place, so to speak. When we attempt to make a description of
this integration of those ultimate processes which we can describe
in terms of physical chemistry, physiology fails us. “At present,”
says Morgan, “we cannot see how any known principles of chemistry or
of physics can explain the development of a definite _form_ by the
organism or by a piece of the organism.” It is true that we can attempt
to imagine a physico-chemical mechanism which is the organisation
of the developing embryo; but this must be a logically constructed
mechanism, not only incapable of experimental verification, but which
can also be demonstrated, purely by physical arguments, to be false.
This conclusion may, without exaggeration, be said to be that of modern
experimental embryology.
There have always been (in modern times) two views as to the nature
of the embryological process: (1) that the egg contained the fully
formed organism in a kind of rolled-up condition, and that the process
of development consisted merely in the unfolding (_evolution_)
of this embryonic organism, and in the increase in volume of its
parts. This was the hypothesis of preformation held in the beginning
of embryological science. It involved various consequences: the
limitation, for instance, of the duration of a species, since each
generation of female organisms contained in their ovaries all the
future generations; with other consequences which the preformationists
did not hesitate to accept. (2) The other view was the later one of
epigenesis: the egg was truly homogeneous and the embryo grew from it.
Obviously the acceptance of this hypothesis led to vitalism, and we
find that it was abandoned just as soon as the embryologists recognised
that physics provided a corpuscular theory of matter, when a return
was made to the preformation views of earlier times; views which
lent themselves to the construction of a mechanistic hypothesis of
development.
[Illustration: FIG. 12.]
We may state very briefly the main facts of the development of a
typical animal ovum, such as that of the sea-urchin.
The fertilised ovum divides into two (2), and then each of these
blastomeres divides again in a plane perpendicular to the first
division plane (3). The third division plane is at right angles to
the first two, and it cuts off a tier of smaller blastomeres from the
tops of the first four. There are now (4) two tiers of blastomeres, a
lower tier of large blastomeres and an upper tier of smaller ones. This
is the 8-cell stage. Next, each of these blastomeres divides in two
simultaneously so that the embryo now consists of sixteen cells. After
this the divisions proceed with less regularity, but after about ten
divisions the embryo consists of about 1000 cells (2^{10}), and these
are arranged to form a hollow sphere consisting of a single layer of
cells. The latter are furnished with cilia, and the whole embryo, now
known as the blastula, can swim about by the movements of these cilia.
Further development results in another larval form--the gastrula, and
yet another, the pluteus larva. After this the transformation into the
fully formed sea-urchin occurs.
With various modifications this scheme represents the early development
of a very large number of animals belonging to most groups.
If we study the process of cell-division we shall find it very
complicated. The ovum, immediately after fertilisation, consists of two
main parts, the nucleus and the cytoplasm.
[Illustration: FIG. 13.]
Within the nucleus is a substance distinguishable from the rest; it is
distributed in granules and is called the chromatin (1). When the cell
is about to divide this chromatin becomes arranged in a long coiled
thread (2), and then (3) this chromatic thread breaks into short rods
called chromosomes. Two little granules now appear, one at each end of
the nucleus, and very delicate threads, the asters, appear to pass from
each of these bodies towards the chromosomes (4). Each of the latter
then splits lengthways into two, and a half chromosome appears to be
drawn by the asters towards the poles of the nucleus. The latter then
divides (5) and then the whole cell divides. What thus, in essence,
happens in nuclear divisions is that the chromatin of the nucleus is
more or less accurately halved. Apparently this substance consists of
very minute granules and the whole process is directed towards the
splitting of each of these granules into two. A half-granule then goes
to each of the daughter nuclei. Every time the embryo divides this
process is repeated. Thus each of the (theoretically) 1028 cells of the
blastula contains 1/1028th of the substance of each chromatic granule
in the fertilised ovum.
Pfluger and Roux (in 1883 and 1888 respectively) were the pioneers in
the experimental study of the development of the ovum, and the results
of their work and that of their successors has, more than anything else
in biology, modified and shaped our notions of the activities of the
organism. Roux found, or thought so at least, that the first division
of the frog’s egg marked out the right and left halves of the body, the
one blastomere giving rise to the right half, the other to the left
half. The next division, which separates each of these blastomeres,
marked out the anterior and posterior parts of the embryo. Thus:--
[Illustration: FIG. 14.--The frog’s egg in the 4-blastomere stage seen
from the top.]
Now in an experiment which has become classical Roux succeeded in
killing one of the blastomeres in the 2-cell stage, while the other
remained alive. The uninjured blastomere then continued to develop,
_but it gave rise to a half-embryo only_.
Upon these experiments the Roux-Weismann hypothesis of development--the
“Mosaik-Theorie”--was developed. The lay reader will see how obviously
the facts of nuclear division and the experimental results indicated
above lend themselves to a mechanistic hypothesis. Notice that but
for the physical conception of matter as made up of molecules and
atoms the mosaic-theory would hardly have shaped itself in the minds
of biologists. But this notion of matter consisting of corpuscles must
have suggested that the essential “living material” of the organism
consisted also of corpuscles, as soon as a microscope powerful enough
to see the chromatic granules was turned on a dividing cell prepared
so as to render these bodies visible. Obviously the primordial ovum
contained all the elements of the organisms into which it was going
to develop. But then in the process of division of the ovum all these
chromatic granules are shared out among the cells, and a really very
pretty mechanism comes into existence for this purpose of distribution.
Weismann built up his hypothesis of the germ-plasm upon the
observations we have outlined. The chromatic matter of the nucleus
consists of elements called _determinants_, the determinants themselves
being composed of ultimate bodies called _biophors_. Each determinant
possesses all the mechanism, or factors, necessary for the development
of a part of the body: there are determinants for muscles, nerves,
connective tissues, for the retina of the eye, for hairs of each
colour, for the nails, and so on. All these determinants are contained
in the chromatin of the nucleus of the egg, and in the divisions of the
latter they are gradually separated so that ultimately each cell of
the larva contains the determinants for one individual part, or organ,
or organ-system of the adult body. The right blastomere, for instance,
contains all the determinants for the right side of the frog’s body,
those for the left side being contained in the left half. The process
of cell-division involved in the segmentation of the egg consists
then in the orderly disintegration of this complex of determinants,
and in the marshalling into place of the isolated elements. The cell
body--the cytoplasm--carried out a very subordinate rôle, mainly
that of nourishing the essential chromatic substance. Such was the
Roux-Weismann Mosaic-theory of development in its pristine form.
It is clearly a preformation hypothesis. It is true that the actual
organism is not contained in the germ, but all the parts of the latter,
even the colours of the eyes or hair, are present in it in the form
of the determinants. Obviously it involves a mechanism of almost
incredible complexity. But if we regard it as a working hypothesis of
development this complexity of detail does not matter; its truth would
be indicated by the fact that all analysis of the processes involved
would tend to simplify it and to smooth out the complexity. But this
is exactly what has not happened, for all subsequent investigation has
necessitated subsidiary hypothesis after hypothesis. As a theory of
development it has failed entirely.
If, after one of the blastomeres in the frog’s egg at the 2-cell stage
be killed, the egg is then turned upside down, the results of the
experiment become totally different; the uninjured blastomere develops
into a _whole_ embryo, differing from the normal one chiefly in that it
is smaller. If the uninjured egg in the 2-cell stage be turned upside
down _two whole embryos_, connected together in various ways, develop.
In the frog’s egg the two first blastomeres cannot be separated from
each other without rupturing them, but in the egg of the salamander
they can be separated. After this separation two perfect, but small,
embryos develop. In the egg of the newt a fine thread can be tied
round the furrow formed by the first division. If this ligature be
tied loosely it does not affect development, and then it can be seen
that the median longitudinal plane of the embryo does not correspond,
except by chance, with the first division plane. If the ligature be
tied tightly, then each of the blastomeres gives rise to an entire
embryo. If it is tied in various places monsters of various types are
produced. Therefore there is no segregation of the determinants in the
first two blastomeres. These results, moreover, are not exceptional,
for similar ones have been obtained with other animal embryos, in
fishes, _Amphioxus_, ascidians, medusæ, and hydrozoa, and in some
cases even each of the first four blastomeres develops into an entire
embryo when it is separated from the rest. In the sea-urchin embryo
the blastomeres can be shaken apart; or by removing the calcium which
is contained in sea water the blastomeres can easily be separated from
each other. It was then found by Driesch that each of the blastomeres
in the 16-cell stage could develop into an entire embryo. It is plain,
then, that up to this stage at least there has been no segregation of
the determinants.
Upon the results of these experiments Driesch based his first proof of
vitalism. Let us suppose that there is a mechanism in the developing
egg. Now the embryo which results from the latter sooner or later
acquires a three-dimensional arrangement of parts: head-end differs
from tail-end, dorsal surface differs from ventral surface, and
the parts differ on either side of the median plane. The mechanism
must, therefore, be one which acts in three dimensions, anterior and
posterior, laterally, and dorso-ventrally. We may represent it by a
diagram of three co-ordinate axes, _x_, _y_, _z_; _x_ and _y_ being in
the plane of the paper, and _z_ at right angles to the plane of the
paper. Now in the 2-cell stage the same mechanism must be present,
for this stage develops normally into one entire embryo. But since
_either_ of the blastomeres may develop into an entire embryo, the
mechanism must also be present in each of them, and since in the
16-cell stage each blastomere may develop an entire embryo, it must
be present in each of the sixteen blastomeres. A three-dimensional
mechanism is therefore capable of division down to certain limits.
[Illustration: FIG. 15.]
[Illustration: FIG. 16.]
Suppose now that we allow the sea-urchin egg to develop normally up to
the blastula stage. In this stage it is a hollow sphere, the wall of
which is a single layer of cells. It is similar all round, that is, we
cannot distinguish between top and bottom, right and left, anterior
and posterior regions; but since it develops into a larva in which
all these distinctions become apparent very soon, it must possess the
three-dimensional mechanism, since the activity of the developmental
process is going to produce different structures in each direction. Now
the blastula, by very careful manipulation can be divided, cut into
parts with a sharp knife. Since it is similar all round the direction
of the cut is purely a matter of chance. It can be cut through along
the planes 1 2, 3 4, 5 6, 7 8, for instance; really there are an
infinite number of planes along which the blastula can be cut into
two separate parts, and the direction of the plane is not a matter of
choice, but purely a matter of chance. Nevertheless, each of the parts
into which the larva is cut becomes an entire embryo. For a time the
partial blastula--approximately a hollow hemisphere in form--goes on
developing as if it were going to become a partial embryo, but soon the
opening closes up and development becomes normal. It does not matter
even if the two parts into which it is divided are not alike in size;
provided that a part is not too small, it will follow the ordinary
course of development.
[Illustration: FIG. 17.]
Suppose the blastula opened out on the flat, like the Mercator
projection of a globe on a flat map. Suppose that _a_ is a small element
of it. Suppose that the rectangles _b c d e_, _F G H e_, _I J c L_, _M
N O e_, and as many more as we care to make, represent the pieces of
the blastular wall separated by our operation--they all contain the
element _a_, but this is in a different position in each case. There
are really an infinite number of such parts of the blastula and _a_
occupies an infinitely variable position in each of them.
This demonstration is very important, so let us make it as clear
as possible: Driesch’s logical proof of vitalism may be stated as
follows:--
The different parts of the blastula are going to become different parts
of an embryo.
The part _a_, occupying a definite position in the entire blastula, is
going to become a definite part, having a definite position, in the
embryo;
But each partial blastula becomes an entire embryo and the same part
_a_ occupies a different position in each.
Therefore _any_ part of the blastula may become _any_ part of the
embryo.
Now if a mechanism is involved, it must, according to our ideas of
mechanism, be one which is different in its parts, for each part of it
produces a different result from the others;
But since any part of the mechanism may produce any of the different
results contained in the embryo, every one of its parts must be similar
to every other one.
That is, all the parts of the mechanism are the same, though the
hypothesis requires that they should be different.
We conclude, then, that a mechanism such as we understand a mechanism
to be in the physical sciences cannot be present in the developing ovum.
Nevertheless, an _organisation_, using this term as an ill-defined
one for the present, must exist in the ovum, or the system of
undifferentiated cells into which the ovum divides, during the first
stages of segmentation. In certain animals, Ctenophores (Chun, Driesch,
and Morgan), and Mollusca (Crampton), for instance, separation of the
blastomeres in the first stages of segmentation produces different
results from those mentioned above. In these cases the isolated
blastomeres develop as partial embryos, that is, the latter are
incomplete in certain respects, and this incompleteness corresponds, in
a general way, to the incompleteness of the part of the ovum undergoing
development. We have thus the apparently contradictory results: (1)
each of the first few blastomeres resulting from the first divisions
of the ovum is similar to the entire ovum, and develops like it; and
(2) each of the first few blastomeres is different from the others, and
from the entire ovum, and develops differently from the others, and
from the entire ovum.
Let us try to construct a notion of what this organisation in the
developing ovum must be. In the 16-blastomere stage of the sea-urchin
egg we have a “system” of parts. In the case of normal development each
of these parts has a certain actual fate--it will form a part of the
larva into which the embryo is going to develop: It has, as Driesch
says, a _prospective value_. But let the normal process be interfered
with, and then each of these parts does something else. In the extreme
case of interference, when the blastomeres are separated from each
other, each blastomere, instead of forming only a part of a larva,
forms a whole larva. The _prospective potency_ of the part, that is its
possible fate, is greater than its prospective value. Normally it has
a limited, definite function in development, but if necessary it may
greatly exceed this function.
What any one blastomere in the system will become depends upon its
position with regard to the other blastomeres. When the egg of the frog
is floating freely in water it lies in a certain position with the
lighter part uppermost, and then development is normal, each of the two
first blastomeres giving rise to a particular part of the body of the
larva; that is, each of them is affected by the contact of the other
and develops into whatever part of the normal embryo the other does
not. But let the egg in the 2-cell stage be turned over and held so
that the heavy part is uppermost: the protoplasm then begins to rotate
so as to bring the lighter part uppermost; but the two blastomeres
do not, as a rule, adjust themselves to the same extent, and at the
same rate, and corresponding parts may fail to come into contact with
each other. Lacking, then, the normal stimulus of the other part,
each blastomere begins to develop by itself, and a double embryo is
produced. It is clear, then, both from this case and the last one,
that the actual fate of any one part of the system of blastomeres _is
a function of its position_. What it will become depends precisely on
where it is situated with respect to the other parts.
Driesch, then, calls the system of parts in such cases as the 2-cell
frog embryo, or the 16-cell sea-urchin embryo, an _equipotential
system_, since each part is potentially able to do what any other part
may do, and what the whole system may do. But in normal development
each part has a definite fate and its activity is co-ordinated
with that of all the other parts. It is, therefore, an _harmonious
equipotential system_, each part acting in harmony, and towards a
definite result, with all the others; although if necessary it can take
the place of _any_ or _all_ of the others.
Such an harmonious equipotential system exists only at the beginning
of the development of the egg. It is represented by the 8-cell
stage of Echinus but not by the 16-cell stage, since, though the
1/16-blastomeres produce gastrulæ (the first larval stage), they do
not produce plutei (the second stage). It is represented by the 4-cell
stage of Amphioxus but not by the 8-cell stage. It is not exhibited
even by the 2-cell stage of the Ctenophore egg. What does this mean?
It means that the further development proceeds, the less complete does
the “organisation” inherent in any one part of the system become. “The
ontogeny assumes more and more the character of a mosaic work as it
proceeds” (Wilson).
Or perhaps it means, and this is the better way of putting it, that
the “organisation,” whatever it may be, depends on size. We see this
very clearly in the experiment of cutting in two the blastula of the
sea-urchin. If the pieces are of approximately equal size each will
form an entire Pluteus larva, but if one of them is below a certain
limit of size it will not continue to develop. The “organisation,”
therefore, has a certain volume, and this volume is much greater than
that of any one of the cells of which the fragment exhibiting it is
composed. It is enormously greater than the volume of any group of
determinants which we can imagine to represent the different kinds
of cells composing the body of the Pluteus larva, and still more
enormously greater than the volume of a “molecule” of protoplasm. Now
this association of “organisation” and size is of immense philosophical
importance, for it does away, once and for all, with the idea that
the “organisation” is solely a series of chemical reactions. If
it were, one cell of the blastula would contain it, for on the
mechanistic hypothesis one cell, the egg-cell, contains it, and this
cell can be divided innumerable times and still contain it. The egg
is a _complex equipotential system_ (Driesch), which divides again
and again throughout innumerable generations, and still contains the
“organisation.”
It is in vain that we attempt the misleading analogy of the “mass
action” of physical chemistry, to show that volume may influence
chemical action. In such a mass action what we have is this:--
_A_↓{a} + _B_↓{b} ⇆ _C_↓{c} + _D_↓{d}
the letters _A_, _B_ and _C_ standing for chemical substances present,
and the letters _a_ and _b_, etc., representing the active masses of
these substances. But variations in this active mass affect only the
_velocity_ of the reaction. What we have to account for in our blastula
experiments is the _nature_ of the reaction, and how can velocity or
even nature of reaction affect _form_? If we could show that the form
of the crystals deposited from a solution in some reaction depended on
the volume of the solution, the analogy would be closer, though even
then the difficulties in pressing it would be so enormous as to render
it futile to attempt to entertain it.
A chemical mechanism cannot, then, be imagined, much less described,
and the only other mechanism so far suggested is the Roux-Weismann
one, involving the disintegration of the determinants supposed to be
present in the egg nucleus. Let us suppose (in spite of the incredible
difficulty in so doing) that there is such a mechanism. It must usher
the nuclei containing the determinants of the embryonic structure
into their places: those for the formation of the nerve-centre go
forward; those for the mouth, gut, and anus go backwards and downwards;
those for the arms go forwards, ventrally, and posteriorly, in a very
definite way; and those for the complicated skeleton are distributed
in a variety of directions which defy description. These nuclei are,
in short, moved up and down, right and left, backwards and forwards,
and become built up into a complicated architecture. Suppose we prevent
this. Suppose we compress the segmenting egg between glass plates so
that the nuclei are compelled to distribute themselves in one plane
only: to form a flattened disc in which the only directions are right
and left and anterior and posterior. This has been done by Driesch and
others. On the Roux-Weismann original hypothesis a monstrous larva
ought to result, for the first nuclei separated from each other have
been forced into positions altogether different from those which they
should have occupied had they developed normally. Yet on releasing the
pressure readjustment takes place. New divisions occur so as to restore
the normal form of larva. The Roux-Weismann subsidiary hypothesis is
that the stimulus of the pressure has compelled the nuclei to divide at
first in such a way as to compensate for the disturbance.
Let us remove some of the blastomeres. On the original hypothesis the
determinants for the structures which the nuclei of these blastomeres
contained have been lost. These structures should, therefore, be
missing in the embryo. But nothing of the sort is the result. Other
nuclei divide and replace the lost ones, and the embryo develops as
in the normal mode. The reply is that in addition to the determinants
which were necessary for their own peculiar function, these nuclei
contained a reserve of all others. On disturbance these determinants,
“latent” in all other conditions, became active and restituted the lost
parts.
Let us remove some organ from an adult organism. The most remarkable
experiment of this kind is the removal of the crystalline lens from
the eye of the salamander. Now the lens of the eye develops from the
primitive integument (ectoderm) of the head, but the iris of the eye
develops mainly from a part of the primitive brain. After the operation
a new lens is formed _from the iris_ and not from the cornea. Therefore
the highly specialised iris contains also determinants of other kinds.
Does it contain those for itself and lens only, or others? If it
contains many kinds, then we conclude that even the definite adult
structures contain determinants of many other kinds than their own,
that is, reserve determinants are handed down in all cells capable
of restitutive processes, practically all the cells of the body. Or
does it contain only its own and those of the lens? Then this highly
artificial operation was anticipated, an absurd hypothesis which need
not be considered.
This particular mechanistic process (and no other one is nearly
so plausible) crumbles away before attempts at verification, and
it survives only by the addition of subsidiary hypothesis after
hypothesis. In itself this demonstrates that it is an explanation
incompetent to describe the facts.
What, then, is the “organisation”? It is something elemental, and we
may just as well ask what is gravity, or chemical energy, or electric
energy. It cannot be said to be any of these things or any combination
of them. “At present,” says a skilful and distinguished experimenter,
T. H. Morgan, “we cannot see how any known principle of chemistry or of
physics can explain the development of a definite form by the organism
or a piece of the organism.” “Probably we shall never be able,”
concludes Morgan, who is anything but a vitalist. But does not this
mean just that in biology we observe the working of factors which are
not physico-chemical ones?
We have seen that the physiologist studies something very different
from that which the embryologist or naturalist studies. The former
investigates a _part_ of the animal, arbitrarily detached from the
whole because the complexity of the functions of the simplest organism
is such that all of them cannot be examined at once. He adopts the
methods of physical chemistry in his investigation and whatever
results he obtains are necessarily of the same order. Inevitably,
from the mere nature of his method, he can see, in the organism,
only physico-chemical phenomena. The embryologist, on the other hand,
studies the organism as a whole and seeks to determine how definite
forms are produced, and how a change in the external conditions affects
the assumption of these forms. We have seen with what little success
the attempts to relate embryological processes with physico-chemical
ones alone have met. In all studies of organic form mechanism has
failed. It is useless to attempt to press the analogies of crystalline
form, and the forms assumed in nature by dynamical geological agencies.
If the reader examines these analogies critically he will see that they
are superficial only.
We seem, however, to see in those actions of the organism which are
called “tropistic” or “tactic,” reactions of a purely physico-chemical
nature, and starting with these as a basis a plausible theory of
organic movements on a strictly mechanistic basis might be built
up.[21] A “tropism” is the movement of a fixed organism with respect
to a definitely directed external stimulus. This movement may be that
produced by growth of its parts, or by the differential contraction
or expansion of its parts. A “taxis” we may call the motion of a
freely-moving organism in response to the same directed stimuli. The
movements whereby a green plant turns towards the light are called
heliotropic, and those of its roots in the perpendicular direction are
called geotropic. The motion of the freely-moving larva of a barnacle,
for instance, in swimming towards a source of light are called
“phototactic.”
[21] Many of Jacques Loeb’s remarkable investigations point in this
direction.
In all these cases we have to think of the stimulus as a “field
of energy” in the sense in which physicists speak of electric, or
magnetic, or electromagnetic, or thermal, or gravity fields. In all
these cases the factors affecting the movements of the organism are
directed ones.
An electric field, for instance, (1), is produced by placing
the electrodes of a galvanic cell at opposite extremities of a
water-trough: we imagine the electrons moving from one side of the
trough to the other in parallel lines, and in a certain direction. A
light field (2) would be produced by the radiation of light travelling
in straight lines through the water.
[Illustration: FIG. 18.]
The movements of the organism displaying a tropism or a taxis are not
caused by the stimuli of the field, but are only directed by it. In the
absence of these stimuli it would swim at random. In a field, however,
it will orientate itself in some direction with reference to the lines
of force. A “positively phototactic” animal swims towards the focus
from which the light radiation emanates, and a “negatively phototactic”
one swims in the other direction. On the theory of tropistic and tactic
movements this orientation is produced by the differential stimulation
of the opposite sides of the organism. Let us take as a concrete
example the case of a caterpillar which creeps up the stem of a plant
to feed on the tender shoots near the apex. The animal possesses an
elongated body, with muscles beneath the integument, and sensory
nerve-endings in the latter. Its muscles are in a state of “tone,”
that is, they are normally always slightly tense. The incident rays of
light affect the dermal sense-organs, stimulating ganglionic centres
and setting up efferent impulses which descend to the muscles. Let us
suppose the animal is moving so that the longitudinal axis of its body
is at an angle, say of 45°, to the direction of the incident light:
one side of the body is therefore stimulated and the other is not.
The stimulation of the lighted side sets up efferent nerve impulses
which descend to the muscles of this side and increase their tone
(or else the lack of stimulation of the other side produces impulses
which inhibit the muscular tone, or impulses which would otherwise
preserve the tone cease in the absence of light stimulation). In any
case the muscles of the lighted side contract, and the body of the
caterpillar moves so that it sets itself parallel to the direction of
the radiation. Both sides of the body are then equally stimulated and
the animal moves towards the light.
The animal feeds and it then creeps back down the plant. Why does it do
this? Because, says Loeb, the act of feeding has reserved the “sign” of
the taxis. Before, when it was hungry, it was positively phototactic,
but the act of feeding (all at once, it would appear, before digestion
and assimilation of the food itself) has produced chemical substances
in the muscles which cause the latter to relax in response to an
impulse which previously produced contraction.
The nervous link is not, of course, a necessary one. The stimulation
by the energy of the field may affect the muscle substance directly,
or it may, as in the case of a protozoan animal, affect the general
body protoplasm in the same way. In the majority of cases, however,
the orientation would be affected through the chain of sense-organ,
afferent nerve, nerve centre, efferent nerve, and effector organ. This
is the chain of events which on this hypothesis causes a moth to fly
into a flame, or a sea-bird to dash itself against the lantern of a
lighthouse.
A taxis is, then, an inevitable response by movement in a definite
direction, to a directed stimulus. Including also tropisms it may be
admitted that the movement is a purposeful, or at least, a useful one
in some cases, as for instance the heliotropism and geotropism of the
green plant. If we admit that Loeb’s description of the feeding of the
caterpillar, as a tactic act, is true, we may also call this a useful
act. But in the majority of cases tropisms and tactes are acts which
appear to be of no use to the organism. The invasion of a part of the
body which is irritated by a poison (as in inflammation) by leucocytes,
is useful to the body itself, but we must regard the leucocytes as
organisms, and their tactic motion leads to their destruction, and so
also with other analogous acts. Just because of this we find difficulty
in accounting for their origin in terms of natural selection.
This does not matter so much, since it can hardly be maintained now
that the tropistic or tactic act has any reality except in a very
few cases--the motions of plants, galvano-taxis, the chemico-taxic
movements of bacteria and leucocytes, and some other analogous cases,
perhaps, are these exceptions. It can hardly be doubted that the
extension of the concept to cover the motions of many invertebrates,
and even some vertebrate actions, by Loeb and his school is a straining
after generality which has not been justified. The hypothesis, as Loeb
has stated it, is evidently almost certainly a logical one and was
obviously elaborated as a protest against the anthropomorphism which
saw in the flying of a moth into a flame the expression of an emotion;
or in the movements of a caterpillar on a green shrub the expression
of hunger and satiety and of the inherited experience of the animal;
or in the avoidance by a _Paramœcium_ of a drop of acid the emotion
of dislike of the feeling of pain. Well, let it be granted that this
is so, and that the protest was a useful one, for it is obviously
impossible that these notions as to the causes of the movements can
be verified: does it improve matters to take refuge in an hypothesis
which is just as purely physico-chemical dogmatism as the other is
anthropomorphism? But the former hypothesis is at all events one which
is susceptible of experimental verification and in this lies its
usefulness, inasmuch as it has stimulated investigation. It is evident,
however, that this verification has not yet been made. The differential
afferent impulses set up by the energy-field; the increases or
inhibition of muscular tone; the presence of photo-sensitive substances
in the tissues of tactically acting lower animals; the change of
velocity of chemical reaction, in these cases, which ought to follow
stimulation--all these things _could_ be verified if they possess
reality. Yet it is only indirect proofs, capable perhaps of other
interpretations, and not direct experimental ones, which have so far
been adduced in favour of a general theory of tropisms.
Moreover, the close analysis of the actions of some of the lower
organisms by Jennings has shown that the tactic hypothesis is probably
false in the majority of cases. This observer studied the acting of
the organisms themselves and not the beginning and end of the series,
and he shows that the behaviour of the organisms is far more obviously
described by saying that it adopts a method of “trial and error.” Let
us suppose a number of infusoria (_Paramœcium_) in a film of water, at
one part of which is a drop of acetic acid slowly diffusing out into
the surrounding medium. There is a zone of changing concentrations
round the drop: if we draw imaginary contours through the points where
the concentration is approximately the same (the concentric rings in
the diagram), and then draw straight lines normal to these rings (the
radial lines) we can construct a “field” analogous to an electric or
magnetic field. The animal on approaching the field ought to orientate
itself and take the direction of the “lines of force.” It does not,
however, behave in this way, but only enters the field at random.
Having entered, it remains within a part where the concentration is
within certain limits. If it approaches the margin of this limited
field it stops, swims backwards, revolves round its own axis, and then
turns to the aboral side; and it repeats this series of movements
whenever it approaches (by random) a region where the concentration is
too high, or one where it is too low. In this, and other organisms we
see then what Jennings has called a typical “avoiding reaction,” the
precise nature of which depends on the “motor-system” of the animal.
Its general movements are random ones, but having found a region
of “optimum conditions” (conditions which are most suitable in its
particular physiological state), it remains there.
[Illustration: FIG. 19.]
Suppose (what indeed repeatedly happens) that an extensive “bed” of
young mussels forms on a part of the sea bottom. In a short time the
bed becomes populated by a shoal of small plaice feeding greedily
on the little shellfish. In their peregrinations the fishes must
repeatedly pass out beyond the borders of this feeding-ground. Usually,
however, they will return, for failing to find the food they like they
swim about in variable directions and so re-enter the shellfish bed.
Suppose (this was really a fine experiment made by Yerkes) a crab is
confined in a box from which two paths lead out but only one of which
leads to the water. The animal runs about at random, finds the wrong
path, retraces it, tries again and again, and then finds the right
path and gets back to the water. If the experiment is repeated the
animal finds the right path again with rather less trouble, and after
many trials it ends by finding it at once on every repetition of the
experiment.
All this discussion of concrete cases leads up to our consideration
of the modes of acting in the higher organisms. On the strictly
mechanistic manner of thinking the actions of the organism in general
are based on reactions of the tactic kind--inevitable reactions the
nature of which is determined, and which follow a stimulus with a
certainty often fatal to the organism displaying them. Accepting these
tactic reactions as, in general, truly descriptive of the behaviour
of the organism, we can build up a theory of instincts. In their
simplest form instincts are reflexes--tactic movements. In their more
complex forms they are concatenated reflexes, or tactes. A complicated
instinctive action is one consisting of many individual actions, each
of which is the stimulus for the next one; or, of course, it may
also be complex in the sense that several simple reactions proceed
simultaneously, upon simultaneous stimulation of different receptors.
Now the extension of all this to movements of a “higher” grade is
obvious.
Let us note in the first place, that the stimuli so far considered
in all the examples quoted are simple elemental ones. There are, of
course, relatively few such stimuli: gravity, conducted heat (the
kinetic energy of material bodies), radiated heat (the energy of the
ether), electric energy, chemical energy, and mechanical contact or
pressure (including atmospheric vibrations). In all these cases we
have a definite, measurable, physical quantity, with which we must
relate a definite response in the form of a definite measurable,
physico-chemical reaction. There should be a functionality between the
stimulus and response, a definite, quantitative energy-transformation.
To take a concrete example, a certain quantity of light energy falling
upon the receptor organs of Loeb’s caterpillar ought to transform
into another quantity of “nervous energy,” and this travelling in
an analogous way to a “wave of explosion” ought to transform into
an energy quantity of some kind, which initiates another “wave of
explosion” in the muscle substance. All these transformations must
be quantitative ones, and the energy of the individual light must
be traced from the receptor organ to the points in the muscle where
it disturbs a condition of false equilibrium in the substance of
the latter. Nothing less than this is required to demonstrate the
purely physical nature of a reaction, on the part of the organism,
to an external stimulus. It may safely be said that physiological
investigation has not yielded anything even approximating to such an
experimental demonstration.
What are the stimuli to the actions of a higher organism? It is true
that their elements are energies such as we have indicated, but these
energies are integrated to form _individualised stimuli_ (Driesch).
The stimulus in an experimentally studied taxis is, perhaps, a field
of parallel pencils of light rays of definite wave length; but in the
action of a man, or a dog say, the stimulus is an immensely complicated
disturbance of the ether, producing an _image_ upon the retina of
the animal. A sound stimulus employed in an investigation may be the
relatively simple atmospheric disturbance produced by the sustained
note of a syren or violin-string; but the stimulus in listening to an
orchestra may consist of dozens of notes, with all their harmonies,
sounding simultaneously at the rate perhaps of some hundred or two in
the minute. All these are _integrated_ by the trained listener, and one
or two false ones among the multitude may entirely spoil the effect
of the execution. Surely there is here something more than a mere
difference in degree.
More important still is the strict functionality between stimulus
and action that the theory of tactic responses imposes on itself.
Putting this very precisely (but no more precisely than the theory
demands), we say that [Sigma]_A_ = _f_(_x_, _y_, _z_), that is, the
series of actions [Sigma]_A_ (the dependent variable) is a mathematical
function of the independent variables _x_, _y_, _z_. Now is there
anything like this functionality between the acting of the higher
animal and the stimulus? Evidently there is not. We recognise someone
whom we know very well by any one of a hundred different characters,
mannerisms of walk, speech, dress, etc. He or she is the same person,
whether seen close at hand, or afar off, or sideways, or in any one
of almost infinitely different attitudes, and we respond to each
of these very different physical stimuli by the same reaction of
recognition: pleasure, dislike, avoidance, greeting, or whatever it
may be. To a sportsman shooting wild game the stimulus may be some
almost imperceptible tint or shading in cover of some kind, differing
so little from its environment as hardly at all to be seen, yet, to
his experience, upon this almost infinitesimal variation of stimulus
depends his action with all its consequences. In Driesch’s example
two polyglot friends met and one says to the other, “My brother is
seriously ill,” or “Mon frère est séverèment malade,” or “mein Bruder
ist ernstlich erkrankt.” Here the physical stimulus is fundamentally
different in each case, but the reaction--the expressions of sympathy
and concern, the discussions of mutual arrangements, etc., are
absolutely the same. Or let the one friend say to the other, “My
mother is seriously ill,” and in spite of the very insignificant
difference between the consonantal sound _br_ in this sentence and the
corresponding sound _m_ in the other English sentence, the reaction,
that is, the subsequent conversation, and the arrangements between the
two friends may be entirely different.
Putting this argument in abstract form we may say, generally, that
two stimuli, which are, in the physical sense, entirely different
from each other, may produce absolutely the same series of reactions;
and conversely two stimuli differing from each other in quite an
insignificant degree may produce entirely different reactions. It is
also easy to see, by analysis of the antecedents to the actions of the
intelligent animal, that these stimuli are, in the majority of cases,
not elemental physical agencies, but individualised and integrated
groupings of these agencies; and that the animal reacts, not to their
mathematical sum, as it should do on a purely mechanistic hypothesis
of action, but to the typical wholes which are expressed in these
groupings.[22]
[22] Thus to the ordinary woman the sight of a cow in the middle of a
country road produces a certain definite feeling of apprehension, which
is always the same although the optical image of the animal differs
remarkably in different adventures.
It is no answer to this argument to say that it is not the actual
atmospheric vibrations (in the case of the conversation), nor the
optical image (in the case of the recognition of a friend), which
are the true stimuli, but rather the mental conditions, or states of
consciousness, aroused by these physical agencies. If we are to adopt
a strictly mechanistic method of explaining actions, such a method as
that indicated by Loeb’s hypothesis of the purely tactic behaviour
of his caterpillars, then these atmospheric vibrations and optical
images are most undoubtedly the true stimuli, and the reactions
must be functions of them in the mathematical sense. But since
this strict functionality does not exist in any behaviour-reaction
closely analysed, we must grant at once that it is, indeed, _not_ the
physical series of events that determines the actual response, but
truly the conscious state immediately succeeding to these physical
sense-impressions. Now let us see to what conclusions this admission
leads us.
Between the external stimulus (the atmospheric undulations impinging
on the auditory membranes, or the light radiations impinging on the
retinæ) and the behaviour-reaction something intervenes. This is the
individual history of the organism, the “associative memory” of Jacques
Loeb, the “physiological state” of Jennings, the “historical basis of
reacting” (historische Reaktionsbasis) of Driesch, or the “duration”
of Bergson. The last concept is the most subtle and adequate one
and we shall adopt it. The physical stimulus, then, leads to a state
of consciousness, a perception, and this is succeeded by the action.
What is the perception? There may be no perception in a reflex action;
there is none in a taxis.[23] These kinds of reaction follow inevitably
from the nature of the stimulus--depend upon the latter, in fact;
but we cannot fail to observe that the intelligent behaviour of the
higher animal involves choice between alternative kinds of action. The
perception, then, is this choice, or it is intimately associated with
it. But it is something more than the choice of one among many kinds
of response. The whole past experience of the animal enters into the
perception, or at least all that part of the past experience which
illuminates, in any way, the present situation. What the intelligent
animal does in response to a stimulus depends not only on the stimulus
but on all the stimuli that it has received in its past, and on all
the effects of all those stimuli. Into the perception that intervenes
between the external stimulus, then, and the action by which the animal
responds what we usually call its memory enters. Its _duration_ is
really the something which is changed by the stimulus, and which then
leads to the behaviour-reaction.
[23] We do not find this explicitly stated in this way in mechanistic
biological writings. None the less it is implied, and is the legitimate
conclusion from the arguments used.
Duration, then, is memory, but it is more than memory as we usually
think of this quality. The past endures in us in the form of “motor
habits,” and when we recall it we may act over again those motor
events. Careful introspection will readily convince the reader that in
recalling a conversation he is really _speaking inaudibly_, setting in
motion the nerves and muscles of his vocal mechanisms. Actions that
have been learned endure; in some way cerebral and spinal tracts and
connections become established and persist: undoubtedly when a cerebral
lesion destroys or impairs memory it is these physical nerve tracts and
cells that become affected. But in addition to this we have pure memory
(Bergson’s “souvenir pur”). What, for instance, is the visual image
of some thing seen in the past, which most people can form, but pure
recollection?[24]
[24] A visual image may, of course, be something that has never been
actually seen. But then its elements have had actual perceptual
existence in the past.
All the past experience of the organism--all its perceptions, and
all the actions it has performed--endures, either as motor habits or
mechanisms, or as pure memories. All this need not be present in its
consciousness; the motor habits would not, of course, and only so
much of the past would be recalled as would be relevant to the choice
which the organism was about to make of the many kinds of responses
possible to its motor organisations. Out of this past it would select
all that was connected in any way with the actions which were possible
to it in the present. It would recall all actions previously performed
which resembled the one provisionally decided upon; but recalling also
the other circumstances associated with those past actions, it would
discover something which would lead it to modify that provisional
action. Now in describing the whole behaviour of the acting organism
in this way are we doing any more than simply expressing in more
precise terms the “commonsense” notions of the ordinary person? The
latter would sum up all this discussion by saying that what he would
do in any set of circumstance depended not only on the circumstances
themselves but upon his experience. Physiology shows us as clearly as
possible that in the stimulation of a receptor organ, the propagation
of a nervous impulse along an afferent nerve, the transmission of this
impulse through the cord or brain, or both--in the propagation again of
the impulse through an efferent nerve and the transformation of this
impulse into a releasing agency, setting free the energy potential in
the muscle substance--that in all this there can be nothing more than
physico-chemical energy-transformations. All this is clear and certain.
But why should the same afferent stimuli, entering the central nervous
system at different times by the same avenues, and in the same manner,
traverse different tracts, and issue along different efferent nerves,
producing different results? Or why should different stimuli entering
the central nervous system take the same intra-cerebral paths and then
affect the same efferent nerves and effector organs? It is because
these stimuli lead to perceptions which fuse with, and become part of
the duration of, the organism. And the response then becomes a response
not to the physical stimulus, but to the duration modified in this way.
Can we conceive of any physical mechanism in which the duration of the
organism accumulates? Can we think of any way in which memories are
stored in the central nervous system? When we say “stored,” it is our
ingrained habit of thinking in terms of space and number that makes
us regard memories as laid by somewhere, in the way we file papers
in a cabinet, or store specimens in a museum. Supposing perceptions
are stored in this way, we think of them as stored or recorded in the
same way as a conversation is recorded and stored in a phonograph. The
phonograph can reproduce the conversation just as it was received,
but what we make use of when we utilise our experience is obviously
the elements of that experience, selected and re-integrated as we
require them. There must, then, be something like an analysis of our
perceptions, a dissociation of these into simple constituents, and a
means of restoring and recording these constituents in such a way that
they can be recombined in any order, and again made to enter into our
consciousness.
It is quite possible to imagine such a mechanism. Let us suppose
that an efferent impulse enters the cerebral cortex _via_ any one
axon: there is a perfect labyrinth of paths along which the impulse
may travel. Everywhere in the central nervous system we come upon
interruptions of nervous paths formed by inter-digitating arborescent
formations. The twigs of these arborescences do not, apparently, come
into actual contact with each other and the impulse leaps across the
gap between them. This gap is, of course, exceedingly narrow, and one
can almost speak of it as a membrane, since it must be occupied by some
organised substance. It has been called the synaptic membrane. Let us
suppose that a stimulus of a certain nature passes through the synapse,
modifying it physico-chemically as it passes. Thereafter a stimulus
of similar nature will tend to pass across this particular synapse,
the resistance of the latter having been decreased. It will thus tend
to travel by a definite tract through the central nervous system.
Now the latter we may regard in a kind of way as a very complicated
switchboard, the function of which is to place any one stimulus (or
series of stimuli) out of many in connection with any one motor[25]
mechanism (or series of mechanisms) out of many. A motor habit, or
path, is then established and will persist.
[25] Or more generally _effector_ mechanism. This enables us to include
reactions, such as secretory ones, which are not motor.
Such a conception is clear and reasonable in principle, and all work on
nervous physiology tends to show that it is a good working hypothesis.
We cannot read modern books without feeling that immense advances will
be made by its aid. But the complexity of the brain of the higher
vertebrate is so incredibly great, and the difficulties of imagining
the nature of the necessary physico-chemical reactions in the synapses,
and elsewhere, are so immense that experimental verification may be
impossible. And all that we have said applies to a single elemental
stimulus, yet in any common action the stimulus is a synthesis of
almost innumerable simple ones, while the response is also a synthesis.
The optical image of almost any object contains a very great number of
tints and colours differing almost imperceptibly: there must at least
be as many simple stimuli as there are rod or cone elements in the part
of the retina covered by the image. The motor responses consist of a
multitude of delicately adjusted and co-ordinated muscular contractions
and relaxations. If we are to accept a mechanistic hypothesis of
action, of this kind, and which includes only such processes as
are suggested above, it is not enough that a logical description,
consistent in itself, and consistent with physico-chemical knowledge,
should be formulated. The mere statement of such an hypothesis does not
carry us far. If it is, in essence, mechanistic, it must be capable of
experimental verification in detail.
Even if it were verified experimentally it would still leave untouched
the problem of consciousness. All that we have considered are series of
physico-chemical energy-transformations. How, then, does consciousness
arise? We cannot even imagine its association in a functional sense
with the train of events forming an afferent impulse. In some form or
other mechanism must assume a dualism--a parallelism of physical and
psychical processes. Physical events in the central nervous system
are associated with psychical ones--when the former occur so do the
latter--yet the former are not “causes” in any physical sense of
the latter. Consciousness follows cerebral energy-transformations
as a parallel “epiphenomenon.” At once we leave the province of
mechanism, and how can we remain content with such a limitation of
our descriptions? And if we conclude, as we seem obliged to do, that
consciousness is an affective agency in modifying our responses to
external stimuli, does not this in itself show that our concept of
behaviour as a purely physico-chemical process is insufficient in its
exclusiveness?
We return to a consideration of the main results of experimental
embryology in a later chapter, but let us notice here what is the
direction in which these results, and those of the analysis of
instinctive and intelligent action, carry us. It is towards the
conclusion that physico-chemical processes in the organism are only
the _means_ whereby the latter develops, and grows, and functions,
and acts. In the analysis of these processes we see nothing but the
reactions studied in physical chemistry; but whenever we consider
the organism as a whole we seem to see a co-ordination, or a control
or a direction of these physico-chemical processes. Nägeli has said
that in the development of the embryo every cell acts as it if _knew_
what every other cell were doing. There is a kind of autonomy in the
developing embryo, or regenerating organism, such that the normal,
typical form and structure comes into existence even when unforeseen
interference with the usual course of development has been attempted:
in this case the physico-chemical reactions which proceed in the normal
train of events proceed in some other way, and the new direction is
imposed on the developing embryo by the organisation which we have to
regard as inherent in it. This same direction and autonomy must be
recognised in the behaviour of the adult organism as a whole. What
is it? We attempt to think of it as an impetus which is conferred
upon the physico-chemical reactions which are the manifestations of
the life of the organism. It is the _élan vital_ of Bergson, or the
_entelechy_ of Driesch. What is included in these concepts we consider
in the last chapter of this book; and before so doing it will be
necessary to consider the organism from another point of view, that
of its mutability when it is regarded as one member of a series of
generations.