SEGMENTATION, ORGANIC AND MECHANICAL
Models may be and often have been devised imitating some of the
phenomena of division, but none of them have reproduced the peculiarity
which characterises divisions of living tissues, that _the position
of chemical differentiation_ is _determined by those divisions_. For
example, models of segmentation, whether radial or linear, may be made
by the vibration of plates as in the familiar Chladni figures of the
physical laboratory, or by the bowing of a tube dusted on the inside
with lycopodium powder, and in various other ways. The sand or the
powder will be heaped up in the nodes or regions of least movement, and
the patterns thus formed reproduce many of the geometrical features
of segmentation. But in the segmentations of living things the nodes
and internodes, once determined by the dividing forces, would each
become the seat of appropriate and distinct chemical processes leading
to the differentiation of the parts, and the deposition of the bones,
petals, spines, hairs, and other organs in relation to the meristic
ground-plan. The "ripples" of meristic division not merely divide but
differentiate, and when a "ripple" forks the result is not merely a
division but a reduplication of the organ through which the fork runs.
An example illustrating such a consequence is that of the half-vertebrae
of the Python. On the left side the vertebra is single (Fig. 7) and
bears a single rib, but on the right side a division has occurred with
the result that two half-vertebrae, each bearing a rib, are formed, one
standing in succession to the other. We cannot, indeed, imagine any
operation of physiological division carried out in such an organ as a
vertebra, passing through a plane at right angles to the long axis of
the body, which does not necessarily involve the further process of
reduplication.
As the meristic system of distribution spreads through the body,
chemical differentiations follow in its track, with segmentation and
pattern as the visible result. Could we analyse these simultaneous
phenomena and show how it is that the places of chemical differentiation
are determined by the system of division, progress would then be rapid.
It is here that all speculation fails.
[Illustration: FIGS. 7 and 8. Two examples of imperfect division in the
vertebræ of a python. _I_, the vertebræ 147-150 from the right side,
showing imperfect division between the 148th and 149th. The condition on
the left side of this vertebra was the same. _II_, the dorsal surface of
vertebræ 165-167. On the right side the 166th is double and bears two
ribs, but on the left side it is normal and has one rib only.]
Many attempts have been made to interpret the processes of division
and repetition, in terms of mechanics, or at least to refer them to
their nearest mechanical analogies, so far with little success. The
problem is beset with difficulties as yet insurmountable and of these
one must be especially noticed. In the living thing the process by which
repetition and patterns come into being consists partly in division but
partly also in growth. We have no means of studying the phenomena of
pattern-formation except in association with that of growth. Growth soon
ceases unless division takes place, and if growth is impossible division
soon ceases also. In consequence of this fact that the final pattern
is partly a product of growth, it can never be used as unimpeachable
evidence of the primary geometrical relations of the members as laid
down in the divisions.
In the last chapter in referring to the problem of repetition I
introduced an analogy, comparing the patterns of the organic world
with those produced in unorganised materials by wave-motion. In the
preliminary stage of ignorance, having no more trustworthy clue, I do
not think it wholly unprofitable to consider the applicability of this
analogy somewhat more fully. It possesses, as I hope to show, at least
so much validity as to encourage the belief that morphology may safely
discard one source of long-standing error and confusion.
Those who have studied the structure of parts repeated in series will
have encountered the old morphological problem of "Serial Homology,"
which has absorbed so much of the attention of naturalists and
especially of zoologists at various periods. This problem includes
two separate questions. The first of these is the origin in evolution
of the resemblance between two organs occurring in a repeated series,
of which the fore and hind limbs of Vertebrates are the prerogative
instance. From the fact that these resemblances can be traced very far,
often into minute details of structure, many anatomists have inclined to
the opinion that the resemblance must originally have been still more
complete, and that the two limbs, for instance, must have acquired their
present forms by the differentiation of two identical groups of parts.
Similar questions arise whenever parts are repeated in series, whether
the series be linear or radial, and, though less obviously, even when
the repetition is bilateral only. In each such example the question
arises, is the resemblance between the parts the remains of a still
closer resemblance, or is differentiation original? Sometimes the
view that these parts have arisen by the differentiation of a series
of identical parts is plausible enough, as for example when the
peculiarities of various appendages of a Decapod Crustacean are referred
to modifications of the Phyllopod series. In application to other cases
however we soon meet with difficulty, and the suggestion that the
segments of a vertebrate were originally all alike is seen at once to be
absurd, for the reason that a creature so constituted could not exist,
and that, differentiation of at least one anterior and one posterior
segment, is an essential condition of a viable organism consisting of
parts repeated in a linear series. Between these two terminal segments
it is possible to imagine the addition of one segment, or of a series
of approximately similar segments; but when once it is realised that
the terminals must have been differentiated from the beginning, it
will be seen that the problem of the origin of the resemblance between
segments is not rendered more comprehensible by the suggestion that
even the intervening members were originally alike. Seeing indeed that
some differentiation must have existed primordially it is as easy to
imagine that the original body was composed of a series grading from the
condition of the anterior segment to that of the posterior, as any other
arrangement. The existence of a linear or successive series in fact
postulates a polarity of the whole, and in such a system the conception
of an ideal segment containing all the parts represented in the others
has manifestly no place. The introduction of that conception though
sanctioned by the great masters of comparative anatomy, has, as I think,
really delayed the progress of a rational study of the phenomena of
division. The same notion has been applied to every class of repetition
both in animals and plants, generally with the same unhappy results. In
the cruder forms in which this doctrine was taught thirty years ago it
is now seldom expressed, but modified presentations of it still survive
and confuse our judgments.
The process of repetition of parts in the bodies of organisms is
however a periodic phenomenon. This much, provided we remain free from
prejudice as to the nature and causation of the period or rhythm, we
may safely declare, and a comparison may thus be instituted between the
consequences of meristic repetition in the bodies of living things and
those repetitions which in the inorganic world are due to rhythmical
processes. Of such processes there is a practically unlimited diversity
and we have nothing to indicate with which of them our repetitions
should rather be compared.
[Illustration: FIG. 9. Osmotic growths simulating segmentation. (After
Leduc.)]
In some respects perhaps the best models of living organisms yet made
are the "osmotic growths" produced by Leduc.[1] These curious structures
were formed by placing a fragment of a salt, for instance calcium
chloride, in a solution of some colloidal substance. As the solid takes
up water from the solution a permeable pellicle or membrane is formed
around it. The vesicle thus enclosed grows by further absorption of
water, often extending in a linear direction, and in many examples this
growth occurs by a series of rhythmically interrupted extensions. Some
of the growths thus formed are remarkably like organic structures,
and might pass for a series of antennary segments or many other
organs consisting of a linear series of repeated parts. In admitting
the essential resemblance between these "osmotic growths" and living
bodies or their organs I lay less stress on the general conformation
of the growths, which often as Leduc points out, recall the forms
of fungi or hydroids, but rather on the fact that the interruptions
in the development of these systems are so closely analogous to the
segmentations or repetitions of parts characteristic of living things
(Fig. 9). In the same way I am less impressed by Leduc's models of
Karyokinesis, wonderful as they nevertheless are, for the division is
here imitated by putting separate drops on the gelatine film. What we
most want to know is how in the living creature one drop becomes two.
The models of linear segmentation have the remarkable merit that they do
in some measure imitate the process of actual division or repetition.
So in a somewhat modified method Leduc, by causing the diffusion
of a solution in a gelatine film, produced rhythmical or periodic
precipitations strikingly reminiscent of various organic tissues, for
here also the process of periodic repetition is imitated with success.
It is a feature common to these and to all other rhythmical repetitions
produced by purely mechanical forces that there is resemblance between
the members of the series, and that this similarity of conformation may
be maintained in most complex detail. When however in the mechanical
series some of the members differ from the rest we have no difficulty
in recognising that these differences--which correspond with the
differentiations of the organic series--are due to special heterogeneity
in the conditions or in the materials, and it never occurs to us to
suppose that all the members must have been primordially alike. For
example, in the case of ripple-marks on the sand, which I choose as one
of the most familiar and obvious illustrations of a repeated series
due to mechanical agencies, if we notice one ripple different in form
from those adjacent to it, we do not suppose that this variation must
have been brought about by deformation of a ripple which was at first
formed like the others, but we ascribe it to a difference in the sand at
that point, or to a difference in the way in which the wind or the tide
dealt with it. We may press the analogy further by observing that in as
much as such a series of waves has a beginning and an end, it possesses
polarity like that of the various linear series of parts in organisms,
and even the formation of each member must influence the shape of its
successor. Since in an organism the beginning and end of the series
are always included, some differentiation among the repetitions must
be inevitable. If therefore it be conceded, as I think it must, that
segmentation and pattern are the consequence of a periodic process we
realize that it is at least as easy to imagine the formation of such a
series of parts having family likeness combined with differentiation
as it would be to conceive of their arising primordially as a series
of identical repetitions. The suggestion that the likenesses which we
now perceive are the remains of a still more complete resemblance is a
substitution of a more complex conception for a simpler one.
The other question raised by the problem of Serial Homology is how far
there is a correspondence between individual members of series when
the series differ from each other either in the number of parts, or in
the mode of distribution of differentiation among them. Students, for
example, of vertebrate morphology debate whether the _n_th vertebra
which carries the pelvic girdle in Lizard A is individually homologous
with the _n_ + _x_th vertebra which fulfils this function in Lizard B,
or whether it is not more truly homologous with the vertebra standing in
the _n_th ordinal position, though that vertebra in Lizard B is free.
In various and more complex aspects the same question is debated in
regard to the cranial and spinal nerves, the branches of the aorta,
the appendages of Arthropoda, and indeed in regard to all such series
of differentiated parts in linear or successive repetition. Persons
exercised with these problems should before making up their minds
consider how similar questions would be answered in the case of any
series of rhythmical repetitions formed by mechanical agencies. In the
case of our illustration of the ripples in the sand, given the same
forces acting on the same materials in the same area, the number of
ripples produced will be the same, and the _n_th ripple counting from
the end of the series will stand in the same place whenever the series
is evoked. If any of the conditions be changed, the number and shapes
can be changed too, and a fresh "distribution of differentiation"
created. Stated in this form it is evident that the considerations
which would guide the judgment in the case of the sand ripples are not
essentially different from those which govern the problem of individual
homology in its application to vertebrae, nerves, or digits.
The fact that the unit of repetition is also the unit of growth is the
source of the obscurity which veils the process. When we compare the
skeleton of a long-tailed monkey with that of a short-tailed or tailless
ape we see at once how readily the additional series of caudal segments
may be described as a consequence of the propagation of the "waves" of
segmentation beyond the point where they die out in the shorter column,
and we see that with an extension of the series of repetitions there is
growth and extension of material.
The considerations which apply to this example will be found operating
in many cases of the variation of terminal members of linear series.
Some of these series, like the teeth of the dog, end in a terminal
member of a size greatly reduced below that of the next to it. Even when
there is thus a definite specialisation of the last member of the series
it not infrequently happens that the addition, by variation, of a member
beyond the normal terminal, is accompanied by a very palpable increase
in size of the member which stands numerically in the place of the
normal terminal.[2] So also with variation in the number of ribs, when
a lumbar vertebra varies homoeotically into the likeness of the last
dorsal and bears a rib, the rib placed next in front of this, which in
the normal trunk is the last, shows a definite increase in development.
The consequences of such homoeoses are sometimes very extensive,
involving readjustments of differentiation affecting a long series of
members, as may easily be seen by comparing the vertebral columns of
several individual Sloths[3] (whether _Bradypus_ or _Choloepus_) to take
a specially striking example.
It may be urged that no feature as yet enables us to perceive wherein
lies the primary distinction which determines such variation, whether it
is due to a difference in the dividing forces or in the material to be
divided. If for instance we were to imitate such a series of segments
by pressing hanging drops of a viscous fluid out of a paint-tube by
successive squeezes, the number of times the tube is contracted before
it is empty will give the number of the segments, but their size may
depend either on the force of the contractions or on the capacity of
the tube, or on various other factors. Nevertheless in the case of the
variation of terminal members, whatever be the nature of the rhythmical
impulse which produces the series of organs, the elevation of the
normally terminal member in correspondence with the addition of another
is what we should expect.
If the organism acquired its full size first and the delimitation of
the parts took place afterwards, there might be some hope that the
resemblance between living patterns and those mechanically caused by
wave-motion might be shown to be a consequence of some real similarity
of causation, but in view of the part played by growth, appeal to these
mechanical phenomena cannot be declared to have more than illustrative
value. Similarly in as much as living patterns appear, and almost
certainly do in reality come into existence by a rhythmical process,
comparisons of these patterns with those developed in crystalline
structures, and in the various fields of force are, as it seems to me,
inadmissible, or at least inappropriate.
However their intermittence be determined, the rhythms of division must
be looked upon as the immediate source of those geometrically ordered
repetitions universally characteristic of organic life. In the same
category we may thus group the segmentation of the Vertebrates and of
the Arthropods, the concentric growth of the Lamellibranch shells or of
Fishes' scales, the ripples on the horns of a goat, or the skeletons of
the Foraminifera or of the Heliozoa. In the case of plant-structures
Church[4] has admirably shown, with an abundance of detail, how on
analysis the definiteness of phyllotaxis is an expression of such
rhythm in the division of the apical tissues, and how the spirals
and "orthostichies" displayed in the grown plant are its ultimate
consequences. The problem thus narrows itself down to the question of
the mode whereby these rhythms are determined.
It is natural that we should incline to refer them to a chemical source.
If we think of the illustration just given, of the segmentation of a
viscous fluid into drops by successive contractions of a soft-walled
tube we can, I think, conceive of such rhythmic contractions as due to
summations of chemical stimuli, somewhat as are the beats of the heart.
But when we recognize the vast diversity of materials the distribution
of which is determined by an ostensibly similar rhythmic process it
seems hopeless to look forward to a directly chemical solution. That the
chemical degradation of protoplasm or of materials which it contains is
the source of the energy used in the divisions cannot be in dispute, but
that these divisions can be themselves the manifestations of chemical
action seems in the highest degree improbable.
We may therefore insist with some confidence on the distinction between
the Meristic and the substantive constitution of organisms, between,
that is to say, the system according to which the materials are divided
and the essential composition of the materials, conscious of the fact
that the energy of division is supplied from the materials, and that
in the ontogeny the manner in which the divisions are effected must
depend secondarily on the nature of the substances to be divided.
The mechanical processes of division remain a distinguishable group
of phenomena, and variations in the substances to be distributed in
division may be independent of variations in the system by which the
distribution is effected.
Modern genetic analysis supplies many remarkable examples of this
distinction. When formerly we compared the leaves of a normal palmatifid
Chinese Primula with the pinnatifid leaves[5] of its fern-leaved variety
we were quite unable to say whether the difference between the two types
of leaf was due to a difference in the material cut up in the process
of division or to a difference in that process itself. Knowledge that
the distinction is determined by a single segregable factor tends to
prove that the critical difference is one of substance. So also in the
Silky fowl we know that the condition of its feathers is due to the
absence of some one factor present in the normal form. We may conceive
such differences as due to change of form in the successive "waves" of
division, but we cannot yet imagine segregation otherwise than as acting
by the removal or retention of a material element. Future observation
by some novel method may suggest some other possibility, but such cases
bring before us very clearly the difficulties by which the problem is
beset.
[Illustration: FIG. 10. The palm-and fern type of leaf in _Primula
Sinensis_. The palm is dominant and the fern is recessive.]
In another region of observation phenomena occur which as it seems to
me put it beyond question that the meristic forces are essentially
independent of the materials upon which they act, save, in the remoter
sense, in so far as these materials are the sources of energy. The
physiology of those regenerations and repetitions which follow upon
mutilation supplies a group of facts which both stimulate and limit
speculation. No satisfactory interpretations of these extraordinary
occurrences has ever been found, but we already know enough to feel
sure that in them we are witnessing indications which should lead
to the discovery of the true mechanics of repetition and pattern.
The consequences of mutilation in causing new growth or perhaps more
strictly in enabling new growth to take place, are such that they cannot
be interpreted as responses to chemical stimuli in any sense which
the word chemical at present connotes. Powers are released by mutilation
of which in the normal conditions of life no sign can be detected. All
who have tried to analyse the phenomena of regeneration are compelled
to have recourse to the metaphor of equilibrium, speaking of the normal
body as in a state of strain or tension (Morgan) which when disturbed by
mutilation results in new division and growth. The forces of division
are inacessible to ordinary means of stimulation. Applications, for
example, of heat or of electricity excite no responses of a positive
kind unless the stimuli are so violent as to bring about actual
destruction.[6] These agents do not, to use a loose expression, come
into touch with the meristic forces. Changes in the chemical environment
of cells may, as in the experiments of Loeb and of Stockard produce
definite effects, but the facts suggest that these effects are due
rather to alterations in the living material than to influence exerted
directly on the forces of division themselves.
By destruction of tissue however the forces both of growth and of
division also may often be called into action with a resulting
regeneration. Interruption of the solid connexion between the parts may
produce the same effects, as for example when the new heads or tails
grow on the divided edges of Planarians (Morgan), or when from each half
embryo partially separated from its normally corresponding half, a new
half is formed with a twin monster as the result.
Often classed with regenerations but in reality quite distinct from
them are those special and most interesting examples where the growth
of a _paired_ structure is excited by a simple wound. Some of the best
known of these instances are presented by the paired extra appendages
of Insects and Crustacea. Some years ago I made an examination of all
the examples of such monstrosities to which access was to be obtained,
and it was with no ordinary feeling of excitement that I found that
these supernumerary structures were commonly disposed on a recognizable
geometrical plan, having definite spatial relations both to each other
and to the normal limb from which they grew. The more recent researches
of Tornier[7] and especially his experiments on the Frog have shown
that a cut into the posterior limb-bud induces the outgrowth of such a
_pair_ of limbs at the wounded place. Few observations can compare with
this in novelty or significance; and though we cannot yet interpret
these phenomena or place them in their proper relations with normal
occurrences, we feel convinced that here is an observation which is no
mere isolated curiosity but a discovery destined to throw a new light on
biological mechanics. The supernumerary legs of the Frog are evidently
grouped in a system of symmetry similar to that which those of the
Arthropods exhibit, and though in Arthropods paired repetitions have not
been actually produced by injury under experimental conditions we need
now have no hesitation in referring them to these causes as Przibram has
done.
At this point some of the special features of the supernumerary
appendages become important. First they may arise at any point on the
normal limb, being found in all situations from the base to the apex.
Nor are they limited as to the surface from which they spring, arising
sometimes from the dorsal, anterior, ventral, or posterior surfaces, or
at points intermediate between these principal surfaces.
With rare and dubious exceptions, the parts which are contained in these
extra appendages are only those which lie _peripheral to their point of
origin_. Thus when the point of origin is in the apical joint of the
tarsus, the extra growth if completely developed consists of a double
tarsal apex bearing two pairs of claws. If they arise from the tibia,
two complete tarsi are added. If they spring from the actual base of
the appendage then two complete appendages may be developed in addition
to the normal one. We must therefore conclude that in any point on a
normal appendage the power exists which, if released, may produce a bud
containing in it a paired set of the parts peripheral to this point.
[Illustration: FIG. 11. Diagrams of the geometrical relations which are
generally exhibited by extra pairs of appendages in Arthropoda. The
sections are supposed to be those of the apex of a tibia in a beetle.
_A_, anterior, _P_, posterior, _D_, dorsal, _V_, ventral. _M_^{1},
_M_^{2} are the imaginary planes of reflexion. The shaded figure is in
each case a limb formed like that of the other side of the body, and the
outer unshaded figures are shaped like the normal for the side on which
the appendages are. On the several radii are shown the extra pairs in
their several possible relations to the normal from which they arise.
The normal is drawn in thick lines in the center.]
Next the geometrical relations of the halves of the supernumerary pair
are determined by the position in which they stand in regard to the
original appendage. These relations are best explained by the diagram
(Fig. 11), from which it will be seen that the two supernumerary
appendages stand as images of each other; and, of them, that which is
adjacent to the normal appendage forms an image of it. Thus if the
supernumerary pair arise from a point on the dorsal surface of the
normal appendage, the two _ventral_ surfaces of the extra pair will
face each other. If they arise on the anterior surface of the normal
appendage, their morphologically posterior surfaces will be adjacent,
and so on.
These facts give us a view of the relations of the two halves of a
dividing bud very different from that which is to be derived from the
exclusive study of normal structures. Ordinary morphological conceptions
no longer apply. The distribution of the parts shows that the bud or
rudiment which becomes the supernumerary pair may break or open out in
various ways according to its relations to the normal limb. Its planes
of division are decided by its geometrical relations to the normal body.
Especially curious are some of the cases in which the extra pair are
imperfectly formed. The appearance produced is then that of two limbs
in various stages of coalescence, though in reality of course they are
stages of imperfect separation. The plane of "coalescence" may fall
anywhere, and the two appendages may thus be compounded with each other
much as an object partially immersed in mercury "compounds" with its
optical image reflected from the surface.
Supernumerary paired structures are not usually, if ever, formed when
an appendage is simply amputated. Cases occasionally are seen which
nevertheless seem to be of this nature. Borradaile,[8] for example,
described a crab (_Cancer pagurus_) having in place of the right chela
three _small_ chelae arising from a common base, where the appearances
suggested that the three reduced limbs replaced a single normal limb.
From the details reported however it seems still possible that one of
the chelae (that lettered F. I in Borradaile's figure) may be the normal
one, and the other two an extra pair. The chela which I suspect to be
the normal is in several respects deformed as well as being reduced in
size, and this deformity may perhaps have ensued as a consequence of the
same wound which excited the growth of the extra pair. Its reduced size
may be due to the same injury, which may quite well have checked its
growth to full proportions.
Admitting doubt in these ambiguous cases it seems to be a general rule
that for the production of the extra pair the normal limb should persist
in connexion with the body. Moreover it is practically certain that
in no case can a _single_, viz. an unpaired, duplicate of the normal
appendage grow from it. Many examples have been described as of this
nature, but all of them may be with confidence regarded as instances of
a supernumerary pair in which only the two morphologically anterior or
the two morphologically posterior surfaces are developed. We have thus
the paradox that a limb of one side of the body, say the right, has in
it the power to form a pair of limbs, right and left, as an outgrowth of
itself, but cannot form a second left limb alone.
A very interesting question arises whether it is strictly correct to
describe the extra pair as a right and a left, or whether they are not
rather two lefts or two rights of which one is reversed. This question
did not occur to me when in former years I studied these subjects. It
was suggested to me by Dr. Przibram. The answer might have an important
bearing on biological mechanics, but I know no evidence from which the
point can be determined with certainty. In order to decide this question
it would be necessary to have cases in which the paired repetition
affected a limb markedly differentiated on the two sides of the body,
and of course the development of the extra parts in order to be decisive
must be fairly complete. One example only is known to me which at all
satisfies these requirements, that of the lobster's chela figured (after
Van Beneden) in _Materials for the Study of Variation_, p. 531, Fig.
184, III.
Here the drawing distinctly suggests that one of the extra
dactylopodites, namely that lettered R, is differentiated as a left
and not merely a reversed right. For the teeth on this dactylopodite
are those of a cutting claw, not of a crushing claw, whereas the
dactylopodites R' and L' bear crushing teeth. The figure makes it fairly
certain also that the limb affected was a crushing claw. Accepting this
interpretation, we reach the remarkable conclusion that the bud of new
growth consisted of halves differentiated into cutter and crusher as the
normal claws are, and that the extra crusher is geometrically a left
but physiologically a right. Though shaped as a left in respect of the
direction in which it points, the extra crusher is really an optically
reversed right, while the dactylopodite R, which is placed pointing like
a right, is really a reversed left (Fig. 12).
[Illustration: FIG. 12. Right claw of lobster bearing a pair of extra
dactylopodites (after van Beneden). The fine toothing on R suggests that
this is part of a cutting claw, though the limb bearing it is a crusher.]
If these indications are reliable[9] and are established by further
observation we shall be led to the conclusion that the bud which
becomes an extra pair of limbs does not merely contain the parts proper
to the side on which it grows, but is comparable with the original
zygotic cell, and consists not simply of two halves, but of two halves
differentiated as a right and a left like the two halves of the normal
body.
Phenomena of this kind, evoked by mutilation or injury, together with
the cognate observations on regeneration throw very curious lights
on the nature of living things. To an understanding of the nature of
the mechanics of living matter and its relation to matter at large
they offer the most hopeful line of approach. I allude especially to
the examples in which it has been established that the part which is
produced after mutilation is a structure different from that which was
removed. The term "regeneration" was introduced before such phenomena
were discovered, and though every one recognizes its inapplicability
to these remarkable cases, the word still misleads us by presenting
a wrong picture to the mind. The expression "heteromorphosis" (Loeb)
has been appropriately applied to various phenomena of this kind, and
Morgan has given the name "morphallaxis" to another group of cases in
which the renewal occurs by the transformation of a previously existing
part.[10] But we must continually remember that all these occurrences
which we know only as abnormalities and curiosities must in reality be
exemplifications of the normal mechanics of division and growth. The
conditions needed to call them forth are abnormal, but the responses
which the system makes are evidences of its normal constitution. When
therefore, for example, the posterior end of a worm produces a reversed
tail from its cut end we have a proof that there must be in the normal
body forces ready to cause this outgrowth. The new structure is not
an ill-shaped head-end, for, as Morgan shows, the nephridial ducts
have their funnels perforating the segments in a reversed direction.
The "tension" of growth is actually reversed.[11] So also when in a
Planarian amputation of the body immediately behind the head leads to
the formation of a new reversed head at the back of the normal head,
while amputation further back leads to the regeneration of a new tail,
these responses give indications of forces normally present in the
body of the Planarian. Such facts open up a great field of speculation
and research. Especially important it would be to determine where the
critical region may be at which the one response is replaced by the
other. I suppose it is even possible that there is some neutral zone in
which neither kind of response is made.
Physical parallels to the phenomena of regeneration are not easy
to find and we still cannot penetrate beyond the empirical facts.
Przibram has laid stress on the general resemblance between the new
growth of an amputated part in an animal and the way in which a broken
crystal repairs itself when placed in the mother-solution. That the
two processes have interesting points of likeness cannot be denied.
It must however never be forgotten that there is one feature strongly
distinguishing the two; for I believe it is universally recognized
by physicists that all the phenomena of geometrical regularity which
crystals display are ultimately dependent on the forms of the particles
of the crystalline body. This cannot in any sense be supposed to hold
in regard to protoplasm or its constituents. The definiteness of
crystals is also an unlikely guide for the reason that it is absolute
and perfect, or in other words because this kind of regularity cannot
be disturbed at all without a change so great that the substance itself
is altered; whereas we know that the forms of living things are capable
of such changes, great and small, that we must regard perfection of
form, whether manifested in symmetry or in number, as an ideal which
will only be produced in the absence of disturbance. The symmetry of the
living things is like the symmetry of the concentric waves in a pool
caused by a splash. Perfect circles are made only in the imaginary case
of mathematical uniformity, but the system maintains an approximate
symmetry though liable to manifold deformation.
Since the geometrical order of the living body cannot be a direct
function of the materials it must be referred to some more proximate
control. In renewing a part the body must possess the power of seizing
particles of many dissimilar kinds, and whirl them into their several
and proper places. The action in renewal, like that of original growth,
may be compared--very crudely--with the action of a separator which
simultaneously distributes a variety of heterogeneous materials in an
orderly fashion; but in the living body the thing distributed must
rather be the _appetency_ for special materials, not the materials
themselves.
If the analogy of crystals be set aside and we seek for other parallels
to regeneration there are none very obvious. I have sometimes wondered
whether it might not be possible to institute a fruitful comparison
between the renewal of parts and the reformation of waves of certain
classes after obliteration. In several respects, as I have already said,
some curious resemblances with the repetitions formed by wave-motion
are to be traced in our organic phenomena, and though admitting that
I cannot develop these comparisons, I think nevertheless they may be
worth bearing in mind. When, after obliteration, an eddy in a stream,
or a ripple-mark (a more complex case of eddy-formation) in blown sand
is re-formed, we have an example in which pattern is reconstituted and
growth takes place not by virtue of the composition of the materials--in
this case the water or the sand--but by the way in which they are acted
upon by extraneous forces.
A feature in the actual mode by which ripple-marks are reconstituted may
not be without interest in connexion with our phenomena of regeneration.
When, for example, the wind is blowing steadily over a surface of fine,
dry sand, the familiar ripple-marks are formed by a heaping of the sand
in lines transverse to the direction of the wind. The heaping is due to
the formation of eddies corresponding with positions of instability.
When the wind is steady and the sand homogeneous, the distances between
the ripples, or wave-lengths, are sensibly equal. If while the wind
continues to blow, the ripples are obliterated with a soft brush they
will quickly be re-formed over the whole area, but I have noticed that
at first their wave-length is approximately half that of the ripples
in the undisturbed parts of the system.[12] The normal wave-length is
restored by the gradual accentuation of alternate ripples. Of course
the sand-ripples are in reality slowly travelling forward in the
direction towards which the wind is blowing, and for this our living
segmentations afford no obvious parallel, but the appearances in the
area of reformation, and especially the forking of the old ridges where
they join the new ones, are curiously reminiscent of the irregularities
of segmentation seen in regenerated structures. The value of the
considerations adduced in the chapter is, I admit, very small. The
utmost that can be claimed for them is that mechanical segmentations,
like those seen in ripple-mark, or in Leduc's osmotic growths, show
how by the action of a continuous force in one direction, repeated
and serially homologous divisions can be produced having features of
similarity common to those repetitions by which organic forms and
patterns are characterised. The analogy supplies a vicarious picture of
the phenomena which in default of one more true may in a slight degree
assist our thoughts. It suggests that the rhythms of segmentation may be
the consequence of a single force definite in direction and continuously
acting during the time of growth. The polarity of the organism would
thus be the expression of the fact that this meristic force is
definitely directed after it has once been excited, and the reversal
seen in some products of regeneration suggest further that it is capable
of being reflected. This polarity cannot be a property of the material,
as such, but is determined by a force acting on that material, just as
the polarity of a magnet is not determined by the arrangement of its
particles, but by the direction in which the current flows.
To some it may appear that even to embark on such discussions as this is
to enter into a perilous flirtation with vitalistic theories. How, they
may ask, can any force competent to produce chemical and geometrical
differentiation in the body be distinguished from the "Entelechy" of
Driesch? Let me admit that in this reflexion there is one element of
truth. If those who proclaim a vitalistic faith intend thereby to
affirm that in the processes by which growth and division are effected
in the body, a part is played by an orderly force which we cannot
_now_ translate into terms of any known mechanics, what observant man
is not a vitalist? Driesch's first volume, putting as it does into
intelligible language that positive deduction from the facts--especially
of regeneration--should carry a vivid realisation of this truth to any
mind. If after their existence is realised, it is desired that these
unknown forces of order should have a name, and the word entelechy is
proposed, the only objection I have to make is that the adoption of a
term from Aristotelian philosophy carries a plain hint that we propose
to relegate the future study of the problem to metaphysic.
From this implication the vitalist does not shrink. But I cannot find in
the facts yet known to us any justification of so hopeless a course. It
was but yesterday that the study of _Entwicklungsmechanik_ was begun,
and if in our slight survey we have not yet seen how the living machine
is to be expressed in terms of natural knowledge that is poor cause for
despair. Driesch sums up his argument thus:[13]
"It seems to me that there is only one conclusion possible. If we are
going to explain what happens in our harmonious-equipotential systems
by the aid of causality based upon the constellation of single chemical
factors and events, there _must_ be some such thing as a machine. Now
the assumption of the existence of a machine proves to be absolutely
absurd in the light of the experimental facts. _Therefore there can
be neither any sort of a machine nor any sort of causality based upon
constellation underlying the differentiation of harmonious-equipotential
systems._"
"For a machine, typical with regard to the three chief dimensions
of space, cannot remain itself if you remove parts of it or if you
rearrange its parts at will."
To the last clause a note is added as follows:
"The pressure experiments and the dislocation experiments come into
account here; for the sake of simplicity they have not been alluded to
in the main line of our argument."
I doubt whether any man has sufficient knowledge of all possible
machines to give reality to this statement. In spite also of the
astonishing results of experiments in dislocation, doubt may further be
expressed as to whether they have been tried in such variety or on such
a scale as to justify the suggestion that the living organism remains
itself if its parts are rearranged at will. All we know is that it can
"remain itself" when much is removed, and when much rearrangement has
been affected, which is a different thing altogether.
I scarcely like to venture into a region of which my ignorance is so
profound, but remembering the powers of eddies to re-form after partial
obliteration or disturbance, I almost wonder whether they are not
essentially machines which remain themselves when parts of them are
removed.
Real progress in this most obscure province is not likely to be made
till it attracts the attention of physicists; and though they for long
may have to forego the application of exact quantitative methods, I
confidently anticipate that careful comparison between the phenomena
of repetition formed in living organisms and the various kinds of
segmentation produced by mechanical agencies would be productive of
illuminating discoveries.
FOOTNOTES:
[1] Stéphane Leduc, _Théorie Physico-Chymique de la Vie_, Paris, 1910.
[2] _Materials for the Study of Variation_, No. 249, p. 217; and p. 272.
[3] _Materials_, p. 118.
[4] Church, A. H., _On the Relation of Phyllotaxis to Mechanical Laws_,
London, 1904.
[5] It is a question whether the dominance of the palmatifid leaf over
the pinnatifid is not really an example of the dominance of a lower
number of segmentations over a higher. From the uncertainty whether two
given leaves of two separate plants are actually comparable one cannot
institute quite satisfactory numerical comparisons, but I think the view
that the "Fern" leaf has more lobes than an otherwise similar "Palm"
leaf may be fairly maintained. If this be admitted, the "Palm" leaf
represents the dominant low number and its round shape is a consequence
of the greater powers of growth which are so often possessed by the
members of a shorter series.
[6] It is perhaps of importance to remember that in certain species
of bacteria (e. g. _Bacillus Anthracis_) division may cease where the
organism is cultivated under certain artificial conditions though growth
continues. In this way very long unsegmented threads are produced.
[7] _Arch. f. Entwm._, XX, 1905, p. 76; _Sitzungsb. d. Ges. Naturf._,
Berlin, 1907, p. 41, etc.
[8] Borradaile, L. A., _Jour. Marine Zool._, 1897, No. 8.
[9] Dr. Przibram, I should mention, concludes that on the whole the
facts are against this interpretation, but as more evidence is certainly
required, I call attention to the possibility.
[10] Morgan, T. H., _Regeneration_, 1901.
[11] It would be interesting to know whether growth continues at the
original posterior end after the new "posterior" end has been formed in
front.
[12] In the actual case observed, the ripples unsmoothed had a
wave-length of about 2-1/2 inches; and when the new ones were first
formed, there were about 30 ridges in the length originally traversed by
15 or 16.
[13] _The Science and Philosophy of the Organism_; Gifford Lectures,
1907. London, 1908, p. 141.