THE BUILDING OF REFLEX ARCS.
Assuming the foregoing origin of the innervation of the skin, I submit
that between this rudimentary process and the building of sensori-motor
arcs in the spinal cord and brain there is a field, almost unlimited,
for initiative in the construction of new forms of animal life. The
former is nothing without the latter. To leave it without proceeding
further is to leave it “in the air” as military writers say. The
formation of Receptors, then, both in the skin field and in the higher
sense-organs, leads of necessity to the formation, multiplication and
co-ordination of reflex arcs. As in an imperfectly organised telephone
service after many a repeated stimuli or “rings” the messages begin to
reach their destinations, and as by practice the operators better and
better learn their business, so the impulses passing through receptors
and nerve-fibrils become organized into more or less efficient systems
of arcs, and response is secured to them by some effector of gland or
muscle. It is not true of man alone that practice makes perfect.
A certain feature of higher animals which distinguishes them from lower
must be remembered, and that is that among them the individual becomes
increasingly important. Speaking generally, the latter are born and die
in large groups, and their lives resemble those of their group more
closely than in the former. The struggle of the individual is vividly
pictured by Professor Woods Jones in his description of the baby of the
perfected arboreal animals. He shows how they and the roaming Ungulates
and Pelagic Cetacea cannot indulge in large families, and that it is
only those forms which have a safe retreat for their young which can
avoid reduction of the size of their families, and how the higher apes
still more resemble in these respects mankind, as we know it. For
the proper study of the “synthesis of the individual” organism this
essential fact must be kept in mind.
Some Illustrations.
It will be expected of course that for the claim here advanced on
behalf of the predominant influence of the nervous system in the
initiative of the evolutionary process some experimental or other
evidence should be produced. Before entering upon this, I think some
analogous facts from the story of man, in accordance with the principle
laid down in the first chapter should be stated, so as to illustrate
the line of thought. These will be in the nature of analogies, and
whether or not the accepted accounts of the chosen examples agree
precisely with the last word of the critics is immaterial, for if not
they will equally well serve the purpose of illustration.
Abraham.
When from his Mesopotamian home an opulent and successful farmer
decided for reasons sufficient to himself that he would leave his
present prosperity for a promised land, and went out not knowing
whither he went, it is manifest that the construction and organization
of Abraham’s cerebral cortex was the motive power which led to this
step so fraught with change to himself, his descendants, and the
world. By his choice he showed the inherited structure of his brain,
its nature, and perhaps its nurture, to be different from those of his
family and tribe. Implicit in this venture was the introduction of a
new group of people into a new environment, and their reaction to it
through many generations is written before our eyes to-day in indelible
characters. It was neither stature, muscular development, colour of
hair, skin or eyes, properties of digestive or circulatory organs,
keenness of sight, hearing, taste, smell, or touch which led to this
result even though without a high degree of efficiency of these he
could never have “arrived” as he did.
Mohammed.
The conjunction of environment with a certain organized complexity of
grey matter was hardly ever more important to the world than that of
Mohammed. The powerful frame, abundant black hair, wonderful dark eyes,
and great imposing head may well have attracted the rich widow who
“made his fortune” by marrying him, and they stood him in good stead in
his later adventurous career. But nothing short of a unique arrangement
of his reflex-arcs, chiefly in the association-areas of his brain,
could have opened up to him the world of Asia and Europe.
Columbus.
Who can doubt that it was ultimately to the inherited structure of the
convolutions of his brain that Columbus owed his great achievement
in opening up a New World; or that to the reactionary and intense
“character” of Philip’s brain the persecutions in the Netherlands were
due; and on the other hand that to the brain of William of Orange
with its liberal and enlightened “character” the Seven Provinces that
resisted Philip owed their freedom; the results in the two cases being
the decay of Spain from that time forward, and the final success in
the struggle for religious liberty. In such a view of historical facts
it is not necessary either to follow Carlyle in his extreme claims for
the influence of great men and heroes, nor to look upon the hero as an
epiphenomenon. It is certain that eventually some other great man would
have arisen to do what the great Genoese did, if he had not done it,
and as it is claimed that Amerigo di Vespucci did, and it is certain
that Philip was only the last of the Hapsburg sovereigns who determined
the fall of Spain, and that Huss, Jerome, Wycliffe and Luther in their
days initiated the struggle for religious liberty which Holland brought
to success. But the facts referred to can hardly be disputed, and the
men and their “characters” did certainly determine permanent changes in
the world.
Napoleon.
Among individual men of modern times none strikes the imagination
as does Napoleon. Without ignoring the tremendous outburst of the
soul of down-trodden France at the Revolution, it cannot be denied
that the “character” or grey matter of brain of the man of whom it
is said “nothing where he had passed was as it had been before,” was
the dominant and natural fact that changed the face of Europe. What
physical quality had Napoleon, except those of his grey cells, which
could have led him to such results on the environment into which he was
cast?
Migrations.
Similar results in nations and tribes can easily be supplied from
the great migrations of the past. The wider movements are but due to
comparatively small aggregates of adventurous men, in other words to
the aggregation of many similar central nervous systems. The great
Western and Southern adventures of the Scythian Tribes had many
contributing causes on which the historian has much to say, and they
were physically highly efficient for their new career, but, reduced
to the simplest elements, it was neither their great stature, strong
muscles, flaxen hair, nor blue eyes, but the cerebral constitution of
a comparatively small group of them which brought part of the nation
to the promised land, and left another and large part in their homes
beyond and along the Danube. The subsequent story of the latter may
well be compared with the invaders of Gaul and Italy in connection with
initiative in evolution.
The successive invasion of Britain by Low German tribes in the fifth
century, and the Scandinavian hordes of Swedes, Danes, Norwegians,
Letts and Finns in the eighth and ninth teach the same lesson. The
later condition and development of the Northmen in France, Italy,
Spain, Sicily and Britain have only to be compared for a moment with
that of their races who remained in Norway, Sweden and Denmark and
their descendants, to bring clearly before one’s mind the profound
influence exerted by the cerebral constitution of the original Viking
hosts on their career in their new environments, and, indeed, on the
environments themselves; as in intermarriage with their conquered foes.
These examples have been chosen for the reason that one feature is
common to them all, the introduction of an individual or group into
new environments by reason of the constitution of their brains,
irrespective of the contributing factors. If these be sound analogies
they bear closely on the matter of initiative in the evolution of new
forms of life. The men in question came to their task, in their day,
with a certain equipment of brain derived from many ancestors and much
nurture. Unconscious arbiters of their fate and that of multitudes who
should follow them, they initiated a course of physical and cerebral
evolution of which we can see much revealed before our eyes. The motive
power of their conduct bears a relation to their physical forms that
the engines of a motor-car do to its varied forms of body. The latter
are modified indefinitely to suit convenience, comfort and grace, but
fundamentally they exist and are energised by the former, just as
structure is modified for the performance of function.
This fact is occasionally brought vividly to the mind of an observer
when he first passes a Rolls-Royce car in all its glory and
magnificence, and then a rough squalid kind of trolly in which the
engine-parts of a similar future Rolls-Royce are out for trial. In
principle it is not a long step from these illustrations to the diverse
environments of animals in which their lot is cast, and their reaction
to them as to behaviour and structural change.
Some Changes in Habits of Man.
There are two current views as to the present erect posture of man,
one which traces it to the adoption of a new posture by a pronograde
four-footed ancestor, and the other that man’s ancestors were “never
typically pronograde with four supporting limbs,” but derived from an
arboreal stock in which the forelimbs were mobile rather than stable.
Whenever or wherever man became orthograde he opened up for himself
and his descendants immense regions of structural and functional
change and became increasingly dominant over his environment. Changes
in muscles, joints, bones, bursæ, lungs, heart, and vessels occurred
through his employing in new modes the muscles, joints, bones, bursæ,
lungs, heart and vessels he already possessed, and the resemblance
between these structures of man and the great apes has given to the
latter the name of anthropoid, and this similarity of structures
in the highest Primates has done much to support in the past that
Simian origin of man which is at present questioned. The behaviour
of the apes and early man were sufficiently alike to lead either to
a parallel or genetic similarity. This point is, perhaps, irrelevant
in considering the great field for initiative in the formation of
new physical characters, and chief among these new reflex-arcs which
have built up the marvellous organ of man’s glory and greatness; but
no one can dispute the elementary fact that the ancestor of man who
adopted terrestrial bipedal locomotion and became orthograde, owed it
to his growing brain and the higher integration of his organs for that
function. But besides the new posture he had adopted he learned to talk
articulately, to make tools, and to use stereoscopic vision. None of
these could have been started on the upward way without a long process
of trial and error in the course of his total experience and practice
of his powers. The results that followed from these three properties of
his are inconceivably great, and it is unnecessary to enlarge on such a
theme or to add to the number of examples.
Leaving, then, the immediate ancestor to work out his own destiny in
his new terrestrial home, we must as before proceed backward in the
history of animal life in the line of Primate ancestry.
Primate Ancestry.
It is generally agreed to trace the Primates back to an active pioneer
animal form which took to the trees, and which arose out of the
widely-spread Insectivores. This derivation will probably satisfy
any reasonable genealogist. But, if we may use a parallel in human
families, this active animal was as different from its congeners as
Napoleon was from his four brothers who played a part in European
history, and it is not necessary to say more as to the significance of
this fact than that the relative importance of “chassis” and “body” is
again a useful analogy. But we need to ask what those congeners did
if we are to succeed in understanding the Napoleon-like course of him
who became our Primate ancestor. From the original widely-spread and
plastic raw material of the Insectivores allied forms took different
lines, and their stories are written at great length in one small
and the three other great orders of Bats, Carnivores, Ungulates, and
Rodents. As it has been pointed out, Carnivores took to attacking
larger prey, including their less fortunate relatives, and stepped
into the arena as carnivorous animals; the Ungulates-to-be became
herbivorous and developed into two great groups of hoofed animals,
relying mainly on flight for safety; Rodents took to burrows for
defence, ceased to trouble much about attack, and became gnawing
animals; Bats adopted an aerial life--a poor form of it indeed like
that of the aeroplane--and acquired a degraded fore-limb. Before
leaving these great orders of animals, whom I do not desire to compare
unfavourably with poor Louis, Jerome, Joseph or Lucien Bonaparte, it is
convenient here to refer to a fact which comes to light immediately one
looks into such a piece of classification as this of the orders arising
out of the loins of the early Insectivores, and that is the functional
conception underlying it. Doubtless pure functional “characters”
could never supply a whole system of classification in the light of
the modern doctrine of descent with modification, and of zoological
affinities. This is shown in a change from division of six orders of
Birds-Running, Swimming, Wading, Climbing, Predatory and Perching
Birds, to that of a few old-fashioned Ratite Birds, and all the rest,
one which seems the best that can be offered at present.
Insects, Mollusca, Birds.
The grouping of animals by structural characters, and by affinities
which are assumed, though based on almost undeniable evidence, whether
into species, families, classes, phyla or sub-phyla, has its apotheosis
in Mollusca and Insects. As to the second of these immense groups
it has always seemed strange that their colourings and structural
characters should have received such intensive study from Weismann to
the exclusion of Mollusca, when he set out to prove his stupendous
negative, and still more that of Vertebrates, among which his chief
difficulty and desired triumph would seem to have lain. Mollusca though
invertebrate are held by many to be in the line of ancestry of the
highest forms of life, and at any rate insects are not. They are most
fruitful fields indeed in which Nature has been able to show what she
could do by her stern selective powers, but, from the point of view of
descent with modification, may be fairly compared to a review of an
army in time of peace, or the Kriegspiel of a German military staff. He
who concerns himself with the fundamental difficulties of the problems
at issue in evolution must make his notes of what experts tell him of
such groups as those of Insects, Mollusca and Birds, and pass on to the
higher forms in which on the one hand function becomes the predominant
partner, and on the other individual experience becomes more and more
important. He feels indeed at liberty to wish the entomologist and
ornithologist all success, and to leave him at peace, in his siding, to
pursue his delightful and interminable studies far from the dust and
din of controversy.
Insectivores.
The critical territory of vertebrate, and still more of Mammalian
forms, in which the genealogist pictures the five main groups of
Insectivores, looking about them, if one may so speak, in the world
around and pondering which of many paths they shall pursue, resembles
certain centres that may be seen in towns where three, four, five,
or seven different roads are open to the traveller, each with its
incalculable effects on his ultimate career. If one may change here the
metaphor it may be said that the Insectivores are the watershed of the
Five Rivers of higher life. However much the wayfaring insect-feeders
have diverged from this broad centre in structure, and however much
the laws of genetics have widened this divergence, the facts of
function stare one in the face when such descriptions of three of the
four orders outside the Primate stock are pondered--Flesh-feeders,
Herbivorous animals, Burrowers and Gnawers. These time-honoured names
appealed strongly to older zoologists, and in them is implicit a large
body of evidence for initiative in their evolution by pioneering work
on the part of their ancestors. Though in these days Prototheria
include Monotremes, One-vent animals, Metatheria, Marsupials or pouched
animals, and Eutheria Insect-feeders, and though Mammals derive
their indispensable name from the function by which they feed their
young, the most severe of systematists cannot clear his mind from
the old leaven of function in all these terms. They imply momentous
potentialities prior to new structures, and the modern fails to ban
entirely such functional names. I believe there is here no juggling
with names and words on my part, but a stone in the foundation of the
unambitious building which I am seeking to rear. It is ultimately
connected with a directive power as well as the formation of
sensori-motor arcs in the central nervous system.
Is it possible or probable that the factors which led some group to the
water alone, some to a life in water and on land at different parts
of their lives, some to a crawling life on land and partly in water,
some to the air and trees, some to nocturnal, some to hybernating, some
to burrowing life, some to a diet of flesh, some to one of plants,
some to the trees alone, some to the trees and land, some to the
land by night and trees by day, and some for ever and wholly to the
land--is it probable that any process of selection of suited structures
with countless ages of trial and error, could have determined these
changes of habit and habitat? At least one may claim that the balance
of probabilities is heavily against that view, and that the forging
of reflex-arcs, with all it means to the career of an individual,
affords a more intelligible hypothesis, and that this is strongly
supported by modern discoveries and doctrines arising from the work of
physiologists, as will appear later.
The Place of the Nervous System in Evolution.
The constitution of the nervous system is conditioned by conduction,
its fundamental and primary function. Its processes consist in the
transmission of impulses from receptive fields to effective reactions
through devious paths in a region which, even to-day, is a jungle, with
many further secrets for physiology to reveal. From this point of view
the nervous system may be looked at as a clearing-house and storehouse
of impulses _on their way in_, _on their way through_, and _on their
way out_. If so, the making of new reflex-arcs is a process which has
gone on simultaneously with the formation of receptors in the skin,
the higher sense-organs and such deep structures as muscles, and that
of effectors of infinite variety--and these are called conveniently
adaptations. When we hear from Professor Sherrington that the afferent
fibres with their private paths which enter the spinal cord outnumber
three times those which leave it, and that those of the cranial nerves
should be added, so that the afferent fibres may be reckoned as five
times more numerous than the efferent, we get a vivid idea of the
fundamental importance of the formation and compounding of reflex-arcs
into systems. Without that the most sensitive receptors and the widest
range of structures and organs, small and great, would be as nothing
and things of naught.
A neurone is the anatomical, as the reflex-arc is the functional unit
of a central nervous system. Just as it is profitless to consider apart
the engines and body of a motor car, as working machine, so is it to
picture neurones and reflex-arcs separately in the living nervous
system except for the purpose of an ideal construction. In common with
the organs and structures of higher animals they have to pass, as
historical structures, through the stages of initiation, repetition
of rudimentary function, and selection by trial and error, till the
“canalizing force of habit” issues in rudimentary and increasingly
efficient effectors. It is in this final stage where the triumphs of
selection have been won, and where their undeniable value and interest
has led some exponents of the distributional laws of genetics to
disregard, or accept as data, the early and formative stages. Theirs
is a mental state which resembles that of Darwin, who, for once in a
moment of haste, declared the question of the origin of life to be
rubbish.
In the foregoing consideration of the formation of receptors of the
skin it was assumed that certain common stimuli of the environment
hammer out for themselves paths in the nerve-fibrils of the skin and
by ceaseless repetition lay down not only the receptor, which may
be called the _terminus a quo_, but also the afferent fibres which
ultimately find their way into the grey matter of the cord and brain.
That this is the initial stage of the construction of the higher
nervous system can hardly be denied. But it carries the problem of the
synthesis of the organism but a little way unless it be coincident
with the construction of new reflex-arcs and their co-ordination
into systems. Till this stage be reached in a rudimentary form the
most cunning and exact adaptations and structures, or, as they may
be broadly called effectors, will not advance the efficiency of the
organism in the smallest degree. If the receptor be the _terminus
a quo_ the effector is the _terminus ad quem_. This is so obvious
that it may be waved aside as a truism not worth the notice of a
zoologist concerned with the major problems of biology. It may seem to
challenge in a highly speculative region and manner the labours of the
biometrician and Mendelian, but, if fairly met it no more encroaches
on their territory than do the labours of the engineers who invented
the first and crudest chassis of a motor car upon the elaborate and
brilliant ingenuity, taste and skill of the coachbuilders who turn
out the “body” of a sumptuous Rolls-Royce of 1920. But the latter
would never have “arrived” if the former had not made his slow and
arduous trials and errors and final success. So here, as in many other
subjects, a truism has its use. If the biometrician and Mendelian will
only abstain from erecting notice-boards to proclaim “No thoroughfare
here,” we shall not be put down as trespassers or poachers on their
ground and may range at large in certain fields of speculation.
Some Neural Phenomena.
Among numerous phenomena of nervous reactions discovered by the
research of physiologists certain have a close bearing on the formation
of receptors, afferent fibres and reflex-arcs, especially those
of Delay, Summation, Fatigue, Block or Resistance, Localization,
Facilitation and Inhibition.
Facilitation.
But of all these important reactions in nervous tissues none bears
so closely on the problem of the formation of reflex-arcs as that of
Facilitation. This is equivalent to the Law of Neural Habit of the
physiological psychologist, and is bound up with the highly important
Law of Forward Direction, which Professor Starling says might as well
be spoken of as the Irreciprocal conduction of nerve-arcs. The Law of
Forward Direction of sensori-motor arcs is too well known to need here
any description. But when this law is taken into account the phenomenon
of Facilitation is seen to throw a strong light upon the earliest
and rudimentary formation of specialized nerve-fibres, reflex-arcs
and Final Common Paths leading to the effector glands or muscles.
Facilitation is described shortly by Professor Starling as follows.
If the passage of a nervous impulse across a synapse or series of
synapses in the central nervous system be too often repeated, fatigue
is produced, and there is an increase of the block at each synapse.
If, however the stimulus be not excessive and the impulse not too
frequently evoked, the effect of a passage of an impulse once is to
diminish the resistance, so that a second application of the stimulus
provokes the reaction more easily, and he adds that the result of
summation of stimuli is in fact in the direction of removal of block.
When an impulse has passed once through a certain set of neurones to
the exclusion of others it will tend, other things being equal, to
take the same course on a future occasion, and each time it traverses
this path the resistance in the path will be smaller. Education then
is the laying down of nerve-channels in the central nervous system,
while still plastic, by this process of Facilitation along fit paths,
combined with inhibition (by pain) in the other unfit paths. He makes
the important statement that Facilitation is of great interest in
connection with the development of “long paths” in the central nervous
system and, _more especially with the acquirement of new reactions by
the higher animals_. (Italics not in the original).
Raw Materials of the Central Nervous System.
The raw materials of higher central nervous systems are furnished
even in lowly Vertebrates by the neurones and their processes, and
the pathways into the grey matter by the “canalizing force of habit”
in the receptors and afferent fibres. Facilitation, discovered in
higher Vertebrates, such as dogs and cats, throws backwards a light on
the earliest struggles towards success and integration among phyla,
sub-phyla and smaller groups, and here again the well-known may lead
to the less-known. We may then frame a legitimate hypothesis, or at
least an ideal construction of trials and errors and success, if those
of lower levels were ever to be introduced to the career of progress
and achievement. But to make good this claim it is necessary that it
be based on the important doctrine taught by Hughlings Jackson of the
three (or more) levels of sensori-motor arcs--those of the spinal or
lowest, of the sensory or intermediate, and those of the third or
highest level, in which the association-areas of the Primate brain are
at once the means and the title to his primacy, or headship of the
sentient world. The light of this doctrine guides the mind backwards to
the frog-stage of animal evolution with its highly organized congenital
system of arcs of the spinal level, so efficient for its life that,
even when the brain is removed, the frog can execute under certain
stimuli a purposeful complicated movement such as that of trying to
wipe away with its foot an irritant drop of acid applied to its head
or back; or, still more, if touched lightly between the scapulæ, will
“lower its head at the first touch, and again more so at a second,
and at a third will, besides lowering the head, draw the front half
of its trunk slightly backwards; at a fourth the same movement with
stronger retraction; at a fifth give an ineffectual sweep with its
hind or fore-foot; at a sixth a stronger sweep; at a seventh a feeble
jump; at an eighth a free jump, and so on.” Probably such an animal as
the frog has all its reflexes congenitally organized, whereas a dog,
reaching the sensory level, has added countless reflex-arcs to those
inherited from its early ancestors of the Insectivores which had long
emerged from the spinal level, retaining its old, perfecting its new
inheritance, and eliminating the unfit. Perhaps a faint picture of this
long process may be afforded by watching an experienced mountain guide
ascending an ice-slope with the aid of ice-axe, hand and foot.
Integration of Raw Materials.
Every group of animals in the higher ranks has its own entailed
property of innate reflexes, for example, the reflexes which subserve
the reflex functions of the cord: those of locomotion, muscular and
vascular tone, micturition, defæcation, impregnation and parturition.
These exist in an animal of the spinal level whether or not it
remains purely aquatic, partly aquatic, partly terrestrial, arboreal
or terrestrial. As the progressive groups ascend the ladder of life
they add to this inalienable heritage, gained we need not here ask
how, fresh reflex-arcs by response to new initial stimuli, forging
them by the incident of use. So, the original acquirements in the
past levels serve as starting points for raising the degree of their
nervous integration with growing control over their environments. The
long story from the simple central nervous system of a fish, with
a few or no association-areas, to that of man with his extensive
frontal, parietal, parieto-occipital association-areas, could never be
deciphered, even with the light of the laws of genetics turned on full,
without a protracted process of construction of fresh arcs. A common
illustration of such a series of changes and results may be seen in the
building of a house. Bricks, foundation-stones, walls and a roof may
serve some of the elementary requirements of a house and much less than
these were of use to early man for his shelter. Without them we cannot
call any structure a modern house; but also without floors, staircases,
windows, chimneys, division into rooms, some degree of decoration by
paint or paper, and a supply of water, we should refuse in these days
the name of house to that rough structure, apart from beauty of design,
decoration, within and without, and some addition of modern appliances
of comfort and convenience. In the history of house-building the stages
of supply of raw materials, adaptation to needs guided by selection,
initiation, trial and error have their counterpart in the construction
of higher animals.
Evidence.
It will be asked what evidence there is for the view here put forward
that such is the order and method of the construction of the central
nervous system. There are two classes of evidence. The first direct,
and the second indirect and resting on inference. The well-known leads
to the less-known and inferred. Direct evidence of the foundation
of new reflex-arcs and their organization is of course small. The
conditions, such as the duration of human life, preclude any extensive
formation under experiment of new reflex-arcs, but enough is known to
enable one to follow the backward way with some confidence. As to the
inheritance of these, the evidence rests on opinion and tremendous
probability, but as the only problem with which I am concerned here
is that of initiative I think it better to leave the matter of
transmission to a dispassionate consideration of the probability of its
occurrence.
Direct Evidence.
The prolonged researches of over twelve years of Professor Pawlow and
his colleagues on dogs afford a body of evidence as to the possibility
of producing new reflexes in the life of an individual which have
never been questioned. In 1913 at Groningen, before the International
Congress of Physiologists, he gave a brief account of this work. His
previous work on the digestive glands carried on by delicate operations
in which the œsophagus was diverted from the stomach and made to open
externally, and in which a portion of the stomach was diverted from the
rest and a new “small stomach” was formed, gave him the opportunity
of immensely important insight into the factors governing the work of
the various glands of the stomach. The work of others showed similar
results in the pancreas. I only refer to these because they lead up to
the special artificial results with new reflexes which he described in
1913. He states that the nervous system besides the primitive function
of reproducing innate reflexes, possesses another prime function-namely
the formation of new reflexes; and that the living thing is enabled
to respond, by definite and suitable activities to agencies to which
it was formerly indifferent. His experiments on the formation of
“conditional reflexes,” as he calls them rather than “acquired” as
opposed to “innate,” are grouped around the feeding of the animal
and mainly deal with the salivary glands, because they are in direct
connection with the external world and their reactions are simply and
easily observed. An indifferent stimulus is chosen for the reflexes
which it is desired to build up, and this is applied at the same time
as food or acid is introduced in the mouth. After a few sittings it
is found that this indifferent stimulus _alone_ is now capable of
calling forth a secretion of saliva. “The conditional reflex has been
formed; the formerly indifferent stimulus has now found a path to the
requisite part of the central nervous system. The reflex-arc has now
a different afferent neurone.” He gives a good example of this in the
result of the application of painful stimuli by a strong electrical
current to the skin, systematically accompanying each feeding of the
animal. He finds that the strongest electrical stimuli applied to the
skin give rise merely to the “feeding reaction,” that is, the secretion
of saliva, and no indications of any fright or pain appear. “The skin
of a dog can be subjected to cutting, pinching or burning, and the
only result we shall obtain will be the manifestation of what, judging
from our own experience, we should call the symptoms of the keenest
appetite; the animal follows the experimenter about, licks himself,
and saliva flows in abundance.” This, it must be remembered, occurs in
the absence of the offer or sight of food, _at the time in question_.
He adds: “In this way we have been able to divert the impulses from
one path to another according to the conditions, and we cannot avoid
the conclusion that the diversion of an impulse from one path to
another represents one of the most important functions of the highest
parts of the central nervous system.” The presence of certain special
conditions, he points out, causes the indifferent stimulus, which would
otherwise be dispersed in the higher centres, to be directed to a
particular focus, and _eventually to lay down for itself a path to that
part_. A very interesting detail of such a building of a new reflex is
that “the stimuli from which the new reflex is to be worked out shall
be rigidly isolated.” Therefore to avoid any interference with the
certainty of the experiment, such matters as a personal bodily odour
or kind of movement, or even such a slight fact as a change in the
mode of breathing familiar to the dog on the part of the experimenter,
has in the latest experiments been removed by the application of the
stimuli by mechanical devices worked from another room, with results
similar to the earlier ones. Conditional reflexes can also be obtained
from stimuli arising from the locomotor apparatus, as the joints,
eliminating the stimuli arising from the skin. Also certain parts of
the frontal lobes were extirpated and “when one part is extirpated
the reflex is obtained from the flexion of the joint, but not from
the skin; if a different part be removed we can get the skin-reflex,
but not the reflex from the joint.” He extirpated in one case the
greater portion of the posterior part of the brain and the dog lived
for several years after this in complete health. It was found easy to
obtain a conditional reflex for various intensities of illumination,
also for sound, and even a fine differentiation of tones. In another
dog the anterior half of the brain was removed and all the reflexes
before worked out in this animal disappeared, and yet in this helpless
condition of the dog he could train it to give that response of the
salivary glands which he called the “water-reflex,” in which first of
all an irritating acid was introduced into the mouth and the subsequent
administration of water provoked an abundant secretion of saliva which
does not occur when water is poured into the mouth of a normal dog.
This was confirmed in another example in which alone the centre for
smell had been spared, and yet it was possible in it to train the
smell-reflexes also. I add one striking sentence from Pawlow’s address
which, though an opinion, must be received with the respect it deserves
from such a source. “It is perhaps not rash to think that some of the
newly-formed conditional reflexes can be transmitted hereditarily and
become unconditional thereby.”
Indirect Evidence.
From these limited but cogent pieces of evidence I turn to the
larger but confirmatory lines of indirect evidence and inference, of
which such works as those of Professors Sherrington, Bayliss, and
Starling, the notable address of Professor Macdonald at Portsmouth
in 1911, as well as the recent work of Professor Woods Jones on
Arboreal Man, are full. Indeed if the construction of new reflexes
and reflex-arcs in organic evolution “forged by an incident of use”
as Professor Macdonald puts it, were expunged from these works, their
treatment of the physiology of the central nervous system of higher
animals would be emasculated, to say the least of it. And yet not
one of these eminent men is writing _ad hoc_, or for the confusion
of Weismann and his followers. At this point it may perhaps gain for
the remaining pages a little more consideration from opponents if I
give a few quotations from these writers in support of the foregoing
statement--perhaps the breeze of authority may then carry my little
bark a little further on its perilous voyage. Professor Sherrington
remarks on the first page of his well known work, in reference to the
cell-theory, “with the progress of natural knowledge, biology has
passed beyond the confines of the study of merely visible form, and
is turning more and more to the subtle and deeper sciences that are
branches of energetics. The cell-theory and the doctrine of evolution
find their scope more and more, therefore, in the problems of function,
and have become more and more identified with the aim and incorporated
among the methods of physiology.” Again, “Mere experience can apart
from reason mould nervous reactions in so far as they are plastic.
The ‘bahnung’ (or facilitation) of a reflex exhibits this in germ.”
He uses more than once the pregnant phrase, “The canalizing force of
habit”; again, “Progress of knowledge in regard to the nervous system
has been indissolubly linked with the determination of function in
it.” Speaking of the receptive-field he says of the central nervous
system, “To analyse its action we turn to the receptor organs, for to
them is traceable the initiation of the reactions of the centres”;
of the extero-ceptive field he says, “facing outwards on the general
environment it feels and has felt for countless ages the full stream of
the varied agencies for ever pouring upon it from the external world,”
page 20, and “each animal has experience only of those qualities of
the environment which as stimuli excite its receptors, it analyses
its environment in terms of them exclusively. The integration of the
animal associated with these leading segments can be briefly with
partial justice expressed by saying that the rest of the animal, so far
as its motor machinery goes, is but the servant, of them. Volitional
movements can certainly become involuntary, and conversely, involuntary
movements can sometimes be brought under the subjection of the will.
From this subjection it is but a short step to the acquisition of
co-ordinations which express themselves as movements newly acquired by
the individual,” and, “The integrating power of the nervous system
has, in fact, in the higher animal more than in the lower, constructed
from a mere collection of organs and segments a functional unity, an
individual of more perfected solidarity,” also “a single momentary
shock produces in the nervous arc a facilitating influence on a
subsequent stimulus applied even 1400σ later.” I will give but one more
statement from this work which seems to tell against my humble position
of initiative in evolution. Professor Sherrington says at the end of
his book, speaking of the adjustments of nervous reactions in the
lifetime of the individual: “These adjustments though not transmitted
to the offspring yet in higher animals form the most potent internal
condition for enabling the species to maintain and increase in sum
its dominance over the environment in which it is immersed.” A little
care in reading the foregoing chapters will show that this in no way
contradicts the views expressed.
Facilitation.
From Professor Starling’s Principles of Human Physiology I may again
quote part of his account of Facilitation or “Bahnung.” “When an
impulse has passed through a certain set of neurones to the exclusion
of others it will tend, other things being equal, to take the same
course on a future occasion, and each time it traverses this path the
resistance in the path will be smaller. Education is the laying down
of nerve-channels in the central nervous system, while still plastic,
by the process of ‘Bahnung’ along fit paths combined with inhibition
(by pain) in the other unfit paths. Memory itself has the process of
facilitation for its neural basis,” again, “stimulation of one anterior
root produces no definite movement of a group of muscles, but partial
contraction of a number of muscles which do not normally contract
simultaneously. Thus, stimulation of a sensory nerve may provoke either
flexion or extension of a limb, not both simultaneously. Stimulation
of the motor roots will cause simultaneous contraction of both flexor
and extensor muscles. It is _this subordination of morphological to
physiological arrangements_ in the limbs which has necessitated the
foundation of limb-plexuses.” (Italics not in the original). Professor
Graham Kerr in his work on Embryology before mentioned says: “In early
stages of Evolution, whether phylogenetic or ontogenetic, we may take
it that vital impulses flitted hither and thither in an indefinite
manner within the living substance and that one of the features of
progressive evolution has been the gradual more and more precise
definition of the pathways of particular types of impulse, as well as
the transmitting and receiving centres between which they pass. We may
then regard the appearance of neuro-fibrils within the protoplasmic
rudiment of the nerve-trunk as the coming into view of tracks, along
which, owing to their high conductivity, nerve-impulses are repeatedly
passing. It may be that as each successive passer-by causes a
jungle-pathway to become more clearly defined so each passing impulse
makes the way easier for its successors and makes it less likely for
them to stray into the surrounding substance” (p. 112).
Professor Macdonald, in the Portsmouth address referred to, speaking of
the states of the cells under excitation, rest, and inhibition, says
“_excitation_ is associated with an increase in pressure of certain
particles within the cells; in _rest_ these particles are in their
normal quantity and have their normal number. During _inhibition_ they
are decreased in number or have a retarded motion. Thus it happens
that the excited cell tends to grow in size, on the other hand the
inhibited cell tends to diminish, and the resting cell to remain
unaltered in the nervous system. Structure is everywhere the outcome
of function.” Speaking of the relationship of parts within the nervous
system, “In so far as it is fixed, it is a sign of the orderly action
of circumstance upon the structures of the body, and the result rather
than the cause of the monotony of existence. I hold it as probable
that all the individual structures of the nervous system, and so in
the brain, have just so much difference from one another in size and
shape and in function as is the outcome of that measure of physical
experience to which each one of them has been subjected; and that the
physiological function of each one of them is of the simplest kind.
The magnificent utility of the whole system, where the individual
units have such simplicity, is due to the physically developed
peculiarities of their arrangement in relation to one another, and
to the receptive surfaces and motor-organs of the body.” As to the
lens-system of the eyeball he remarks, “Surely there is no escape from
the statement that either external agency cognisant of light, or light
itself has formed and developed to such a state of perfection this
purely optical mechanism, and that natural selection can have done no
more than _assist_ in this process.” He applies the same conclusion
to the formation of the sound-conducting and resonant portion of the
ear as well as the semi-circular canals and to the cerebellum. These
statements are not strictly associated with this chapter but bear by
analogy very strongly on the matter at issue. Indeed the whole of this
address might be utilised by a junior counsel for Lamarck if he rested
alone on the authority of a leading physiologist. The same may be said
of the anatomist whose _Arboreal Man_ has attracted so much attention.
Speaking of the arboreal habit in the phylogenetic history of mammals
he asks the question, “How did this factor enable that particular stock
to acquire supremacy?” and says that it will be answered as far as it
is possible, by the study of the influence of the arboreal habit upon
the animal body; which may be put in another way as the production of
reflex-arcs suited thereto (p. 3.) Of the muscle groups of fore and
hind limbs he says, “With a simple arrangement of anatomical parts
a slight shifting of muscular origins has turned a perfectly mobile
second segment into a supporting segment constructed upon very simple
lines: that these changes are those produced by the demands of support
from the hind-limbs in tree-climbing seems obvious” (p. 6); of the
position of uprightness upon a flexed thigh of an arboreal man, “It is
tree-climbing which makes this posture a possibility” (p. 63). “But it
is not to be doubted that the underlying principle is clear enough,
that the arboreal habit develops the specialised and opposable thumb
and big toe” (p. 71). “Even before the power of grasp is developed,
we may imagine the dawn stages of educational advances initiated by
hand-touch” (p. 159). “Tactile impressions gained through the hand are
therefore perpetually streaming into the brain of an arboreal animal
and new avenues of learning about its surroundings are being opened up
as additions to the olfactory and snout-tactile routes” (p. 160). He
asks also the pertinent question, and says at least a partial answer
to it can be given, “Did the cerebral advance create the physical
adaptations, or did the physical adaptations make possible a cerebral
advance?” (p. 196). Two more statements from this chapter show what
the answer to this question from the _anatomist_ would be--“and again
in the evolutionary story we are forced back to consider a combination
of seemingly trivial, and apparently chance associations: in this case
the dawning possibilities of neo-pallial developments combined with
the physical adaptations _due directly to environmental influences_”
(p. 198). I have ventured to underline this passage.
I regret the necessary length of these quotations but, on account of
them, can the better be suffered to finish this study, when I briefly
consider certain well-known nervous reactions in the cat and dog as
to their probable origin. It would be a highly interesting thing to
hear an exposition by an expert of all the reflexes and reflex-arcs of
such a system as those which in a cat, dog, ape, or man are concerned
with the passage of a morsel of food from the mouth through all its
chequered and varied career till it undergoes metabolism and excretion,
but I could not do it if I would, and would not here if I could,
because of their fundamental fixed and innate character, and I think
it simpler and safer to refer to such minor reflex-arcs as those which
govern the scratch-reflex in a dog, the pinna reflexes in a cat, and a
few smaller ones, on the principle of _ex uno disce omnes_. Such minor
nerve-mechanisms as these in a pair of well-known domesticated animals
will suffice for evidence on behalf of initiative in evolution.
The Scratch Reflex.
The scratch-reflex in the dog, which like the tendon-reflex in man was
in my youth a subject for schoolboy tricks, has received a vast amount
of attention and research from physiologists to whom it has brought
valuable fruit. It is a familiar phenomenon in a familiar friend of
man. There is a saddle-shaped area on the back of the dog over which
it was found empirically that even a light stimulus when applied
rhythmically, produces the “scalptor-reflex” or a reflex rhythmical
action of the flexor muscles of the leg on the same side, calculated to
remove the irritating causes of the stimulus. This includes a series
of receptors in the skin leading to a spinal segment in the region of
the shoulder, a long neurone in the cord, then a motor neurone, the
axon from which activates the flexor muscles of the leg and produces
scratching. It is described as an efferent arc from receptor to the
motor neurone, from which the Final Common Path supplies the motor
apparatus or effector. Professor Sherrington says that in this reflex
a single stimulus which is far below threshold intensity is found
on its fortieth repetition and nearly four seconds after its first
application to become effective and provoke the reflex and that its
frequency is about 4.5 per second. The reflex movement remains rhythmic
and clonic under the strongest as under weaker stimulation. When it is
easily elicitable the scratch-reflex can be evoked by various forms
of electrical as well as mechanical stimulation, but, when not easily
elicitable, electrical stimulation fails whereas rubbing or other
mechanical forms of stimuli still evoke it, though less vigorously than
usual. This reflex can also be set aside by the “nociceptive arc from
the homonymous foot” or, in other words, a nocuous stimulus to the leg
of that side produces “interferences which amounts to inhibition.”
Empirically it is easy to notice also that if the “scalptor-reflex”
can be elicited on both sides of the body, the dog when standing will
momentarily lose the power in the hind legs.
_Note._--The rhythm of this reflex act is so special even to the
layman that lately I had a singular confirmation of its stereotyped
character, when lying awake at night and being puzzled by a curious
rhythmical scratching sound coming from my next door neighbour’s back
yard. It might have been taken by a wakeful person for some mechanical
work on the part of a burglar, but after listening repeatedly to the
apparently familiar sound I found that it came from the kennel of a fox
terrier kept by my neighbour.
Purposes of Reflexes.
All reflexes being purposive this particular innate reflex is
acknowledged to have for its purpose the grooming or cleaning of the
skin over its hereditary territory. This introduces its connection
with initiative here propounded, and the justification for its
introduction is contained in Professor Sherrington’s statement that
“In the analysis of the animal’s life as a machine in action there can
be split off from its total behaviour fractional pieces which may be
treated conveniently, though artificially, apart, and among these are
the reflexes we have been attempting to decipher”--scratch-reflexes and
others. There seems to be no reason for the existence and stereotyped
character of this reflex except the need or rather the desire (if
one may use a convenient but inaccurate term) on the part of the dog
to remove an irritant which disturbs its comfort when at rest. Some
“minor horrors,” probably fleas moving across the skin-receptive
field of its shoulder and back, must be assumed to be the irritant in
question. This touches the great question of the initiative of this
remarkable reflex, which seems more fixed and powerful in the dog as
we know him than that other reflex which leads him to turn tail and
flee immediately he sees a boy stoop down as if to pick up a stone. I
dare say a clever advocate on the opposite side might impress a jury by
building up a case under which an adaptation to a protective need would
be conceived as responsible for the rapid flight at the sight of the
threatening attitude of the boy. Such a reconstruction is not required,
for it is perfectly clear that in the history of the domesticated
dog the selection of such an adapted reflex could have no place. The
survival-value of this reflex would be nil, for the number of dogs
killed by a stone or maimed for life would be so negligible that the
production of a specialised reflex for the purpose by selection or
survival of the fittest would not arise. Obviously the danger would
be intermittent and rare; and dead dogs tell no tales. On the other
hand it would be highly unpleasant for dogs to be hit by stones and
educability would lead them to avoid the stooping attitude associated
with missiles.
We are told on high authority that not education but educability
is transmissible, and yet this humble reflex appears in very young
dogs that could hardly if ever have known the impact of a stone.
Incidentally we are compelled to remember how in past battles of our
youth the aim both of “ourselves and the enemy” was deplorably poor,
and not from want of practice. This school-boy-stone reflex is either
an example of educational effects transmitted or of a minute bit of the
unpacking of an original complexity which it would require the brain of
a de Quincey to work out. But if we suppose the initial stages of such
a stimulus as the occasional impact of a stone in many generations to
be slowly ingrained in the skin-receptors, reflex-arcs and receptors we
do not need opium either for the acceptance of orthodox dogma or to aid
us in the Mendelian alternative to a very simple ideal construction.
This digression bears on the initiative of the more important
scratch-reflex, and it is profitable to ask “are not both of these
reflexes in dogs examples of Evolution of the Indifferent?” Is it
possible to imagine that from its inception to its fully-formed state,
with a specialised territory of skin-receptors accurately mapped
out, with receptor neurones, reflex-arcs and adapted effectors, this
scratch-reflex can have arisen through Germinal Selection or selective
processes within the germ? At no stage can anything more than a
contribution to more or less comfort to the animal be held to result
from its operation. It is strangely reminiscent of the proceedings
of an elderly man after lunch on a hot day when he protects his head
against house-flies with a handkerchief. I am aware that it is but one
of a large number of reflexes produced for the purpose of grooming
the trunk head or limbs of animals as low down in the scale as the
house-fly or grasshopper, many of which were beautifully described
a few years ago by Miss Frances Pitt in the _National Review_ in an
article dealing with small mammals, chiefly rodents. But I have availed
myself here as elsewhere, of the liberty of doing what Professor
Sherrington says we may do, and consider this scratch-reflex as
split off from the rest of the animal’s behaviour for the purpose of
analysis. He also says in discussing the subject of parasites moving
across the receptive surface of the skin that the ulterior purpose
may be the removal of what “would confuse its function as a receptive
surface to more significant environmental stimuli.” This statement is
hypothetical and the problem obscure; but at any rate we know this that
the removal of the parasite must conduce to the greater comfort of the
dog without any more recondite purpose. The one suggested by Professor
Sherrington would in some possible but very vague manner be referable
to selection, but, whether the suggestion be valid or not, it is almost
impossible to suppose that a saddle-shaped area of the kind described
could be under the guidance of selection. The law of Parcimony forbids.
There is a close similarity between this saddle-shaped area in the dog
and that on the cow’s trunk described in Chapter X. It is difficult
to believe that from man downwards to grasshoppers relief from mild
irritating causes such as this is not enjoyable to the particular
animal, and yet indifferent altogether as to its survival in the
struggles of life for food and mates. The “scalptor-reflex” only
reaches the limits of the receptive field of the scratch-reflex and it
is contrary to observed facts that parasites confine their depredations
just to the region where the formidable scalptor-reflex can reach. The
wicked flea knows better than that. The initiative of this reflex can
well be pictured as taking place in domesticated dogs and their wild
ancestors whose habitats in prehistoric times were probably infested
with these irritants to such a degree that no modern mind can conceive,
and the adequate stimuli, leading to receptors after ages of impact and
consequent hammering out pathways through certain reflex-arcs until
the required weapons of offence or effectors were organised into a
_defensive-offensive system_--were there in profusion. But a great and
fundamental principle of the evolutionary process such as Selection
is not honoured by being dragged in, even for forensic purposes,
to account for results which owe to the search for comfort their
perfection of organisation. I have personally seen in some professional
invalids of the softer sex nearly as perfect adaptations to their
comfort which in no way contributed to their length of life. This may
be put aside as irrelevant but it is at least suggestive.
I submit the statement as to the scratch-reflex in the dog that from
beginning to end it is an _indifferent_ mechanism and the probability
is immense that its initial stages were governed alone by repeated
stimuli from parasites which produced receptors, conducting fibres
afferent neurones and efferent neurones, leading into the Final
Common Path controlling the flexors of the hind limb. It would then
come under the Law of Subjective or Hedonic Selection formulated by
Professor Stout in the words: “Lines of action, if and so far as they
are unsuccessful, tend to be discontinued or varied; and those which
prove successful to be maintained. There is a constant tending to
persist in those movements and motor attitudes which yield satisfactory
experiences, and to renew them when similar conditions recur; on the
other hand those movements and attitudes which yield unsatisfactory
experiences tend to be discontinued at the time of their occurrence,
and to be suppressed on subsequent similar occasions.”
In this connection a statement from Professor McDougall’s work may be
advantageously quoted. He says that “It is characteristic of those
(arcs) of the higher or third level that their organisation, their
interconnections, by means of which the simpler neural systems of
great complexity, is _congenitally determined in a very partial degree
only_, and is principally determined in each individual by the course
of its experience. The arcs of the higher level thus constitute the
physiological basis or condition of docility, the power of learning by
experience.”[88] (My italics)
[88] _Op. cit._ p. 21.
Scratch Reflex of the Cat.
There is a notable difference between the scratch-reflex of the dog
and that of the cat, especially as to the site of its receptive-field.
That of the dog has been referred to, but it appears to be generally
accepted that the cat has no such saddle-shaped or indeed other area of
skin receptive-field on its back or flanks. I have repeatedly tried by
various mechanical stimuli, applied both irregularly and rhythmically,
to evoke a scratch-reflex in a cat, young or adult, on the surface
corresponding to that of the dog, and have found no response. This
has been tried both when the animal was awake and when asleep. But
the receptive field of the cat’s scratch-reflex has received careful
and elaborate attention, which is described in a paper by Professor
Sherrington in the _Journal of Physiology_, Vol. LI. No. 6. By means of
delicate stimuli, mechanical and electrical in a decerebrate cat, the
receptive-field of the scratch-reflex has been accurately delineated
in the pinna, and several other pure reflexes have been obtained.
These are protective of the pinna; some, the retraction and folding
reflexes seem directed against irritant touches, _e.g._ the settling
of fleas--or against exposure to injury in fighting; others, the cover
and head-shake and scratch-reflexes against the ingress of foreign
matter, such as dust, water, insects, into the meatus and ampulla. The
threshold for their elicitation is extremely low, that is to say, they
require very gentle stimuli to evoke them, while with the exception of
the scratch-reflex they are elicited with difficulty and uncertainty
_by electrical stimuli_ (My italics) to which the animal has been
subjected in the course of its total experience. He adds that the
pinnal reflexes are readily obtained in the normal animal, and I may
allude here to some small observations I made on a normal young cat
during profound sleep, recorded in _Nature_, Vol. 106, Sept. 2, 1920.
Light mechanical stimuli, applied during this state of deep sleep to
the internal surface of the pinna, especially close to the meatus,
produced first, twitching of the facial muscles on the same side;
second, as this ceased the fore foot was moved irregularly towards the
ear, and third, as this ceased a rhythmical scratching action of the
hind foot took its place, the rate of which seemed to be exactly the
same as that of the scratch-reflex in the dog evoked from stimulation
of the flank and back. I had not then, unfortunately read more than
an abstract of the above paper, but if the full account be followed
it will be seen that the various “territories” belonging to all the
former-reflexes are now known as well as the frontiers of a European
Kingdom. All I was able to do with this unusual opportunity of a heavy
sleep in a normal young cat was to verify more roughly Professor
Sherrington’s observations and slightly to extend them _in respect of a
sleeping animal_.
In the course of these observations on a young cat I examined the
various regions of the back and flanks with mechanical stimuli of
different degrees of strength. These were applied during sleep and I
found that it was more often during a moderate than a light or deep
sleep that the following results were shown--chiefly under the stronger
stimuli the tail was raised sharply and swept in a circular way, and
this would be repeated according as the stimulus was applied; but at
the same time there was shown a strong, irregular twitching along the
flank, extending forwards to a point near the level of the shoulder.
This latter reflex would appear to be a reaction on the part of the
panniculus carnosus. Both the reflex of the muscles of the tail
and this of the flanks appear to be connected in their origin with
movements of parasites in their respective territories.
In considering the scratch-reflex in the cat a subtle bit of adjustment
is found. That coarse and simple scratching of its ear, which we see
so often in the cat, must have often astonished us for its vigour and
yet its bloodless character. This action is of course a purposeful
one, for it goes on when the animal is awake. Here if anywhere this
profoundly hedonistic animal shows that for it the laws of comfort
are its laws of conduct. It is clear that there may be two processes
or conditions involved in its bloodless violence. On the one hand the
reflex retractile mechanism of the claws may be kept in abeyance by
another reaction which is pre-potent; on the other, it is a fact that
the hind foot in the cat is furnished with claws which are much blunter
than those of the fore foot. As far as I have been able to examine cats
of different ages I have found the claws of the hind foot more like
the blunt claws of a dog than the familiar sharp claws of the Felidæ.
So in the violent scratching referred to there may be a double reason
associated in the process. As to the difference in the sharpness of the
fore and hind claws it would appear to be remarkably like a transmitted
bit of adaptation initiated and kept in being by use and habit in
progression, for the hind foot in such animals as the cat has a larger
share in this action than the fore foot. But here it is difficult as so
often to assign to selection its possible share of the adjustment.
Certain minor but persistent reflexes may be briefly mentioned in
support of this side of the evolutionary process. In the dog and cat,
as we know them, the action of the muscles of the tail by which it is
elevated during the act of defæcation is very suggestive of a reflex
acquired by a very small degree of physical comfort and repeated in
countless individuals, wild and domesticated. I have seen not only this
but a few small scratches made by a cat before defæcation in a kitten
as young as three weeks old. It is also mentioned in illustration of a
vestigial character that a horse will paw the ground with no immediate
apparent object, the act being derived from ancestors which thus
cleared away snow from the ground. This is claimed, doubtfully I think,
as a vestige of a formerly _useful_ habit but seems more probably to
be one of these indifferent reflexes connected with comfort than with
survival-value.
It will be observed that in this branch of the case for Lamarck _v._
Weismann the indirect evidence from inference far exceeds in amount
that of direct experimental evidence, but from the nature of the
problem under consideration this could not be otherwise.
If we may again look back in thought over the long series of animals,
from man downwards, we shall picture those of the spinal level
striving (with apologies for the use of an anthropomorphic word) to
reach the sensory level and finding out the fact that few there be
that enter therein. Again we see in vision the higher creatures of
the sensory level reaching forwards to the strait paths of primate
existence, and again finding the difficulty of self-advancement that
their predecessors found. We see the elect few of these, by a happy
combination of nature and nurture, uprearing to glory and honour the
primate stock with its culmination in man. A long vista indeed and a
vision, but assuredly no mere figment of the imagination, as some of
the slender facts and arguments here would seem to show. With Professor
Bateson we personify Nature in the story, with her wonted coyness
betraying the fact that though she is stern she has her tolerant
moods; that she allows her children, even that “insurgent son” who
calls himself Homo Sapiens, a genial liberty to frame new reflex-arcs
which make for his enjoyment of life in indifferent fields, and _that
the great neural process of Facilitation is the leading factor in
their constructions and probably also in more deeply-based systems of
sensori-motor arcs_.
SUMMARY.
Though it be true that _dolus latet in generalibus_, it is a more
important truth that “without premature generalisations the true
generalisation would never be arrived at.”[89]
[89] Herbert Spencer, _Essays_, II, 57.
Therefore I conclude:--
1. That Plasto-diēthēsis, or the moulding and sifting processes
experienced by organisms, represents the beginning and end of higher
animal evolution; and that its wide hyphen stands for the provinces
where Mendelism, Mutationism, Tetraplasty, Orthogenesis, and the
dynamical work of growth on Form, as well as other factors yet to be
discovered, can range at large.
2. That personal selection is the leading form of that process in
higher animals, whereas among Invertebrates, especially unicellular
forms, selection of groups is the rule.
3. That Initiative in animal evolution comes by stimulation,
excitation, and response in new conditions, and is followed by
repetition of these phenomena until they result in structural
modifications, transmitted and directed by selection and the laws
of genetics--a series of events which agree with Neo-Lamarckian
principles.
4. That undesigned experiments in the arrangement of the Mammalian
hair, and the production of new bursæ, as well as the designed
experiments of Pawlow, support the foregoing claims, with which agree
the converging facts of--varieties of epidermis, arrangement of the
papillary ridges, flexures of the palm and sole, the formation of the
plantar arch, the origin of certain muscles, the innervation of the
human skin, and the building of reflex-arcs.
5. That there is a large place in higher animals for the Evolution of
the Indifferent through the action of use and habit.
6. That the position for Initiative in Evolution here advanced is no
bar to unlimited research.
INDEX.
Adami, Professor. “Medical Contributions to the Study of Evolution,”
21.
Ancestry, primate, 235, 236.
Anthropology, 4.
Ape: Arrangement of hair on forearm of, 43.
----Papillary ridges on hand and foot of, 164.
----Bursæ of, 184–186.
----Muscles of, 213, 214.
Artists, Evidence from, 66.
Ass: Hair-patterns of, 82.
Baboon, Chacma, papillary ridges on hand and foot of, 165, 166.
Bartholinus, Erasmus, 126.
Bateson, Professor, 9, 20, 22, 33, 149, 255.
----“Materials for the Study of Variation,” 9–13, 22, 210.
Bayliss, Professor, 244.
Bear, Parti-coloured (_Æluropus melanoleucus_): Hair-patterns of,
121, 122.
Beddard, Mr., 118.
----“Animal Colouration,” 206.
Bell, Sir Charles, 205.
Bergson, 205.
Bongo (_Tragelaphus euryceros_): Hair-patterns of, 118.
Bonney, Professor. “The Story of Our Planet,” 3.
Bower, Professor, 21.
Brooks, Professor, W. K., 20, 202.
Bursæ, description of, 179, 180.
----Human, enumerated, 180–183.
----Experiments as to, 186–190.
Canidæ: Hair-patterns of, 98–102.
Capybara, epidermis of, 151.
Chimpanzee, papillary ridges on hand and foot of, 164, 167, 168, 176.
----Bursæ of, 184, 185.
Clark, Sir Andrew, 30.
Cold and warmth sensations of human skin, 220–230.
Coote, Captain, 194.
Correlation, 28.
Correns, 11.
Cow, hair and habits of, 87–91.
----Fly-shaker muscles of, 90, 211, 212.
Crime, detection of a, 5.
Cunningham, J. T., 21.
Darwin, 1, 2, 15, 35, 39, 40, 139, 145, 147, 239.
----“Origin of Species,” 2, 8.
----“Descent of Man,” 2.
----Three Blows to, 9.
----On human eyebrows, 64, 65.
Darwin, Sir Francis, 20, 21.
----On Mnemonic theory of Heredity, 20.
Darwinism, 9, 15, 25, 145.
Dendy, Professor. “Outlines of Evolutionary Biology,” 21.
de Vries, 11, 24, 27, 145, 190.
Dog: Arrangement of hair of, 27, 28, 34, 100–102.
----Habits of, 98, 99.
Dyer, Professor Thiselton, 210.
Earth Wolf, epidermis of, 150.
Echidna, epidermis of, 151.
Elliott, Professor Scott, 98, 126.
----“Prehistoric Man and his Story,” 43, 47, 226.
Environments, Discontinuous, 31–33.
Epidermis: Varieties of, found in mammals, 145.
----Stimuli and response, 145–153.
Eyebrows, hairs of human, 64–73.
----Interpreted by wrinkles, 67.
Facilitation, 240, 246–248.
Felidæ: Hair-patterns of, 92–97.
----Snout of, 94.
Flexures of hand and foot, description of, 170–172.
----Chief types of, 172.
----Meaning of, 173–177.
Foot of Man, 155, 156.
----Papillary ridges on, 159, 160.
----Flexures of, 176, 177.
Foot of Man, Plantar arch of, 192–194.
----Muscles of, 214–217.
Forearm, arrangement of hair on, 41.
Galton, 157; On chiromantic creases, 170.
Galvani, 126.
Geikie, Sir Archibald, 3.
Genealogy, 4.
Germinal Selection, 19, 20.
Gibbon, flexures of foot of, 176.
----Bursæ of, 185.
Gibbon, Hainan, papillary ridges on hand and foot of, 164, 165.
Giraffe: Habits of, 115.
----Hair-patterns of, 117.
Gorilla, papillary ridges on hand and foot of, 164.
Haeckel: Pithecoid Ancestors of Man, 1.
Hair-direction, causation of, 140–144.
----Summary of conclusions with regard to, 141.
----Phenomena of, 37, 38.
----Experimental Inquiry into, 125, 126.
----Steps of Inquiry into, 40, 124, 125.
Hair-pattern, Dynamics of, 44, 45, 46, 50.
Hand of Man, 155, 156.
----Papillary ridges on, 157–159.
----Flexures of, 176, 177.
----Muscles of, 214–217.
Harris, Dr. H. Wilder, 147, 154.
Harris, Mrs. Wilder. _See_ Whipple, Miss Inez.
Hartmann, 213.
Hedgehog, epidermis of, 152.
----Papillary ridges on hand and foot of, 162, 163, 166.
----Flexures on hand and foot of, 173.
Hepburn, Dr., 148, 159, 162.
Heredity, Mnemonic theory of, 20.
Herschel, Sir John, 126.
Hill, Professor Leonard, 224.
Historian a biologist, 2.
Horse: Arrangement of hair on side of neck of, 51–63.
----Habits of, 75.
----Hair-patterns of, 75–82.
----Compared with Zebra, 83–85.
----Effect of harness upon hair of, 126–136.
----Fly-shaker muscles of, 211.
Howes, G. B., 12.
Hutchinson, Professor Jonathan, 188.
Hutton, 3.
Huxley, 5, 8, 126, 192, 200, 217.
Insectivores, 237, 238.
Jackson, Hughlings, 241.
Jevons, 124, 125, 126, 140.
Johnston, Sir H. H., 96, 201.
Jones, Professor Wood, 230.
----“Arboreal Man,” 195, 245, 248.
Kammerer, 21.
Keith, Professor, 112, 188, 189, 190, 195, 196, 208, 215, 216, 217.
----On functions of platysma, 113.
Kerr, Professor Graham, 224.
----On Embryology, 246, 247.
Kiang (Thibetan Wild Ass): Hair-patterns of, 119.
Kinkajou, epidermis of, 153.
Kropotkin, Prince, 21.
Lamarck, 13, 21, 22, 33, 35, 145, 147, 247, 255.
Lamarckian hypothesis of organic evolution, 19, 20, 22, 24, 25, 26,
28, 30, 31, 138, 205.
Lankester, Sir E. Ray, 20, 23, 45, 141, 190, 204.
Lemur: Arrangement of hair on forearm of, 43.
----Hair-pattern of, 46.
----Papillary ridges on foot of, 161, 162, 164, 165.
----Black-headed, epidermis of, 153.
----Ring-tailed, epidermis of, 153.
----Flexures of foot of, 176.
Lion: Hair-patterns of, 92–97, 207.
Livingstone, 92.
Llama: Hair-patterns of, 119, 120.
Loris, Slow: epidermis of, 154, 155.
----Papillary ridges on foot of, 161, 166.
Lydekker, 86, 92, 94, 115, 118, 119, 168, 195.
Lyell, 3, 178, 192.
----“Principles of Geology,” 3, 46.
Macacus, flexures of hand and foot of, 176.
Macalister, 215, 217.
McBride, Professor, 21.
Macdonald, Professor, 190, 244, 245, 247.
McDougall, Professor: On Physiological Psychology, 24, 25, 225, 253.
MacEwen, Sir W., 188.
McTaggart, Dr., 142.
Malthus, 2, 15.
Malus, 126.
Mammals, palms and soles of, 150–153.
Man: hair and habits of, 103.
----Arrangement of hair on back of, 104.
----Passive habits of, 106, 107.
----Arrangement of hair on chest of, 108, 109.
----Palm and sole of, 155, 156.
----Papillary ridges on hand of, 157–159.
----Papillary ridges on foot of, 159, 160.
----Flexures of palm and sole of, 176, 177.
----Plantar arch of, 192–194.
----Muscles of hand and foot of, 214–217.
----Changes in habits of, 234, 235.
Marmoset, Papillary ridges on foot of, 161.
Mendel, 11, 15, 26, 145, 190.
Mercier, Dr.: On Causation, 141, 142.
Mill, John Stuart, 141, 142.
Mole, epidermis of, 150.
Monkey, papillary ridges on hand and foot of, 164.
Mule: Hair-patterns of, 82.
Murphy, Dr. John B., 189.
Muscles: Anatomists’ views of, 201, 202.
----Initiative in, 202, 203.
----New, 205.
----Unstriped, 205, 206.
----Facial, of expression, 207–209.
----Fly-shaker, 210–212.
----Skeletal, 212, 213.
----Skeletal, of Primates, 213, 214.
Neural phenomena, 239.
Nervous System: Some aspects of the, 223–225.
----Place of the, in Evolution, 238, 239.
----Raw materials of the, 240, 241.
----Integration of raw materials of the, 241, 242.
Oken, 126.
Ollier, 188.
Onager, Hair-patterns of, 82.
Opossum, American, epidermis of, 151.
----Azara’s, epidermis of, 153.
Orang, papillary ridges on hand and foot of, 164.
----Bursæ of, 185.
Organic Evolution, consideration of problems of, 24–28.
----Factors in, 27.
----Cross-roads in, 203–205.
Owen, Richard, 10.
Oxen: Hair-patterns of, 87–91.
Palm, skin of, 147–156.
----of Man, 155–156.
Papillary ridges, some undersigned experiments in, 166–169.
Paterson, Dr. C., 188.
Pearson, Professor Karl, 34, 141, 142.
Phalanger, epidermis of, 152.
Pitt, Miss Frances, 94, 95, 251.
Plantar arch, of man, 192–194.
----How it was built, 194, 195.
----Equipment of, 196, 197.
----Description of, 197–199.
Plasto-diēthēsis, 34, 145, 195.
Platysma, Struggles of the, 111–114.
Playfair, 3.
Pocock, Roger, 75.
Poincaré, Henri, 157.
----Principles of method, 36.
Porcupine, Canadian Tree, epidermis of, 151, 153.
Poulton, Professor: “Essays on Evolution,” 86.
Prescott’s “Conquest of Peru,” 120.
Pressure, Examples of the effects of, upon hair-direction, 127–136.
Primates, epidermis of, 153.
Rabbit, epidermis of, 151.
Records, Interpretation of, 110.
Reflex arches, formation of, 231.
----Some historical illustrations of, 231–234.
----Of Insects, 236, 237.
----Of Mollusca, 236, 237.
----Of Birds, 236, 237.
----Evidence of production of new, 242–246.
Reflexes: Stimuli of, 249–256.
----Scratch, of the dog, 249.
----Purposes of, 250–253.
----Scratch, of the cat, 253.
Rivers, Sources of, 4.
Romanes: 19, 39, 40, 145.
----On Weismann, 17, 18.
Roux, 19, 145.
Russell, The Hon. Bertrand, 142.
Russell, E. S. On Lamarck’s theory, 20.
Saint-Hilaire, Geoffrey, 10.
Sapajou, Brown, papillary ridges on hand and foot of, 165.
Schafer’s “Text Book of Physiology,” 219, 220, 226.
Scratch reflex; of the dog, 249.
----Of the cat, 253.
Selous, 92, 115, 118.
Sherrington, Professor, 218, 226, 229, 238, 244, 245, 249, 250, 251,
253, 254.
Skin, Human: Distribution of Touch Corpuscles, 219.
----Distribution of Touch Spots, 219.
----Distribution of Cold and Warmth Sensations, 220.
----Distribution of Cold and Warmth Spots, 220, 221.
----Stimuli of pressure, 222, 226–230.
----Stimuli of cold, 226–230.
----Stimuli of pain, 226–230.
----Stimuli of warmth, 226–230.
Sloth, Two-toed (_Cholæpus didactylus_): Hair-patterns of, 122, 123.
Smith, Dr. E. Barclay, 201, 216.
Sole, skin of, 147–156.
----Of Man, 155, 156.
Spencer, Herbert, 28, 177.
Squirrel, epidermis of, 152.
----Papillary ridges on hand and foot of, 162, 166.
----Flexures of hand and foot of, 175.
Squirrel-monkey, papillary ridges on foot of, 161.
----Flexures of foot of, 176.
Starling, Professor, 227, 244.
----On Facilitation, 240, 246.
Still-born children, subcutaneous bursæ of two, 183, 184.
Stimuli, 30, 31.
----Of touch, 219, 226–230.
----Of cold, 220, 226–230.
----Of warmth, 220, 226–230.
----Of pressure, 222, 226–230.
Stout, Professor, 252.
Summary of conclusions arrived at, 257, 258.
Thomson, Professor J. Arthur, 20, 22, 23, 24, 25, 28, 29, 34.
----On Heredity, 22.
Touch Corpuscles, 219, 227, 228.
Touch spots, 219, 225, 227, 228.
Tschermak, 11.
Ungulates, even-toed, 86–91.
----odd-toed, 74–85.
Vernon, Dr.: “Variations in Animals and Plants,” 28.
Vulpine phalanger, flexures of foot of, 173, 175.
Wallace, Professor, 2, 39, 40, 46, 145.
Weber’s Law, 222.
Weismann, 9, 11, 14, 16, 17, 18, 21, 22, 24, 25, 26, 86, 139, 177,
210, 245, 255.
----Twelve points, 15, 16.
Weismannism, 17, 18, 19, 26, 145.
Welton, Mr., 142.
Whipple, Miss Inez (Mrs. Wilder Harris), 154, 162.
----Criticism of “The Direction of Hair in Animals and Man,” 137–139.
Wolff, 189: “Law of Bone Transformation,” 205.
Young, Arthur, 143.
Zebra: Comparisons between horse and, 83, 84.