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.