LECTURE XXIII
EXAMINATION OF THE HYPOTHESIS OF THE
TRANSMISSIBILITY OF FUNCTIONAL MODIFICATIONS
Darwin's Pangenesis—Alleged proofs of functional inheritance—Mutilations not transmissible—Brown-Séquard's experiments on Epilepsy in guinea-pigs—Confusion of infection of the germ with inheritance, Pebrine, Syphilis, and Alcoholism—Does the interpretation of the facts require the assumption of the transmission of functional modifications?—Origin of instincts—The untaught pointer—Vom Rath's and Morgan's views—Attachment of the dog to his master—Fearlessness of sea-birds and seals on lonely islands—Flies and butterflies—Instincts exercised only once in the course of a lifetime.
As I have already said in an earlier lecture, Darwin adhered to Lamarck's assumption of the transmission of functional adaptations, and perhaps the easiest way to make clear the theoretical difficulties which stand in the way of such an assumption is to show how Darwin sought to present this principle as theoretically conceivable and possible.
Darwin was the first to think out a theory of heredity which was worthy of the name of theory, for it was not merely an idea hastily suggested, but an attempt, though only in outline, at elaborating a definite hypothesis. His theory of 'Pangenesis' assumes that cells give rise to special gemmules which are infinitesimally minute, and of which each cell brings forth countless hosts in the course of its existence. Each of these gemmules can give rise to a cell similar to the one in which it was itself produced, but it cannot do this at all times, but only under definite circumstances, namely, when it reaches 'those cells which precede in order of development' those that it has to give rise to. Darwin calls this the 'elective affinity' of each gemmule for this particular kind of cell. Thus, from the beginning of development there arises in every cell a host of gemmules, each of which virtually represents a specific cell. These gemmules, however, do not remain where they originated, but migrate from their place of origin into the blood-stream, and are carried by it in myriads to all parts of the body. Thus they reach also the ovaries and testes and the germ-cells lying within these, penetrate into them, and there accumulate, so that the germ-cells, in the course of life, come to contain gemmules from all the kinds of cells which have appeared in the organism, and, at the same time, all the variations which any part may have undergone, whether due to external or internal influences, or through use and disuse.
In this manner Darwin sought to attribute to the germ-cells the power of giving rise, in the course of their development, to the same variations as the individual had acquired during its lifetime in consequence of external conditions or functional influences.
I abstain from analysing the assumptions here made; their improbability and their contradictions to established facts are so great that it is not necessary to emphasize them; the theory shows plainly that it is necessary to have recourse to very improbable assumptions, if an attempt is to be made to find a theoretical basis for the transmission of acquired (somatogenic) characters. Even when Darwin formulated his theory of Pangenesis his assumptions were hardly reconcileable with what was known of cell-multiplication; now they are above all irreconcileable with the fact that the germ-substance never arises anew, but is always derived from the preceding generation—that is, with the continuity of the germ-plasm.
If we were now to try to think out a theoretical justification we should require to assume that the conditions of all the parts of the body at every moment, or at least at every period of life, were reflected in the corresponding primary constituents of the germ-plasm and thus in the germ-cells. But, as these primary constituents are quite different from the parts themselves, they would require to vary in quite a different way from that in which the finished parts had varied; which is very like supposing that an English telegram to China is there received in the Chinese language.
In spite of this almost insuperable theoretical obstacle, various authors have worked out the idea that the nervous system, which connects all parts of the body with the brain and thus also with each other, communicates these conditions to the reproductive organs, and that thus variations may arise in the germ-cells corresponding to those which have taken place in remote parts of the body.
Even supposing it were proved that every germ-cell in ovary or testis was associated with a nerve-fibre, what could be transmitted to it by the nerves, except a stronger or weaker nerve-current? There is no such thing as qualitative differences in the current; how then could the primary constituents of the germ be influenced by the nerve-current, either individually or in groups, in harmony with the organs and parts of the body corresponding to them, much less be caused to vary in a similar manner? Or are we to imagine that a particular nerve-path leads to every one of the countless primary constituents? Or does it make matters more intelligible if we assume that the germ-plasm is without primary constituents, and suppose that, after each functional variation of a part, telegraphic notice is sent to the germ-plasm by way of the brain as to how it has to alter its 'physico-chemical constitution,' so that the descendants may receive some benefit from the acquired improvement?
I am not of the number of those who believe that we already know all, or at least nearly all, that is essential, but am rather convinced that whole regions of phenomena are still sealed to us, and I consider it probable that the nervous system in particular is not yet exhaustively known to us, either in regard to its functioning or in regard to its finest structural architecture, although I gratefully recognize the advances in this domain that the last decades have brought about. In any case, such assumptions as I have just indicated, or similar ones, seem to me quite too improbable to furnish any foothold for progress. Yet we must always remain conscious that we cannot decide as to the possibility or impossibility of any biological process whatever from a purely theoretical standpoint, because we can only guess at, not discern, the fundamental nature of biological processes. At the close of this lecture I shall return to the question of the theoretical conceivability of an inheritance of functional adaptations; but first of all we must consider the facts and be guided by them alone. If they prove, or even make it seem probable, that such inheritance exists, then it must be possible, and our task is no longer to deny it, but to find out how it can come about.
Let us therefore investigate the question whether an inheritance of acquired characters, that is, in the first place, of functional adaptations, is demonstrable from experience. We shall speak later on of the effect of climatic and similar influences in causing variation; the case in regard to them is quite different, because they undoubtedly affect not only the parts of the body but the germ-cells as well.
When we inquire into the facts which have been brought forward by the modern adherents of the Lamarckian principle as proofs of the inheritance of acquired characters in this restricted sense, we shall find that none of them can withstand criticism.
First, there are the numerous reputed cases of the inheritance of mutilations and losses of whole parts of the body.
It is not without interest to note here how opinion in regard to this point has altered in the course of the debate.
At the beginning of the discussion they were all brought forward as evidence of undoubted value for the Lamarckian principle.
At the Naturalists' Congress in Wiesbaden in 1887, kittens with only stumps of tails were exhibited, and they were said to have inherited this peculiarity from their mother, whose tail, it was asserted, had been accidentally amputated. The newspapers reported that the case excited great interest, and biologists of the standing of Rudolf Virchow declared it to be noteworthy, and regarded it as a proof, if all the details of it were correct. From many sides similar cases were brought forward, intended to prove that the amputation of the tail in cats and dogs could give rise to hereditary degeneration of this part; even students' fencing-scars were said to have been occasionally transmitted to their sons (happily not to the daughters); a mutilated or torn ear-lobe in the mother was said to have given rise to deformity of the ear in a son; an injury to a father's eye was said to have caused complete degeneration of the eyes in his children; and deformity of a father's thumb, due to frostbite, was said to have produced misshapen thumbs in the children and grandchildren. A multitude of cases of this kind are to be found in the older textbooks of physiology by Burdach, and above all by Blumenbach, and the majority have no more than an anecdotal value, for they are not only related without any adequate guarantee, but even without the details indispensable to criticism.
As far back as the eighteenth century the great philosopher Kant, and in our own day the anatomist Wilhelm His, gave their verdict decidedly against such allegations, and absolutely denied any inheritance of mutilations; and now, after a decade or more of lively debate over the pros and cons, combined with detailed anatomical investigations, careful testing of individual cases, and experiment, we are in a position to give a decided negative and say there is no inheritance of mutilations.
Let me briefly explain how this result has been reached.
In the first place, the assertion that congenital stump-tails in dogs and cats depended on inherited mutilation proved to be unfounded. In none of the cases of stump-tails brought forward could it even be proved that the tail of the relevant parent had been torn or cut off, much less that the occurrence, in parents or grandparents, of short tails from internal causes was excluded. At the same time anatomical investigation of such stump-tails as occur in cats in the Isle of Man, and in many Japanese cats, and are frequently found in the most diverse breeds of dogs, showed that these had, in their structure, nothing in common with the remains of a tail that had been cut off, but were spontaneous degenerations of the whole tail, and are thus deformed tails, not shortened ones (Bonnet).
Experiments on mice also showed that the cutting off of the tail, even when performed on both parents, does not bring about the slightest diminution in the length of tail in the descendants. I have myself instituted experiments of this kind, and carried them out through twenty-two successive generations, without any positive result. Corroborative results of these experiments on mice have been communicated by Ritzema Bos and, independently, by Rosenthal, and a corresponding series of experiments on rats, which these two investigators carried out, yielded the same negative results.
When we remember that all the cases which have been brought forward in support of an inheritance of mutilations refer to a single injury to one parent, while, in the experiments, the same mutilation was inflicted on both parents through numerous generations, we must regard these experiments as a proof that all earlier statements were based either on a fallacy or on fortuitous coincidence. This conclusion is confirmed by all that we know otherwise of the effects of oft-repeated mutilations, as for instance the well-known mutilations and distortions which many peoples have practised for long, sometimes inconceivably long, ages on their children, especially circumcision, the breaking of the incisors, the boring of holes in lip, ear, or nose, and so forth. No child of any of these races has ever been brought into the world with one of these marks: they have to be re-impressed on every generation.
The experience of breeders agrees with this, and they therefore, as Wilckens remarks, have long regarded the non-inheritance of mutilations as an established fact. Thus there are breeds of sheep in which, for purely practical reasons, the tails have been curtailed quite regularly for about a century (Kühn); but no sheep with a stump-tail has ever been born in this breed. This is all the more important because there are other breeds of sheep (fat-rumped sheep) in which the lack of the tail is a breed character; it is thus not the case that there is anything in the intrinsic nature of the tail of the sheep to prevent it becoming rudimentary. The artificially rounded ear of fox-terriers, too, though cut for generations, never occurs hereditarily. Mr. Postans of Eastbourne informs me that the cocks which are to be used for cock-fighting are docked of their combs and wattles beforehand, and that this had been done for at least a century, but that no fighting cock without comb and wattles has been reared. In the same way various breeds of dog, such as the spaniels, have had their tails cut to half their length regularly and in both sexes for more than a century, yet in this case there is no hereditary diminution of the length of tail. Deformed stump-tails do indeed occur in most breeds of dog, but, as I said before, their anatomical character is quite different from that of artificially shortened tails, moreover they may occur in breeds whose tails have not suffered from the fashion of docking, as, for instance, in the Dachshund.
We may therefore affirm that an inheritance of artificially produced defects and mutilations is quite unproved, and in no way bears out the supposed inheritance of functional changes.
This is now admitted by the great majority of the adherents of the Lamarckian principle, and we may now regard this kind of 'proof' as disposed of.
In addition to the above, various sets of facts have been brought forward as proofs, and in particular the much discussed experiments of Brown-Séquard on guinea-pigs, from which it was inferred that epilepsy artificially induced could be transmitted. But these experiments do not really prove anything in regard to the question at issue, because epileptic-like convulsions may have very various causes, and these are, for the most part, quite unknown. Since artificial epilepsy can be induced in guinea-pigs by the most diverse injuries to the central or peripheral parts of the nervous system, this of itself points to the fact that it is not a question of the mere lesion of anatomical structure, I mean, of the breaking of the continuity of a definite part, and of its transmission. The result would, in any case, differ according to whether certain centres of the brain, or half the spinal cord, or the main nerve-trunks were cut through. There must, therefore, be something more needed to produce the appearance of epilepsy—some morbid process which may arise at different parts of the nervous system, and be continued from them to the brain-centres. This is corroborated by the fact that it takes at least fourteen days, and often from six to eight weeks, for epilepsy to develop after the operation, and that in many cases it does not develop at all. I have made the suggestion that, during or after the operation, some kind of pathogenic micro-organism might easily reach the wounded parts, and there excite inflammation, which may extend centripetally to the brain. Similar processes have been observed in connexion with lymph-vessels, and why should they not occur in connexion with nerves?
It has been objected to this that the guinea-pig's epilepsy may be produced by blows on the skull, and also by a destructive compression of the nervus ischiadicus through the skin, and that in both cases the epilepsy may reappear in the following generation; and this, it is supposed, shows that the intrusion of microbes is excluded. If this were so beyond a doubt, and if we could exclude the possibility that there were previously various microbes within the body, which could only penetrate into the nervous substance after the cutting or destruction of the neurilemma, nothing would be gained that would in any way support the Lamarckian principle. One could only say: Certain injuries to the nervous system give rise secondarily in guinea-pigs to morbid phenomena like epilepsy, and all sorts of functional disturbances of the nervous system often appear in the next generation, including in rare cases even the phenomena of epileptic convulsions. That this is a case of the transmission of an acquired anatomical modification brought about by the injury is not only unproved, but is decidedly negatived, for the injuries themselves are never transmitted. Thus what is transmitted must be quite different from what was acquired, for no one has ever detected in the offspring the lesion of the nerve-trunk which was cut through in the parent, or any other result except the disease to which the original injury gives rise. Moreover, the inheritance of these morbid phenomena has been again brought into dispute quite recently owing to the investigations of such experts in nervous diseases as Sommer and Binswanger, and the correctness of Brown-Séquard's results, which have dragged through the literature of the subject for so long, has been emphatically denied[11].
[11] See H. E. Ziegler's report in Zool. Centralblatt, 1900, Nos. 12 and 13.
Clearly formulated problems, like that of the inheritance of acquired characters, should not be confused by bringing into them phenomena whose causes are quite unknown. What do we know of the real causes of those central brain-irritations which give rise to the phenomena of epilepsy? It is certain enough that there are diseases which are acquired and are yet 'inherited,' but that has nothing to do with the Lamarckian principle, because it is a question of infection of the germ, not of a definite variation in the constitution of the germ. We know this with certainty in regard to the so-called Pebrine, the silkworm disease which wrought such devastation in its time; the germs of the pebrine organism have been demonstrated in the egg of the silk-moth; they multiply, not at once but later, in the young caterpillar, and it is the half-grown caterpillar, or even the moth, that succumbs to the disease.
Whether in this case also the disease germs are transmitted through the male sex-cells is not proved, as far as I am aware, but that this can happen is shown by the transmission of syphilis from father to child. That in this case, also, the exciting cause of the disease is a micro-organism cannot be doubted, although it has not yet been proved. Thus even the minute spermatozoon of Man can contain microbes, and transmit them to the germ of a new individual.
This discussion of scientific questions ought not to be brought down to the level of a play upon words, by bringing forward cases like the above as evidence for the inheritance of 'acquired characters,' as was done, for instance, by M. Nussbaum, who cited as a proof of this the migration of the alga-cells which live in the endoderm of the green freshwater Hydra into the ovum, which is originally colourless, and originates in the ectoderm of the animal ([Fig. 35B, p. 169, vol. i]). It seems to me better to make a precise distinction between the transmission of extraneous micro-organisms through the germ-cells and the handing on of the germ-plasm with the characters inherent in its structure. Only the latter is inheritance in the strict scientific sense, the former is infection of the germ.
Still less than the cases of inherited traumatic epilepsy can the morbid constitution of the children of drunkards be regarded as a proof of the inheritance of somatogenic characters, though this has often been maintained. I will not lay any stress on the fact that the allegation itself is, according to the most competent observers, such as Dr. Thomas Morton[12], far from being established. But even if it were quite certain that the numerous diseases of the nervous system, amounting sometimes to mania, which are frequently observed in the children of drunkards, were really caused by the drinking of the parents, it ought not to be overlooked that we have here to do not with the hereditary transmission of somatic variations, but of variations directly induced in the germ-plasm of the reproductive cells, for these are exposed to the influence of the alcohol circulating in the blood, just as any other part of the body is. That by this means variations in the germ-plasm can be brought about, and that these may lead to morbid conditions in the children cannot be denied, and ought not on a priori grounds to be called in question. For we are acquainted with many other influences—climatic, for instance—which directly affect and cause variation in the germ-plasm. Whether this is so in the case of drunkenness, and in what manner it comes about, whether through direct action of the alcohol, or through infection of the germ with some microbe, we must leave to the future to decide; the whole question is out of place here; it can in no way help us to clear up the problem with which we are now occupied.
[12] Morton, 'The Problem of Heredity in Reference to Inebriety,' Proceed. Soc. for the Study of Inebriety, No. 42, Nov. 1894.
But even if there were not a trace of proof of the transmissibility of functional modifications, that alone would not justify us in concluding that the transmission is impossible, for many things may happen that we are not in a position to prove at present. If it could be shown that there was a whole group of phenomena that could not be explained in any other way than on the hypothesis of such inheritance, then we should be obliged to assume that it really occurred, although it was not demonstrable, and, indeed, not even theoretically conceivable. This is the standpoint of the adherents of the Lamarckian principle at present.
They say there are a great number of transformations which are simply and easily explained, if we regard them as the effects of inherited use or disuse, but which admit only of a strained explanation, and sometimes of none at all, on the basis of natural selection, and these are not a few isolated cases, but whole categories of them.
I will submit a few of these, and show at the same time why I cannot regard them as convincing, even if it be the case that we are not at present in a position to explain them without the aid of the Lamarckian principle. But let me hasten to add that it is my belief that we can do this, although certainly not without first giving a somewhat extended application to the principle of selection.
It has often been maintained that the existence of animal instincts is in itself enough to prove that the Lamarckian principle is operative. In one of the earlier lectures I showed that at least the greater number of instincts must have originated in purely reflex actions, and therefore, like these actions themselves, can only be explained through natural selection. A reflex action, such as coughing, sneezing, shutting of the eyelids, and so on, differs from an instinctive action in the lesser complexity and shorter duration of the series of movements liberated by a sense-impression, and also in that it does not require to enter into consciousness at all; but no very precise boundary can be drawn between the two, and, in any case, both depend, as we have already seen, on a quite analogous anatomical basis. It is only a difference in degree whether, at the sight of a rapidly approaching object, the muscles of the eyelids contract, and by shutting the lids, protect the eye, or whether the fly, which we intend to seize with our hand, is impelled by the sight of the rapidly approaching shadow of the hand to fly quickly up. The action of the fly may be regarded as reflex, or equally well as instinctive. But there is also only a difference in degree, not in kind, between this simple action and the complex and protracted behaviour of a mason-bee, the sight of whose colony impels her to fly out and fetch clay, with it gradually to build a neat cell, to fill this with honey, to lay an egg in it, and finally to furnish the cell with a roof of clay. Since all reflex mechanisms, and all the natural instincts of animals, contribute to the maintenance of the species, and are therefore useful, their origins must be referable to natural selection, and we have only to ask whether they must be referred to it always, and to it alone.
It cannot be doubted that, in Man, and in the higher animals voluntary actions which are often repeated gradually acquire the character of instinctive actions. The individual movements pertaining to the particular action are no longer each guided by the will, but a single exercise of will is enough to liberate the whole complex action, such as writing, speaking, walking, or the playing of a whole piece of music; frequently the will-impulse may be absent altogether, and the action be set going simply by an adequate external stimulus, as in the case of sleep-walking, which is observed in fatigued children and soldiers, and in somnambulists. The external stimulus is transmitted to the proper group of muscles as unfailingly as in the case of true instincts, and this happens not only in regard to actions which, like walking, are essential to the life of the species, but also in regard to those which have arisen from chance habits or exercises. Often a short practice is sufficient to make an action in this sense instinctive, and the complexity of the instinct-mechanism gained by such practice is often astounding. Under some circumstances a person may play a piece on the piano from the score, and yet be thinking intently of other things, and be quite unconscious of what is played. In the same way it may happen that a person dominated by violent emotion, when trying to free himself from it by reading, may read a whole page, line by line, without understanding in the least what has been read. In the last case it is not directly demonstrable that the reader has made all the complex delicate eye-movements which would be liberated by the sight of the words, but in the case of playing, the listeners can perceive that the piece is correctly played, and thus that the stimulus exercised by each note on the retina of the eye is translated into the complex muscular movement of arm and finger, corresponding both to the pitch and the duration of the note, and to the simultaneousness of several notes.
In all these cases it is probably not always quite new paths which are established in the brain, but use is made of particular tracks in the innumerable nerve-paths already existing in the nerve-cells (neurons) which are 'more thoroughly trodden' by practice, so that the distribution of the nerve-current takes place more easily along them than along others[13]. This much-used metaphor does not indicate the actual structural changes which have taken place, but it serves at least to indicate that we have to do with material changes in the ultimate living elements of the nerve-substance (nerve-biophors) whether these changes be in position or in quality. Now, if such brain-structures and mechanisms acquired through exercise in the individual life could be transmitted, new instincts would certainly arise in this way, and many naturalists hold this view still.
[13] This, however, is by no means intended to cast doubt on the possibility that quite new paths may arise during the individual life, as is made probable by the recent investigations of Apáthy, Bethe, and others.
If the inheritance of acquired characters had already been proved in other ways, we could not refuse to admit that it might play a part in the higher animals in the modification and new formation of instincts. We should then have to admit that habits can be inherited, and that instincts actually are or may be, as they have often been said to be, inherited habits. But to make the converse conclusion, and to infer from the result of the brain-exercise in the individual life and its similarity to inborn instincts that the latter also depend on inherited exercise, and that there must therefore be inheritance of acquired characters, is hardly admissible.
It might be all very well if there were no other explanation! But as instincts depend on material brain-mechanisms which are variable, like every other part of the body, and as, furthermore, they are essential to the existence of the species, and, down to the minutest detail, are adapted to the circumstances of life, there is no obstacle in the way of referring their origin and transformation to processes of selection.
It has been asserted that the results of training, for instance in dogs, can be inherited, since the untaught young pointer points at the game, and the young sheep-dog runs round and barks at the flock of sheep without biting them. It is, however, often forgotten that, not only have these breeds arisen under the influence of artificial selection by Man, but that they are even now strictly selected. My colleague and friend, Dr. Otto vom Rath, who unhappily died all too soon for Science, and who was not only a capable investigator, but an experienced sportsman, told me that huntsmen distinguish very carefully between the better and the inferior young in a litter, and that by no means every whelp of a pair of pointers can be used for hunting game-birds. Lloyd Morgan points out the same thing, and he is undoubtedly a competent judge in the domain of instinct; he confirms the statement that the pointer 'often points at the quarry, it may be a lark's nest, without instruction,' but he says at the same time, that the power is inborn in very varying degrees, and that, in his opinion, selection undoubtedly plays a part.
It must not, therefore, be believed that the habit of the pointer depends on training; it is only strengthened in each individual by training, but it depends on an innate predisposition to creep up to the game, and is thus a form of the hunting instinct. Man has taken advantage of this, and has increased it, but has certainly not ingrafted it into the breed by whipping. And something similar will be found to be true in all cases of so-called inheritance of the effects of training. It must not be forgotten what astounding results can be achieved in the individual by training. The elephant is the best example of this, for it only exceptionally breeds in captivity, and all the thousands of 'domesticated' elephants in India are tamed wild elephants. Yet they are as gentle and docile as the horse, which has been domesticated for thousands of years; they perform all kinds of tasks with the greatest patience and carefulness, in many cases without being under constant superintendence. They are indeed animals of great intelligence; they understand what is required of them, and they accommodate themselves readily to new conditions of life.
The attachment of the dog to its master and to Man generally has often been cited as a proof of the origin of a new instinct by the inheritance of acquired habitude; but the dog is a sociable animal even in a wild state, and by living in co-operative association with Man it has transferred its sociable affections to him. We find exactly the same thing in the elephant which has been caught wild and tamed. It is particularly emphasized by those who have accompanied animal transports in Africa that the young elephants are wild and malicious towards the blacks who teased and maltreated them, but complaisant and harmless towards the whites who treated them kindly. The attachment of elephants to their keepers and to every one who shows them kindness is familiar enough; it does not depend on a newly acquired impulse, but on the sociable impulse inherent in the species, which, in the wild state, causes them to live in fairly large companies, and on their inoffensive, timid, and, we may almost say, affectionate disposition.
Of course it is easy enough to give an imaginative theoretical interpretation of the origin of a new instinct from a newly acquired habit. We have often heard that sailors have found the birds in distant uninhabited islands quite free from fear; they let themselves be struck down with cudgels without attempting to escape. The extermination of the Dodo three centuries ago is a well-known example of this. Chun, in his magnificent work on the German Deep Sea Expedition of 1898, has recently communicated numerous interesting examples of the indifference of birds towards Man when they have not learned what his presence means: thus the sea-birds of Kerguelen, penguins, cormorants, gulls, 'kelp-pigeons' (Chionis), and others, behaved towards Man very much like the tame geese of our poultry yards. Even enormous mammals like the 'sea-elephant,' a seal with a proboscis-like prolongation of the nose, neither attempted to escape nor showed any hostility to man, but quietly let itself be caught. Similar tales were told by Steller in 1799, after he had been obliged to pass a winter with his sailors on an island in the Behring Straits. The numerous gigantic sea-cows (Rhytina stelleri) which lived there were so confiding that they allowed the boat to come quite up to them, and the sailors were able to kill many of them from time to time, using their flesh for food. But towards the end of the winter the animals began to be shy, and, in the following winter, when other sailors to the polar regions endeavoured to hunt them too, it was very difficult to secure them; they had recognized man as an enemy, and fled from him when they saw him from afar. Thus the same individuals which had earlier carelessly allowed man to come up to them now avoided him as an enemy. This was not instinct, it was a behaviour controlled by the will and founded on experience. But it would soon become 'instinctive' if the meeting with the enemy were often repeated, just like the winding-up of a watch, which is often done at a wrong time, for instance, on changing clothes during the day, and thus without reflection. It is quite easy to conceive that if the material brain-adaptation which causes flight without reflection at the sight of man were transmissible, the flight-instinct might become a congenital instinct in the species in question. But this assumption is unfounded; for, as is shown by the case of the sea-cow, we do not require it where the animal is of sufficient intelligence to perform by its own discernment the action necessary to its existence. The action may thus become 'instinctive' through exercise and imitation in the individual life, without however attaining to transmissibility.
But in many cases this is not enough, namely, in all cases in which the degree of intelligence is not sufficiently high, or where the flight movement must follow so rapidly that it would be too late if it had to be regulated by the will, as, for instance, the shutting of the lids when the eye is threatened, or the flight of the fly or the butterfly when an enemy approaches. Both fly and butterfly would be lost in every case if they had voluntarily to set the flight-movement going after they became conscious of danger. If they had first of all to find out from whom danger threatened no individual would escape an early death, and the species would die out. But they possess an instinct which impels them to fly up with lightning speed, and in an opposite direction, whenever they have a visual perception of the rapid approach of any object of whatever nature. For this reason they are difficult to catch. I once watched the play of a cat, ordinarily very clever at catching, as she attempted to seize a peacock-butterfly (Vanessa Io), which settled several times on the ground in front of her. Quietly and slowly she crept within springing distance, but even during the spring the butterfly flew up just before her nose and escaped every time, and the cat gave it up after three attempts.
In this case the beginning of the action cannot lie in a voluntary action, for the insect cannot know what it means to be caught and killed, and the same is true of innumerable still lower animal forms, the hermit-crabs and the Serpulids, which withdraw with lightning speed into their houses, and so forth. It seems to me important theoretically, that the same action can be liberated at one time by the will, at another by the inborn instinct-mechanism. In both cases quite similar association-changes in the nerve-centres must lie at the root of the animal's action, but in the first case these are developed only in the course of the individual life by exercise, while in the second they are inborn. In the former, they are confined to the individual, and must be acquired in each generation by imitation of older individuals (tradition) and by inference from experience, in the latter they are inherited as a stable character of the species.
It has been maintained by many that the origin of instincts through processes of selection is not conceivable, because it is improbable that the appropriate variations in the nervous system, which are necessary for the selective establishment of the relevant brain-mechanism, should occur fortuitously. But this is an objection directed against the principle of selection itself, and one which points, I think, to an incompleteness in it, as it was understood by Darwin and Wallace. The same objection can be made to every adaptation of an organ through natural selection; it is always doubtful whether the useful variations will present themselves, as long as they are due solely to chance, as the discoverers of the selective principle assumed. We shall attempt later to fill up this gap in the theory, but, in the meantime, I should like to point out that the process of selection offers the only possible explanation of the origin of instincts, since their origin through modifications of voluntary actions into instinctive actions, with subsequent transmission of the instinct-mechanism due to exercise in the individual life, has been shown to be untenable.
If any one is still unconvinced of this, I can only refer to the cases we have already discussed of instincts which are only exercised once in a lifetime, since, in these, the only factor that can transform a voluntary action into an instinctive one is absent, namely, the frequent repetition of the action. In this case, if any explanation is to be attempted at all, it can only be through natural selection, and as we have assumed once for all that our world does admit of explanation, we may say, these instincts have arisen through natural selection.
Even though it may be difficult to think out in detail the process of the gradual origin of such an instinctive activity, exercised once in a lifetime, such as that, for instance, which impels the caterpillar to spin its intricate cocoon, which it makes only once, without ever having seen one, and thus without being able to imitate the actions which produce it, we must not push aside the only conceivable solution of the problem on that account, for then we should have to renounce all hope of a scientific interpretation of the phenomenon. We may ask, however, whether there is not something lacking in our present conception of natural selection, and how it comes about that useful variations always crop up and are able to increase.
But if we must explain, through natural selection, such complex series of actions as are necessary to the making of the cocoon of the silkworm or of the Saturnia moth (Saturnia carpini), what reason have we for not referring other instincts also to selection, even if they be repeated several times, or often, in the course of a lifetime? It is illogical to drag in any other factor, if this one, which has been proved to operate, is sufficient for an explanation.
Thus, as far as instincts are concerned, there is no necessity to make the assumption of an inheritance of functional changes, any more than there is in regard to any purely morphological modifications. As the instincts only exercised once show us that even very complicated impulses may arise without any inheritance of habit, that is, without inheritance of functional modification, so there are among purely morphological characters not a few which, though effective, are purely passive, which are of use to the organism only through their existence, and not through any real activity, so that they cannot be referred to exercise, and therefore cannot be due to the transmission of the results of exercise. And, if this be the case, then transformations of the most diverse parts may take place without the inheritance of acquired characters, that is, of functional modifications, and there is no reason for dragging in an unproved mode of inheritance to explain a process which can quite well be explained without it. For if any part whatever can be transformed solely through natural selection, why, since there is general variability of all parts, should this be confined to the passive organs alone, when the active ones are equally variable, and equally important in the struggle for existence?
There are, indeed, many of these passive parts among animals; I need only recall the coloration of animals, the whole set of skeletal parts, so diversely formed, of the Arthropods, the legs, wings, antennæ, spines, hairs, claws, and so on, none of which can be changed by the inherited results of exercise, because they are no longer capable of modification by exercise; they are ready before they are used; they come into use only after they have been hardened by exposure to the air, and are no longer plastic; they are at most capable of being used up or mutilated. Finally, even so convinced an advocate of the Lamarckian principle as Herbert Spencer has stated that among plants the great majority of characters and distinctive features cannot be explained by it, but only through the principle of selection; all the diverse protective arrangements of individual parts, like thorns, bristles, hairs, the felt-hairs of certain leaves, the shells of nuts, the fat and oil in seeds, the varied arrangements for the dispersal of seeds, and so on, all operate by their presence alone, not through any real activity which causes them to vary, and the results of which might be transmitted. An acacia covered all over with thorns seldom requires to use its weapons even once, and if a hungry ruminant does prick itself on the thorns it is only a few of these which are thus 'exercised,' the rest remain untouched.
But since all these parts have originated notwithstanding their passivity, there must be a principle which evokes them in relation to the necessities of the conditions of life, and this can only be natural selection, that is, the self-regulation of variations in reference to utility. And if there is this principle, we require no other to explain what is already explained.
I can quite well understand, however, that many naturalists, and especially palæontologists, find it difficult to accept this conclusion. If we think only of those parts that actively function, and thus change by reason of their function, being strengthened by use and weakened and diminished in size by disuse, and if, further, we follow these parts through the history of whole geological epochs, we may certainly get the impression that the exercise of the parts has directly caused their phyletic evolution. The direction prescribed by utility in the course of the individual life and in the phylogeny is the same, and the intra-selection, that is, the selection of tissues within the individual animal, leads towards the same improvements as the selection of 'persons.' Thus it appears as if the phyletic variations followed those of the individual life, while in reality the reverse is true; the changes arising from variations in the germ are primary, and they determine the course of phylogenesis, while the tissue-selection in the individual life only elaborates and improves, according to the demands made upon it, the material afforded by the primordial equipment of the germ.
The American palæontologist, Osborn, cites the case of the horse's feet as an example in support of his view that modification brought about by use in the individual life must be transmitted in order that the phyletic transformations may be brought about, but this example is perhaps the best that could be chosen to prove the contrary. He supposes that, in every young horse, the means of locomotion are improved at every step, so to speak, through the contact with the ground, and I am quite willing to admit that this is so. But that only proves that, even now, an elaboration and improvement of the equipment which the germ affords is indispensable, as it has been at all times and in all animals, and thus that, notwithstanding the enormous number of generations which our modern horse has behind it, the functional acquirements of the individual have not yet been impressed upon the germ. Why not? Because the horse becomes perfect without this, and there was no reason why personal selection should perfect the primary constituents of the germ still further, since the finishing touch of perfection through use is readily afforded by the conditions of each individual life.
Moreover, when Osborn, Cope, and other palæontologists emphasize that, in phyletic evolutionary series, definite paths of evolutionary change are adhered to, and are not deviated from either to right or to left, they are undoubtedly right, but the conclusion which they draw is not justifiable, whether they assume with Nägeli that there is a power of development, a principle of perfecting, or whether, as Osborn does, they assume the transmission of the modifications brought about through use in the individual life. There remains a third possibility, that the quiet and constant evolution in a definite direction is guided by selection, and as, in passively useful parts, that principle alone is admissible, I see no justification for assuming it to be inoperative in regard to those which are actively functional. All these variations which have led up, for instance, to the modern form of the horse's foot are useful; if they were not, they could not have been produced either by use or by disuse in the individual life.
At the same time, here again, we are justified in inquiring whether the assumption of 'chance' germinal variations, which we have hitherto made with Darwin and Wallace, affords a sufficient basis for selection. Osborn says very neatly in this connexion, 'We see with Weismann and Galton the element of chance; but the dice appear to be loaded, and in the long run turn "sixes" up. Here arises the question: What loads the dice?'
Until recently we might have answered, 'external conditions'; it is they that load the dice one-sidedly, and condition that the same straight path of phylogenesis is adhered to, and exactly the same direction of variations is preferred and maintained. It has to be asked, however, whether this answer, which is certainly not absolutely incorrect, is sufficient by itself, whether the dice are not falsified and one-sidedly loaded in another sense, so that they always throw a preponderating number of the useful variations. We shall attempt very soon to solve this problem, but in the meantime I must refer to another argument in favour of assuming the Lamarckian principle, perhaps the most important and it may be thought the most difficult of all to refute, the so-called co-adaptation of the parts of an organism, that is, the fitting together of many individual organs for a common purposeful functioning.