Popular Science Monthly/Volume 34/April 1889/On the Causes of Variation II
|ON THE CAUSES OF VARIATION.|||
HAVING thus summarily indicated those factors of evolution associated with genesis and which are essentially physiological, however much psychical phenomena may co-operate, we may touch upon the more purely psychical factors or those pertaining to the growth and use of mind, employing the term to express those neural phenomena traceable to the medium of the brain. Their importance in evolution increases with increasing cephalization and complexity of nerve system. For the present purpose, however, it is with the objective side of psychology, or what may be called psycho-physiology, that we must deal.
Psychical—Use and Disuse.—Full consideration of the effect of use and disuse involves a discussion, not only of the question of the transmission of acquired structures, but of the influence of individual effort and of necessity—i. e., a consideration of the essentially Lamarckian factors in evolution. The occasion will not permit me to do full justice to these subjects. That functionally produced modifications are inherited as the great assumption upon which Lamarck founded his theory of evolution. Many able naturalists have insisted on it, and in my judgment there should no longer be any doubt whatever of the fact, not only so far as grosser structure is concerned, but brain-structure likewise. No question is of more moment in the whole range of biology, and especially biologic philosophy, and Spencer has well pointed out that on the answer to it will depend largely the sciences of psychology, ethics, and sociology. Weismann, Lankester, and others deny hereditary power in such modifications, the former believing that hereditary modification can result only from changes in the germ plasma, i. e., are virtually congenital. Natural selection, according to this view, plays upon the germ plasma; but I have never been quite able to comprehend how this view, even if established, militates against the transmissibility of acquired modification; for, whatever theory of heredity we adopt, it shows us rather the manner of the transmission, and therefore confirms its possibility. But the fact of such transmissibility rests neither on embryological nor theoretical grounds. It is a fact so fully demonstrated in the history of our domestic animals and the history of agriculture, that the skepticism of some of our great naturalists and embryologists must be attributed to that ignorance of the farmers' commonest experiences which is, unfortunately, a too frequent attribute of the city-trained investigator. Darwin in the beginning, and while the importance of natural selection was growing in his mind, allowed little importance to use and disuse, for the same reason that he subordinated external agencies; viz., that, in proportion as it acts on masses simultaneously, it must diminish the importance of natural selection. Yet he allowed more weight to it toward the end, and has furnished some of the best evidence drawn from domestic animals of the transmission of acquired characters affecting the dermal, muscular, osseous, and nervous systems. Spencer has shown that inheritance of functional modification is most easily observed and experimentally proved in those parts which admit of easy observation and comparison, as the dermal covering and the bones; and that they for the most part are beyond these tests in the muscular and nervous systems. Yet he logically concludes:
"Considering that unquestionably the modification of structure by function is a vera causa, in so far as concerns the individual; and considering the number of facts which so competent an observer as Mr. Darwin regarded as evidence that transmission of such modifications takes place in particular cases; the hypothesis that such transmission takes place in conformity with a general law, holding of all active structures, should, I think, be regarded as at least a good working hypothesis."
So far as entomology bears evidence, it confirms the fact that modifications of structure due to use or disuse on the part of the individual may be and are transmitted. These are easily observed in the exo-skeleton, and, while the experimental proof is yet limited, it is not wanting, especially in the history of apiculture. Excessive use of any organ will develop or enlarge it at the expense of other organs, just as disuse will cause a diminution or atrophy thereof. The variation in the individual will be within limits, but, when once the variation has set in, the tendency is always to an increased variation in the same direction in the descendants, especially if they continue the same use or disuse. Here, again, however, it is difficult to separate the modification due to individual effort, or want of effort, and the more general modification affecting the mass of individuals of a species through the environment; because the environment affects function, and function in its turn affects form and structure. The life of every individual furnishes an excellent illustration of new action and new uses for organs not previously used, in the striking and sudden employment of post-natal organs, both of respiration and nourishment, which pre-natally had no corresponding action. Romanes has argued that cessation of selection may reduce an organ where use or disuse can have no play, as in the loss of wings in neuter ants; and that by the law of compensation an organ may even be increased, as in the heads of such neuters. He enforces the idea by exampling the blind crabs of our Kentucky caves, where the complex eyes rapidly disappear under cessation of selection, but where the persistence of the foot-stalks indicates that economy of nutrition could have had little play! It is difficult, however, to draw the line between this cause and Lankester's reversal of natural selection; and still more difficult to say wherein either differs from mere disuse.
Degeneration, which has been urged as the true explanation of many of the existing forms of life, is, it seems to me, but a consequence of disuse, and would, therefore, fall into the present category, among causes of variation.
Emotion as affecting the Individual.—I have here considered the factor of use and disuse as a direct cause of variation, from the psychical rather than the physical standpoint—i.e., individual or conscious effort as furnishing food for natural selection, among more highly endowed animals, rather than as effort by species as a whole necessitated by physical conditions, and inducing modification in masses irrespective of selection. This leads us to the consideration of mind as a factor in evolution, and we shall soon see its importance as a fundamental cause of differentiation, among higher organisms at least. I am not sure, even, that its influence can be excluded from among lower animals, however much we may have to exclude its action in so far as plants are concerned; for any new functional effort inducing new use may be looked upon as conscious and intelligent as compared with use fixed by habit and lapsed into automatic action or instinct. The former typifies variability and progress; the latter constancy and stability.
Mind is a comprehensive cause of variation, and may be considered under several categories. We have, for instance, (1) the action of the mind of the individual in willing, or in selecting between differing alternatives that present themselves, as in the choice of means to ends; (2) the direct influence of the emotions on the individual; and (3) the influence of the emotions of the mother on her unborn offspring.
In the first category, the influence of mind in modifying is chiefly confined to man. It must have acted from the time when he first began to prepare his crude weapons of defense and offense to the present day, when some new discovery or some new invention may alter the map of the world, revolutionize society, or give one race or nation the advantage over another; nor can we feel sure that animals below man have not been modified by similar psychical effort. In the second category, the direct influence of the emotions on the individual, it is a psycho-physiological factor involved in the question of use and disuse; for if it be once admitted (and I think the tendency of modern neural science is in the direction of establishing the fact) that strong mental effort may be made to affect special parts of the body—i.e., that an excess of nervous force brought to play on any particular organ, or any particular part of the organism, induces increased growth or development of such parts; we can understand how far desire, especially under the spur of necessity, may be influential in inducing modification. Lamarck's idea, therefore, may not be so ridiculous as it has hitherto been supposed by many. Darwin took no stock in this influence, and referred with some contempt to the views of Lamarck and Geoffroy Saint-Hilaire. He thought it strange that the author of "Les Animaux sans Vertèbres" should have written that insects which never saw their eggs should will them to be of particular form, which he thought hardly less absurd than to believe that the desire to climb should make a pediculus formed to climb hair, or a woodpecker to climb trees.
Emotion of Mother as affecting Offspring.—There may be some doubt about the extent of the influence of the individual mind in inducing direct modification, for the subject is a difficult one to deal with, and we have few exact data to draw from. Since in human affairs we recognize the power of will in affecting purpose and action, and in molding character, it is legitimate to infer that when our knowledge has increased we shall recognize its effect on function. There can be less doubt as to the third category, viz., influence of the mind or emotions of the mother on her unborn offspring in inducing modification both physiological and mental. As a cause of variation, though believed in by J. D. Hooker, as we learn from the "Life and Letters," and by other of Darwin's contemporaries, it was discarded by Darwin himself, his principal reasons being that the results of observations made for him in hospitals were adverse to any such influence. Medical men, as a rule, also discard it as among the mere notions and superstitions of women, and argue its impossibility on the ground that there is no neural connection between mother and fœtus. The ancients practically recognized the influence of the imagination of the mother on her offspring, and belief in it is still very prevalent among women themselves, of all classes. Women alone are able to speak or feel in this matter from experience, and the almost universal belief in the influence, among those who have any experience at all, should make us hesitate to discard it too summarily. From facts within my own personal knowledge I have long believed in this influence, and the more I have been able to collect reliable data bearing upon it, the more confirmed have I become in the conclusion that the emotional experiences of the mother affect the issue in varying degree, according to the intensity of the emotion. When sudden and excessive, as in rage, fright, repugnance, etc., or where prolonged or accumulative, as in continued brooding, it may induce nervous disorders, and even mental aberration, idiocy, or insanity; or, again, physiological change, as atrophy or increase of parts, and other peculiarities which have relation to the form or character of the inducing mental manifestation or shock in the parent. Investigation of this, as of all subtle phenomena, is attended with the difficulty of separating the chaff of fancy from the grain of reality. The method pursued by Darwin is unsatisfactory, as it dealt with normal conditions which furnish no evidence and with the fanciful or notional side of the subject. The literature of the subject is extensive and quite interesting, and I would refer particularly to the work and writings of Viellard, Schönfeld, Demangeon, Lucas, Féré, and Brown-Séquard. Two other difficulties confront the investigator: first, the somewhat unsatisfactory state of neurology and the difficulty of experimental research therein, as indicated by Vice-President Bowditch before this section two years ago; secondly, the aversion, from feelings of delicacy, on the part of the persons concerned, to publicity of the more marked and striking evidence. The phenomena of hypnotism, proving as they do that physiological results may be induced through the imagination of the subject acted on by the mind of the hypnotizer, are suggestive in this connection, the work of Charcot in Paris more particularly showing how powerful the action may be, and how the effects of actual medicines may be produced by the use of imagined ones. The mind of the hypnotized under these conditions is brought into those exceptional and exalted conditions which are necessary in the case of the mother to produce on her offspring the effect which we are discussing. The recent experiments of Mr. C. T. Hodge on the effects of stimulation on the nucleus and cell-body and on protoplasm are also interesting here, showing, as they do, decrease in the two former and vacuolation of the latter as the result.
The history of science is present to tell us that common and persistent belief, based on experience, has not infrequently been met with skepticism and even ridicule on the part of scientific men, only to be vindicated finally by more thorough and exact knowledge. It is too often the case that, where the processes are recondite and difficult to follow, assumption passes for knowledge. The function of some of our own bodily organs yet remains to be established, and we probably assume too much in requiring that all nervous force must be transferred through nerve tissue, or that there may not be protoplasmic filaments which are not resolvable, in their finer ramifications, even with our best microscopes. The very nature of mind and its processes puts it beyond the reach of the scalpel of the anatomist or the physiologist, just as many psychical phenomena baffle the exact methods of science, at least those so far employed. Leaving out of the question the evidence of peculiar marks due to maternal emotion, cases of which are part of the unwritten history of almost every family, the striking cases of which I have authoritative evidence of addition to, subtraction from, or singular modification of, anatomical parts, confirm me in the belief that this is a most important psycho-physiological cause of modification.
In the romance of "Elsie Venner," in which the heroine's strange attributes are connected with pre-natal influence of the mother, who died of the bite of Crotalus, Oliver Wendell Holmes has strongly put forth this doctrine in the form of fiction. I allude to this clever romance because of the medical knowledge of the eminent author, and because, while admitting in the preface that a grave scientific doctrine lies beneath some of the delineations of character, he also affirms that he has had the most startling confirmation of its truth. The data collected on the subject I hope to bring together on some other more fit occasion; and I would take this opportunity of urging any in my hearing, or who may read these lines, if they have had or are aware of any authoritative and illustrative cases, to communicate them to me with as much detail as possible.
This theory once established, its bearing on evolution as a prime cause of variation must at once be manifest; for it gives not only tangibility to the Lamarckian idea of desire influencing modification, but also a conception of how Infinite Mind in nature may act through the finite in directing such modification. No doubt but that there is a great deal of nonsense and superstition mixed with the genuine, and that the idea that every little whim, or fancy, or imagining of the mother will produce record or mark is one of the unjustified outcroppings of the fundamental fact, and helps to explain the difficulty of getting at the real facts and the ease with which Darwin rejected the idea. In my judgment, this factor acts only when, from whatever cause, and particularly under the spur of necessity, the emotions are exceptionally intensified, or the desire strongly centered in some particular object. The conception is perfectly legitimate, for instance, that when a species is subjected to any external modifying cause, affecting all its members alike, the adaptive modifications which natural selection, under such circumstances, would play upon, have their origin in the emotions, or the influences at work on the pregnant females, giving direction in their offspring, to the needed changes. In this way it is probable that only those individuals born under such conditions would be able to survive. Thus this becomes no mere ancillary cause of variation, but one of deepest import and at the very foundation of evolution. The female in this light acquires an increased importance, and evolution finds her not only the essential at the dawn of life upon our planet, but, in its present highest manifestations, she is nearest by instinct, intuition, and aspiration to the Controlling Mind which in the beginning quickened the great womb of Nature and down through all the ages guided the continuous stream of life to designed ends through the womb of the individual mother.
As already remarked, the psychical factors which we have been considering are substantially Lamarckian, and in proportion as we consider them and get to understand the other direct causes of variation, must we give importance to the ideas of Lamarck and, conversely, less importance to the ideas of Darwin.
Did time permit, I should like to go into an analysis of Lamarck's "Philosophie zoologique," and show how the genius of this illustrious French naturalist anticipated a very large part of that which Darwin subsequently so laboriously helped to establish. I must pass the subject, however, and simply record my surprise that one who was otherwise so honest and fair toward other writers, was so evidently unfair in his estimate of the work of Lamarck, as Darwin, in the "Life and Letters," is shown to have been. It is incomprehensible, reading Lamarck with our present knowledge, that Darwin should have found neither fact nor ideas in a book which abounds in both, except on the theory of a poor translation or that strange national antipathy which has so often prevented the people of one country from doing justice to those of the other and which so long prejudiced the French Academy against Darwin's own especial theories.
Darwinism assumes essential ignorance of the causes of variation, and is based on the inherent tendency thereto in the offspring. Lamarckism, on the contrary, recognizes in use and disuse, desire and the physical environment, immediate causes of variation affecting the individual and transmitted to the offspring, in which it may be intensified again both by inheritance and further individual modification. Both represent important principles in evolution and co-operate to bring about the results. The theory I propose gives renewed importance to the Lamarckian factors by showing one manner of their action not previously urged, and it also helps us to a tangible and scientific conception of design.
Acceleration and Retardation.—In this rapid glance at the immediate causes of variation we have discussed some factors which in some degree represent laws rather than inducing causes of variation. This difficulty appertains to all attempts at formulation of the causes of variation, and only as our actual knowledge increases shall we be able succinctly and definitely to classify the factors. There are, however, certain important laws which have influenced modification but in no sense can be looked upon as causes of variation. They are laws or principles of evolution, by which we may account for the formation of types, acting, just as natural selection does, in differentiating rather than in originating the variation. No one can have followed the important and suggestive works of Cope and Hyatt on the subject of acceleration and retardation and not feel that it expresses an important law of this kind. It is, as I understand it, a factor in evolution not comparable with the principle of natural selection, but complementary thereto, much in the same way as physiological selection and sexual selection are. It is an attempt to give expression and form to a set of facts to which palæontology undoubtedly points and which ontogeny substantiates, viz., that certain types may attain perfection in time and then retrogress and finally become extinct, and that existing types which are dying out, or degenerating, exhibit, ontogenically, the culmination of force and complexity, followed by decadence, corresponding to the phylogenic history of the type. We know from the "Life and Letters" that Darwin gave up in despair the attempt to grasp the full meaning of these particular views of our associates, and, in a letter to Hyatt, with characteristic modesty, he attributes this inability to his own dullness rather than to any weakness in the theory. Others have experienced the same difficulty, and believe, with Prof. Morse, that the facts enumerated, as well as the facts of exact and inexact parallelism, are explicable on the doctrine of natural selection. This is true, it seems to me, only on the broader, unjustified interpretation of the doctrine to which I have previously alluded in the opening of these remarks. The law of acceleration and retardation may, perhaps be substantially stated in this wise: that certain groups acquire some characters rapidly, while corresponding groups acquire the same characters more slowly, or never acquire them at all; and this brings us to another important factor of evolution which serves to give force to the law.
Acceleration by Primogeniture.—This has been elaborated by Hubrecht. He argues that so long as the parent form remained most in harmony with the surrounding conditions, it would maintain in the struggle for existence its characteristics against all tendency to vary in its offspring, which is equivalent to saying that it will remain unchanged so long as the environment remains the same. He then shows that in organisms in which the reproductive period covers many years, accelerated development by primogeniture, i.e., as between the first born and the last born of any pair and of their posterity, will, in time, produce differentiation. The series of the first born will, in the course of time, involve many generations at short distances from each other, whereas the series of the last born will, on the contrary, consist of a much smaller number of terms, each separated from its predecessor by a more considerable distance. Any tendency to variation from external or internal influences must needs find more numerous occasions to act in the series of the first born, not only because these have a more composite ancestry, but because they necessarily become the most numerous. In other words, the chances are more numerous for small differences among the first-born series, and, in proportion as such differences are accumulated, intercrossing with the series of the last born will become rarer. This law will gain from physiological selection, and, it seems to me, throws additional light on that of acceleration and retardation. It must act more particularly among higher animals, where the reproductive period is lengthened, and the time between the first and last born is great.
Saltation.—We are thus led to what have been called saltations in evolution. Although the history of palæontology has continually added to our knowledge of past forms, and helped to fill up many gaps in the evolutional series, and although during the last quarter of a century it has particularly vindicated Darwin's prophecy that many links would yet be found, the substantial truth remains that gaps still occur, and that progress, so far as present knowledge indicates, has been made by occasional saltations. There have been, it would seem, periods of rapid movement, and of comparative repose or readjustment of equilibrium. Cope concludes that "genera and higher categories have appeared in geologic history by more or less abrupt transitions or expression-points, rather than by uniform gradual successions."
One of Pictet's strongest points in opposition to Darwin's theory, which struck Darwin himself with much force, was that it ill agreed with the history of organisms with well marked and defined forms, which seem to have existed during but a limited period, as, for instance, the flying reptiles, the ichthyosaurus, belemnites, ammonites, etc. Some authors, who have fully recognized these gaps or leaps in the developmental history of animals, yet believe them to be consistent with the theory of gradual modification. It may be only one individual of many which, becomes modified and transmits the modification to descendants; it may be but one species of a genus which, for similar reasons, supersedes the rest which become extinct in time proportioned to prolificacy.
There is no reason to suppose that the history of organic life has differed in this respect from that of inorganic. We need not discuss here the question of catastrophism and uniformitarianism in geology. However much the latter prevails at the present time, both have doubtless operated in the past. Catastrophism would necessarily produce gaps, or saltations, in the palæontological record, as only the more plastic species would adapt themselves and survive under its influence. It is not gaps due to such causes that are here to be considered, however, but those which occur in uniform strata. Haldeman has most suggestively remarked that the same mineral will crystallize with three, six, or twelve angles, but not with five or seven, and he asks. Are the facts of organic morphism subject to less definite laws? Cope has drawn another illustration from inorganic forces, in the three great changes in water, from solid, liquid, and vapor, which take place suddenly at what may be called two expression-points of the thermometer, the many intervening degrees involving no change. Rhythm or wave movement would seem to be a universal attribute of matter, whether organic or inorganic. The forces of nature are constant, but the phenomena induced are often paroxysmal. The progressive forces accumulate, while the conservative forces resist, until at last resistance gives way with comparative suddenness. There is every reason to believe that the life-movement, in its ascending complexity, has shared this common law. Accumulation is proportioned to the change in environment, and resistance to the age or rigidity of the organism. The latter may be strong enough to end in death or extinction, or it may break down, and, with comparatively sudden yielding and conformity to necessity, burst the confines and begin a new series of variations and adaptations. In either case we have breaks, because the dying or dropping out of one type makes room for another more accommodating. Rapid evolution, from causes already discussed, implies gaps which must be marked according as the strength of the conservative forces and the violence of the final accommodation are great, and because certain breaks are more apt to occur after long periods of stability. The break may be induced by changes in physical environment or without such change; if the latter, it will more likely occur in some individual born with a marked departure from the type that gives it some advantage, and whose issue will in time supplant all other individuals. In either case we shall have, palæontologically, distinct species or genera, one superposed on the other, without links. To the imperfection of the geologic record is to be attributed, no doubt, a large number of these gaps yet existing between types, and many important links or branches are yet to be discovered. Yet the views we have been considering should absolve evolutionists from all necessity of demonstrating the more minute gradations; because, in deposits like the Tertiary, during which we may assume life-conditions to have remained comparatively uniform, these saltations take place. Saltation, or, what is probably a truer expression, wave-movement, would indeed seem to be a prerequisite of progress, and will account for much that is going on even at the present day. In artificial selection by man we find that it is at first comparatively easy to accumulate minute peculiarities and variations by rigid breeding and exclusion of all deviation; but that we soon arrive at a fixed point which is maintained at first with difficulty but with increasing ease with each generation. During these more fixed periods the potentiality for change is doubtless increasing, until at last it is suddenly manifested in renewed variation. Rest is followed by activity just as surely as activity induces and requires rest.
There is a limit to development in organs, just as there is a limit to individual mental growth. Weariness of previous effort comes upon us when the limit of result is attained, accompanied by great longing for change, and not infrequently with revulsion from previous effort. The naturalist who has devoted a part of his life to the persistent accumulation of facts and specimens, has held the imaginative and generalizing powers in abeyance during that period. The reserve brain-force in this direction may be suddenly called into activity by exhaustion in the other, and the process may perhaps be comparable to the exhaustion of the soil for one particular crop, without lessening its fertility for some other, the recognition of which fact is the foundation of all successful agriculture. Excess of development, whether in body or mind, inevitably brings about either wholesome reaction or utter collapse.
How far the rhythmic tendency in the development of animal life may be explained by the rapid change of climate, by migration and the loss of record, or upon the general law that while there has been progress of the whole, there has not necessarily been progress of every part, it would take us too far to discuss in this connection. I think we are safe in saying, however, that the facts justify the belief that, in the evolution of animal life, as in the evolution of everything else, progress has often been made by waves.
Fiske's Law.—With regard to what may be called Fiske's law of correlation between brain development and infantile dependency, Fiske has so admirably elaborated the subject that it needs no further elucidation here as the principal factor in the evolution in man, first of the family relation, then of the clan, the tribe, and the nation. With this factor in mind, and the immense superiority which anthropoid man must have had, when brain development had once induced this fundamental community of interest over the rest of brute creation, the gap between primitive man and the higher anthropoid apes in the past, or between the present lower races of man and the higher existing primates, is easily explained, even if it had not been greatly exaggerated. At the present time we may note and record the further inevitable increase in the gap, for the lower races of man are gradually becoming extinct, and the higher apes can not long hold their own or persist.
Brooks's Hypothesis.—I have already alluded to Brooks's hypothesis under the head of sexual differentiation, and his work on heredity must be so familiar to you that his views need but a passing notice. He believes that sex differentiation means fundamentally a physiological division of labor, and that the male is essentially the progressive or diversifying and the female the conservative agent. As organisms gradually increased in size, as the number of cells in their bodies became greater, and as the differentiation and specialization of these cells became more and more marked, one element, the male cell, became adapted for storing up gemmules, and, at the same time, gradually lost its unnecessary and useless power to transmit hereditary characteristics.
The theory finds support in some of the phenomena of life, and doubtless expresses a law not easily established, for which reason it will not be readily accepted. It leaves entirely out of consideration some of the forces at work which I have already indicated, and in so far must be considered only a law of secondary importance. However much we may admit the general truth that the germ-cell continues the past and the sperm-cell tends to diverge from it, as a purely dynamic proposition, inducing variation for natural selection to play upon, it does not in any way decrease the overwhelming importance of the female in inducing, through psychico-physiological influences, a needed and purposeful modification in the manner which I have already expounded.
- From the address of the Vice-President of Section F of the American Association for the Advancement of Science, delivered at the Cleveland meeting, August, 1888.