AS REGARDS HEREDITY
Problems of Heredity—Physiological Units—A Digression—The Germ-Cells—Transmission of Acquired Characters—Inconceivability—A Priori Argument—Practical Conclusion
Heredity is the relation of genetic continuity which links generation to generation. An inheritance is all that the organism is or has to start with on its life-journey in virtue of the hereditary relation to parents and ancestors. In all ordinary cases, the inheritance has its initial material basis in the egg-cell and the sperm-cell which unite in fertilisation at the beginning of a new life, and these two kinds of germ-cells, which bear the maternal and the paternal contributions, have their peculiar virtue of reproducing like from like, just because they are the unchanged or very slightly changed cell-descendants of the fertilised ova from which the parents arose. A bud or a cutting separated off from a living creature—tiger-lily or potato, polyp or worm—reproduces an entire organism like the parent, if the appropriate nurture-conditions are available; and it can do so because it is a fair sample of the parental organisation. Similarly a germ-cell or two germ-cells in conjunction can develop into a creature like the parent or parents, in virtue of being the condensed essence of the parental organisation. And the germ-cell is this because of its direct continuity through undifferentiating cell-divisions with the original germ-cell from which the parental body developed.
Even in ancient times men pondered over the resemblances and differences between children and their parents—for like only tends to beget like—and wondered as to the nature of the bond which links generation to generation. But although the problems are old, the precise study of them is altogether modern. The first great step towards clearness was the formulation of the cell-theory by Schwann and Schleiden (1838-9), by Goodsir and Virchow, which made it clear that all but the simplest organisms are built up of cells or modifications of cells, and that the individual life usually begins as a fertilised egg-cell which proceeds by division and re-division, by differentiation and integration, to develop a more or less complex "body." It has become gradually clear that while the fertilised egg-cell gives rise to body-cells which become specialised, it also gives rise to unspecialised descendant-cells, which take no share in body-making, but become the germ-cells—the potential starting-points of another generation. A second great step was the accumulation of facts of inheritance showing that all sorts of qualities innate or inborn in the parents, essential and trivial, normal and abnormal, bodily and mental, may be transmitted to the offspring as part of the organic heritage. A third great step was implied in the acceptance which Darwin in particular won for the general idea of descent, for it is hardly too much to say that the scientific study of the problems of heredity began when it was recognised that heredity is a fundamental condition of evolution.
Problems of Heredity.—In regard to Heredity there are three large problems which tower above the crowd of more detailed problems. The first is: In what way are the germ-cells peculiar, how do they differ from ordinary cells, what gives them their unique reproductive power, how do they come to be such marvellous units that their development results in a new organism? Only two answers have been suggested: (1) that the germ-cells become receptacles of representative samples from the different parts of the body (the pangenetic theory), and (2) that the germ-cells owe their unique character to the fact that they are, along lines of undifferentiated cell-lineage, the direct descendants of the fertilised ova of the parents (the theory of germinal continuity). Thanks, largely, to Weismann, the second view has prevailed over the first, for which there is little factual basis.
The second large problem is as to the way in which it may be supposed that the hereditary qualities are represented in the germ-cell. Is the germ-cell an extremely complex chemical mixture without pre-formed architecture, which, as it lives and grows, gradually gives rise to heterogeneous elements, differentiating along diverse lines according to their diverse relations to one another and to their surrounding conditions? Or is it from the first a complex architecture, an intricate organisation of a large number of items representing particular qualities, a mosaic of inheritance-bearers?
The third large problem is as to the modes in which the inheritance, normally bi-parental, and in some sense always a mingling of ancestral contributions, can express itself. Sometimes the expression is one-sided, sometimes it is a blend. The mother may look out of one eye, and the father out of another, or the grandfather may be re-incarnated. By inter-breeding hybrids pure types may be got, or reversions, or "an epidemic of variations." This is the problem of the diverse modes of hereditary transmission, which we know in some cases to be expressible in a formula, such as Mendel's law or Galton's law, and for which we can sometimes hazard a hypothetical physiological interpretation.
Physiological Units.—To each of these three problems Spencer made a contribution. He started with the legitimate and fertile hypothesis of "physiological units"—the ultimate life-bearing elements, intermediate between the chemical molecules and the cell. Just as the same kinds and even the same number of atoms compose by different arrangements numerous quite different chemical molecules, e.g. in the protein-group, so out of similar molecules diversely grouped an immense variety of "physiological units" may be evolved. Out of the same pieces of coloured glass one may get in the kaleidoscope a very large number of distinct patterns, so in the course of nature similar molecules, grouping themselves differently, have formed a very large number of distinct "physiological units." The grouping is not merely positional and static as in the kaleidoscope; it is dynamic and vital. Since Spencer sketched his idea in 1864 many biologists have thought of units intermediate between the chemical molecules and the cell, and the number of different names which have been bestowed upon them is extraordinary, each voyager re-naming his discovery, ignorant of or ignoring those who had previously sailed the same seas. This recognition of "physiological units" was a natural step in analysis as soon as it began to be recognised that the cell was a little world in itself, a "firm" with many partners. While we cannot agree with Delage that "Spencer est le vrai père de la conception initiale," since Brücke expressed the same idea in 1861, Spencer's exposition in 1864 was quite independent, and it has not found the recognition it deserved.
It should be noted that the "gemmules" which Darwin assumed in his provisional hypothesis of pangenesis to be given off by the various cells of the body, were supposed to be of innumerable unlike kinds, whereas in Spencer's argument "the implication everywhere is that the physiological units are all of one kind."
It is admitted that the molecules of a crystallisable substance have more or less mysterious relations to one another—"polarities" as we call them—which result in definite crystalline forms appearing in definite conditions, with a certain amount of diversity as everyone may see in snow-crystals, and as is more precisely known in the case of certain substances which have several forms of crystallisation. But just as chemical molecules have in virtue of their organisation (always dynamic as well as static) certain prescribed modes of relating themselves to others like themselves, and building up a beautiful integrate, a crystal, so, as Spencer pointed out, the "physiological units" have their "polarities," i.e. their inherent constitutional tendencies to build up forms along with their fellows. Here we have two useful suggestions, (1) that development is like an elaborate organic crystallisation, only much more energetically dynamic, and (2) that the big fact of heredity—that like tends to beget like—has its parallel in the way in which a minute fragment of a crystal can in the appropriate environment of a solution of the same substance build up a crystal like the original form from which it was separated. Germ-cells are potential samples of the organisation which is expressed in the parent, but Spencer did not advance to the more distinctively modern position which recognises that they are separated off rather from the fertilised ovum which gave rise to the parent's body than from that body itself. The parental body is the trustee rather than the producer of the germ-cells.
A Digression.—Here we must digress a little to compare Spencer's conception of physiological or constitutional units with Weismann's conception of the Germ-Plasm. According to Weismann, there is in the nuclei of the germ-cells a distinctive physical basis of inheritance, the germ-plasm. It is the vehicle of the hereditary qualities, the architectural substance which enables the germ-cell to build up an organism; it has an extremely complex and at the same time persistent structure. Following a hypothesis of De Vries, he supposed that the readily stainable nuclear bodies (the chromosomes or idants) consist of a colony of invisible self-propagating vital units or biophors, each of which has the power of expressing in development some particular quality. He supposed that these biophors are aggregated into units of a higher order, known as determinants, one for each structure of the body which is capable of independent variation. These determinants are supposed to be grouped together in ids, each of which is supposed to possess a complete complement of the specific characters of the organism and also to have an individual character. The ids are arranged in linear series to form the visible idants or chromosomes, which will be slightly different from one another according to the individualities of the component ids. When the fertilised egg-cell develops, it gives rise (1) to somatic cells which carry with them part of the germ-plasm, and differentiate to form the body, and (2) to the germ cells which reserve part of the germ-plasm in an unchanged state, and eventually give rise in appropriate conditions to new individuals and their germ-cells.
Spencer refused to accept the contrast between body-cells and germ-cells as expressing a fact, and referred for his reasons to the numerous cases in which small pieces of a plant or polyp may grow into an entire organism. But when he represented Weismann as maintaining that the "soma contains in its components none of those latent powers possessed by those of the germ-plasm," he did not do justice to the comprehensive theory of the "Germ-plasm." For Weismann assumes that in certain cases the body-cells, even though differentiated, may carry with them some residual unused-up germ-plasm.
When a lizard regrows a lost tail—effectively responding to a casualty which has been common for untold generations—Weismann interprets the mechanism of this as due to a reserve of tail-determinants resident at or near the place of breakage, and localised there as the result of a long-continued process of selection. A chamæleon does not regenerate its tail, and this may be interpreted in terms of the selection-theory, since the chamæleon with its tail coiled up or embracing a branch has not been, in the course of its evolution, subjected to the frequently recurrent casualty which has beset most other lizards. Spencer said, "We cannot arbitrarily assume that wherever a missing organ has to be reproduced there exists the needful supply of determinants representing that organ," but Weismann made no such arbitrary assumption. Many organs are lost which are not regenerated, even when, as far as materials or differentiation are concerned, it would be easy to replace them. Why the everywhere present uniform physiological units that Spencer believed in should not replace them, we do not know; but if the distribution of regenerative determinants has been wrought out by selection, we understand the facts.
Spencer said that the hypothesis of a supply of determinants lying latent at or near the seat of injury, and able to reproduce the missing part in all its details, and to do this several times over, was "a strong supposition." We venture to think that the hypothesis that the same result is achieved by the "physiological units," which are all of the same kind, is a weak supposition. Spencer said: "Reproduction of the lost part would seem to be a normal result of the proclivity towards the form of the entire organism." But it is difficult to see why "proclivity of the physiological units towards the form of the entire organism" should bring about the regeneration of a tail here and a head there, a claw here and an eye there. But Spencer was too acute a thinker not to feel that if the theory of regenerative determinants was "incompetent," his own theory, which interpreted regeneration as due to the activity of physiological units, "with a proclivity towards the organic form of the species," did not cover the facts; e.g. the establishment of "false-joints," where the ends of a broken bone failing to unite remain movable one upon the other. Therefore he suggested a qualification of his hypothesis.
In "the social organism," it is often seen that the components of an aggregate "have their activities and arrangements mainly settled by local conditions." "A local group of units, determined by circumstances towards a certain structure, coerces its individual units into that structure." In an emigrant settlement, "individuals are led into occupations and official posts, often quite new to them, by the wants of those around—are now influenced and now coerced into social arrangements which, as shown perhaps by gambling saloons, by shootings at sight, and by lynchings, are scarcely at all affected by the central government. Now the physiological units in each species appear to have a similar combination of capacities. Besides their general proclivity towards specific organisation, they show us abilities to organise themselves locally; and these abilities are in some cases displayed in defiance of the general control, as in the supernumerary finger or the false joint. Apparently each physiological unit, while having in a manner the whole organism as the structure which, along with the rest, it tends to form, has also an aptitude to take part in forming any local structure, and to assume its place in that structure under the influence of adjacent physiological units" (Principles of Biology, revised edition, i. p. 364).
The experiments of Born and others have shown that fragments of a young tadpole may go on developing to some extent after they are cut off, and that the undifferentiated rudiment of a limb may be successfully grafted on to another tadpole. "In brief, we may say that each part is in chief measure autogenous." "Though all parts are composed of physiological units of the same nature, yet everywhere, in virtue of local conditions and the influence of its neighbours, each unit joins in forming the particular structure appropriate to its place." This conclusion is very interesting when compared with that reached more inductively by many embryologists (of the so-called epigenetic school), namely, that what a blastomere or cleavage-cell of an egg does, is determined by its intra-embryonic environment, by its relations, both statical and dynamical, to the whole organisation of which it forms a part. As Driesch puts it: "The relative position of a blastomere in the whole determines in general what develops from it; if its position be changed, it gives rise to something different; in other words, its prospective value is a function of its position." But those who assume heterogeneous determinants do not thereby exclude what truth there may be in this view that what an early blastomere does is a function of its inter-relations.
But let us consider how much Spencer puts to the credit of his "constitutional units." (1) They carry within them the traits of the species and even some of the traits of the ancestors of the species, the traits of the parents and even some of the traits of their immediate ancestors, and the congenital idiosyncrasies of the individual itself. In this they resemble the germ-plasm. (2) They "must be at once in some respects fixed and in other respects plastic; while their fundamental traits, expressing the structure of the type, must be unchangeable, their superficial traits must admit of modification without much difficulty; and the modified traits, expressing variations in the parents and immediate ancestors, though unstable, must be considered as capable of becoming stable in course of time." Again they resemble the germ-plasm. (3) Once more, "we have to think of these physiological units (or constitutional units as I would now re-name them) as having such natures that while a minute modification, representing some small change of local structure, is inoperative on the proclivities of the units throughout the rest of the system, it becomes operative in the units which fall into the locality where that change occurs." Here they part company from the germ-plasm, except in so far as it may be said that the development of the distributed determinants is in part dependent on local conditions. (4) Finally, since Spencer supposed "an unceasing circulation of protoplasm throughout an organism," such that "in the course of days, weeks, months, years, each portion of protoplasm visits every part of the body"—a wild assumption—"we must conceive that the complex forces of which each constitutional unit is the centre, and by which it acts on other units while it is acted on by them, tend continually to re-mould each unit into congruity with the structures around: superposing on it modifications answering to the modifications which have arisen in these structures. Whence is to be drawn the corollary that in the course of time all the circulating units—physiological, or constitutional if we prefer so to call them—visit all parts of the organism; are severally bearers of traits expressing local modifications; and that those units which are eventually gathered into sperm-cells and germ-cells also bear these superposed traits."
This theory—which is not unlike a combination of Darwin's pangenesis with De Vries's neo-pangenesis—is very significant, for it discloses Spencer's hypothesis as to the modus operandi of the transmission of acquired characters. But there is unfortunately no factual warrant for the assumption that the constitutional units visit one another in various corners of the body, getting impressions as they go, or for the assumption that they are eventually gathered into the germ-cells—an assumption which shows how far Spencer deliberately stood from the conception of the continuity of the germ-plasm. Even if we suppose an organism to undergo numerous modifications in different parts of its body, as a plant may do when it is transferred from the Alps to the lowlands; even if we suppose the constitutional units—which are all of one kind—to circulate and become bearers of the traits expressing local modifications, we have to face other questions: do they all become remoulded in relation to all the modifications? or do some become remoulded in relation to one modification and some in relation to another? or do all the modifications so hang together that one kind of alteration impressed upon the constitutional units covers them all? The difficulties of the conception of constitutional-units certainly do not seem less than the difficulties of the conception of specific determinants.
Even to the general reader, who is not concerned with the problem of the mechanism of inheritance and development, who has a shrewd suspicion that it is one of those things no fellow can understand, our digression should be interesting, for it illustrates Spencer's fertility of invention and his adroitness in lugging in one hypothesis after another to eke out a theory which in its first statement appears to be very simple. It is instructive to observe how the constitutional units at first so harmlessly simple, grow under the conjurer's hands until they become more marvellous than Clerk Maxwell's "sorting demons."
But it is more instructive still to hear the conclusion of the whole matter. "At last then we are obliged to admit that the actual organising process transcends conception. It is not enough to say that we cannot know it; we must say that we cannot even conceive it. And this is just the conclusion which might have been drawn before contemplating the facts. For if, as we saw in the chapter on "the Dynamic Element in Life," it is impossible for us to understand the nature of this element—if even the ordinary manifestations of it which a living body yields from moment to moment are at bottom incomprehensible; then still more incomprehensible must be that astonishing manifestation of it which we have in the initiation and unfolding of a new organism." "Thus all we can do is to find some way of symbolising the process so as to enable us most conveniently to generalise its phenomena; and the only reason for adopting the hypothesis is that it best serves this purpose."
But the hypothesis only serves the purpose because the constitutional units are gradually invested with the powers of effective response, co-ordination, and the like which remain the secret of the organism as a whole—the secret of life, which many think will never be read until we recognise that it is also the secret of mind.
The Germ-Cells.—According to Spencer, "sperm-cells and germ-cells are essentially nothing more than vehicles in which are contained small groups of the physiological units in a fit state for obeying their proclivity towards the structural arrangement of the species they belong to," and "if the likeness of offspring to parents is thus determined, it becomes manifest, a priori, that besides the transmission of generic and specific peculiarities, there will be a transmission of those individual peculiarities which, arising without assignable causes, are classed as spontaneous." Not only are the main characters transmitted, the same may be true of even minute details—varietal characters, like the taillessness of Manx cats, and individual congenital peculiarities such as a sixth finger; normal qualities such as swiftness in race-horses, abnormal qualities such as nervousness in man. Here Spencer was of course at one with all biologists.
Transmission of Acquired Characters.—He went on, however, to try to substantiate the proposition, which has been the subject of so much discussion, that modifications or acquired bodily characters are also transmissible, and we must follow his argument carefully.
He first points out that when a structure is altered by a change of function the modification is often unobtrusive, and its transmission consequently difficult to detect. "Moreover, such specialities of structure as are due to specialities of function, are usually entangled with specialities which are, or may be, due to selection, natural or artificial. In most cases it is impossible to say that a structural peculiarity which seems to have arisen in offspring from a functional peculiarity in a parent, is wholly independent of some congenital peculiarity of structure in the parent, whence this functional peculiarity arose. We are restricted to cases with which natural or artificial selection can have had nothing to do, and such cases are difficult to find. Some, however, may be noted."
When a plant is transferred from one soil to another it undergoes "a change of habit"; its leaves may become hairy, its stem woody, its branches drooping. "These are modifications of structure consequent on modifications of function that have been produced by modifications in the actions of external forces. And as these modifications reappear in succeeding generations, we have, in them, examples of functionally-established variations that are hereditarily transmitted." But this is a non sequitur, since the modifications may reappear merely because they are re-impressed directly on each successive generation.
Spencer notes that in the domestic duck the bones of the wing weigh less and the bones of the leg more in proportion to the whole skeleton than do the same bones in the wild duck; that in cows and goats which are habitually milked the udders are large; that moles and many cave-animals have rudimentary eyes, and so on. But all these results may be readily interpreted as due to selection of germinal variations.
The best examples of inherited modifications occur, he says, in mankind. "Thus in the United States the descendants of the immigrant Irish lose their Celtic aspect, and become Americanised.... To say that 'spontaneous variation' increased by natural selection can have produced this effect is going too far." But if the vague statement as to the Americanisation of the Irishman be correct, and if it be true that intermarriage is rare, it remains probable that the Americanisation is a modificational veneer impressed afresh on each successive generation.
"That large hands are inherited by those whose ancestors led laborious lives, and that those descended from ancestors unused to manual labour commonly have small hands, are established opinions." But if we accept the fact, it is easy to interpret the size of the hands as a stock-character correlated with a muscularity and vigour, and established by selection. The prevalence of short-sightedness among the "notoriously studious" Germans is a singularly unfortunate instance to give in support of the inheritance of functional modifications, for there is no reason to believe that short-sightedness is primarily an acquired character. Nor is it confined to readers.
Spencer twits those who are sceptical as to the transmission of acquired modifications, for assigning the most flimsy reasons for rejecting a conclusion they are averse to; but when Spencer cites the inheritance of musical talent and a liability to consumption as evidence of the transmission of functional modifications, we are reminded of the pot calling the kettle black.
Spencer made his position stronger by adducing what he calls negative evidence, namely those "cases in which traits otherwise inexplicable are explained if the structural effects of use and disuse are transmitted."
(1) First he refers to the co-adaptation of co-operative parts. With the enormous antlers of a stag there is associated a large number of co-adaptations of different parts of the body, and similarly with the giraffe's long neck and the kangaroo's power of leaping. Spencer argued that the co-adaptation of numerous parts cannot have been effected by natural selection, but might be effected by the hereditary accumulation of the results of use. The difficulty is to discover how much deep-seated co-adjustment can be effected by exercise even in the course of a long time, and the theory requires such data before it can be more than a plausible interpretation, with certain a priori difficulties against it. If an animal suddenly takes to leaping many individual adjustments to the new exercise will arise; if the animals of successive generations leap yet more freely, they will individually acquire more thorough adjustments up to a certain limit; meanwhile there may arise constitutional variations making towards adaptation to the new habit, and under the screen of the individual modifications these may increase from minute beginnings till they acquire selection-value. Professors Mark Baldwin, Lloyd Morgan, and Osborn, have all made the same useful suggestion that adaptive modifications acquired individually may act as the fostering nurses of constitutional variations in the same direction until these coincident variations are large enough in amount to be themselves effective.
(2) Secondly, Spencer dwelt upon the notably unlike powers of tactile discrimination possessed by the human skin, and sought to show that while these could not be interpreted on the hypothesis of natural selection or on the correlated hypothesis of panmixia, they could be interpreted readily if the effects of use are inherited. But the difficulty again is to get secure data. It is uncertain how much of the inequality in tactile sensitiveness is due to individual exercise and experience, though it is certain that tactility in little-used parts can be greatly increased by use. Nor is it certain how much of the apparent unlikeness in tactility is due to unequal distribution of peripheral nerve-endings and how much to specialised application of the power of central perception. As Prof. Lloyd Morgan says: "We do not yet know the limits within which education and practice may refine the application of central powers of discrimination within little-used areas. The facts which Mr Spencer adduces may be in large degree due to individual experience; discrimination being continually exercised in the tongue and finger-tips, but seldom on the back or breast. We need a broader basis of assured fact." Nor, it may be added, is the action of selection to be excluded.
(3) Spencer's third set of negative evidences was based on rudimentary organs which, like the hind limbs of the whale, have nearly disappeared. Dwindling by natural selection is here out of the question; and dwindling by panmixia, i.e. the diminution of a structure when natural selection ceases to affect its degree of development, "would be incredible, even were the assumptions of the theory valid." But as a sequence of disuse the change is clearly explained. Prof. Lloyd Morgan replies: "Is there any evidence that a structure really dwindles through disuse in the course of individual life? Let us be sure of this before we accept the argument that vestigial organs afford evidence that this supposed dwindling is inherited. The assertion may be hazarded that, in the individual life, what the evidence shows is that, without due use, an organ does not reach its full functional or structural development. If this be so, the question follows: How is the mere absence of full development in the individual converted through heredity into a positive dwindling of the organ in question?" Moreover, the convinced Neo-Darwinian is not in the least prepared to abandon the theory of dwindling in the course of panmixia, especially in the light which Weismann's conception of Germinal Selection has thrown on this process.
The inductive evidence in support of the conclusion that bodily modifications due to use or disuse or environmental influence can be as such or in any representative degree transmitted, is very weak. The so-called evidences are often anecdotal and vague, often irrelevant and fallacious, and those Spencer adduced are by no means convincing. Let us consider the question briefly from the a priori side.
The general argument against the hypothesis rests on a realisation of the continuity of the germ-plasm. For if the germ-plasm, or the material basis of inheritance within the germ-cells, be somewhat apart from the general life of the body, often segregated at an early stage, and in any case not directly sharing in the every day metabolism, then there is a presumption against the likelihood of its being readily affected in a specific manner by changes in the nature of the body-cells. The germ-cell is in a sense so apart that it is difficult to conceive of the mechanism by which it might be influenced in a specific or representative manner by changes in the cells of the body.
On the other hand, in many plants and lower animals, the distinction between body-cells and germ-cells is far from being demonstrably marked, and even in higher animals we cannot think of the germ-cells as if they led a charmed life uninfluenced by any of the accidents and incidents in the daily life of the body which is their nurse, though not exactly their parent. No one believes this, Weismann least of all, for he finds one of the chief sources of germinal variation in the nutritive stimuli exerted on the germ-plasm by the varying state of the body. The organism is a unity; the blood and lymph and other body-fluids form a common internal medium; various poisons may affect the whole system, germ-cells included; and there are real though dimly understood correlations between the reproductive system and the rest of the organism.
There are some who pretend to find in this admission "a concealed abandonment of the central position of Weismann," for if, they say, the germ-plasm is affected by changes in nutrition in the body, and if acquired characters affect changes in nutrition, then "acquired characters or their consequences will be inherited." But it is a quite illegitimate confusion of the issue to slump acquired characters and their consequences as if the distinction was immaterial. The illustrious author of the Germ-Plasm has made it quite clear that there is a great difference between admitting that the germ-plasm has no charmed life, insulated from bodily influences, and admitting the transmissibility of a particular acquired character, even in the faintest degree. The whole point is this: Does a change in the body, induced by use or disuse or by a change in surroundings, influence the germ-plasm in such a specific or representative way that the offspring will exhibit the same modification which the parent acquired, or even a tendency towards it? Even when we fully recognise the unity of the organism, or recognise it as fully as we know how, it is difficult to suggest any modus operandi whereby a particular modification in, say, the brain or the thumb can specifically affect the germinal material in such a way that the modification or a tendency towards it becomes part of the inheritance. Did we accept Darwin's provisional hypothesis of pangenesis according to which the parts of the body give off gemmules which are carried as samples to the germ-cells, the possibility of transfer might seem more intelligible. But Darwin's suggestion remains a pure hypothesis, and is admitted by none except in extremely modified form. In fairness, however, we must note how little we understand of the modus operandi of influences which certainly pass in the other direction, from the reproductive organs to the body; we must recall Prof. Lloyd Morgan's warning that although we cannot conceive how a modification might as such saturate from body to germ-cells, this does not exclude the possibility that it may actually do so.
As a matter of fact, Spencer has himself suggested a modus operandi—as already outlined. The constitutional units are supposed to circulate; when they come to a modified organ and visit its modified constitutional units, they are supposed to be themselves impressed; they are supposed to be "eventually gathered into sperm-cells and germ-cells," which thus come to bear the "superposed traits" resulting from modification. But, as we have seen, the difficulty is to find any basis in fact on which these assumptions can rest. Indeed, they are contradictory to well-established physiological facts.
Inconceivability.—In reference to the difficulties which beset theories of heredity, Spencer remarks:—
"If it is said that the mode in which functionally-wrought changes, especially in small parts, so affect the reproductive elements as to repeat themselves in offspring, cannot be imagined—if it be held inconceivable that those minute changes in the organ of vision which cause myopia can be transmitted through the appropriately modified sperm-cells or germ-cells; then the reply is that the opposed hypothesis presents a corresponding inconceivability. Grant that the habit of a pointer was produced by selection of those in which an appropriate variation in the nervous system had occurred; it is impossible to imagine how a slightly different arrangement of a few nerve-cells and fibres could be conveyed by a spermatozoon. So too it is impossible to imagine how in a spermatozoon there can be conveyed the 480,000 independent variables required for the construction of a single peacock's feather, each having a proclivity towards its proper place. Clearly the ultimate process by which inheritance is effected in either case passes comprehension; and in this respect neither hypothesis has an advantage over the other."
Let us consider what Spencer has said in regard to "inconceivability." Most ova are very minute cells, often microscopically minute, and a spermatozoon may be only 1/100,000th of the ovum's size—inconceivably minute, but yet exceedingly real and potent. We cannot conceive how a complex inheritance made up of numerous contributions is potentially contained in such small compass, and yet in some form it must be. Similarly, we cannot conceive how the pin-head like brain of the ant contains all the ant's "wisdom."
Those who find it difficult to believe that items so minute as the germ-cells can have room for the complexity of hereditary organisation which seems to be a necessary postulate may be reminded of three things: (1) They should recall what students of physics have told us in regard to the fineness, or, from another point of view, the coarse-grainedness of matter. They tell us that the picture of a Great Eastern filled with framework as intricate as that of the daintiest watches does not exaggerate the possibilities of molecular complexity in a spermatozoon, whose actual size is usually very much less than the smallest dot on the watch's face.
(2) It should be remembered that in development one step conditions the next, and one structure grows out of another, so that there is no need to think of the microscopic germ-cells as stocked with more than initiatives. (3) It should be remembered that every development implies an interaction between the growing organism and a complex environment without which the inheritance would remain unexpressed, and that the full-grown organism includes much that was not as such inherited, but has been individually acquired as the result of nurture or external influence.
Now, returning to Spencer, we find that by an extraordinary argument he concludes that the number of determinants required for the development of a single feather in the peacock's tail must be 480,000, and he points to the inconceivability of these being contained, along with much else of course, in the spermatozoon. We are not at present concerned with the precise number of determinants, but we can see no reason why a spermatozoon should not contain millions if they were needed. The inconceivability is a general one; it is due to the difficulty of imaging the complexity of matter.
But the inconceivability of a particular modification of the nose affecting the germ-cells in a specific and representative way is a different kind of inconceivability. It is due to our being unable to imagine any reasonable modus operandi consistent with our knowledge of the structure and metabolism of the organism. As we have seen and emphasised Spencer does himself suggest a modus operandi, but it seems to us to make unwarranted assumptions, and is for that reason to us "inconceivable."
A Priori Argument.—But Spencer advanced an a priori argument to strengthen the position which he felt bound to hold—the transmissibility of acquired characters. "That changes of structure caused by changes of action must be transmitted, however obscurely, appears to be a deduction from first principles—or if not a specific deduction, still, a general implication. For if an organism A, has, by any peculiar habit or condition of life, been modified into the form A', it follows that all the functions of A', reproductive function included, must be in some degree different from the functions of A." [This, we venture to think, must depend on the nature and amount of the modification.] "An organism being a combination of rhythmically-acting parts in moving equilibrium, the action and structure of any one part cannot be altered without causing alterations of action and structure in all the rest." [The appreciability of the change will depend on the amount and nature of the modification, and on the intimacy of the correlation subsisting in the organism. Dislodging a rock may alter the centre of gravity of the earth, but it does not do so appreciably.] "And if the organism A, when changed to A', must be changed in all its functions; then the offspring of A' cannot be the same as they would have been had it retained the form A." [Assuming that is to say that the change in the physiological units of the body affects the physiological units in the germ-cells.] "That the change in the offspring must, other things equal, be in the same direction as the change in the parent, appears implied by the fact that the change propagated throughout the parental system is a change towards a new state of equilibrium—a change tending to bring the actions of all organs, reproductive included, into harmony with these new actions." [It seems to us to pass the wit of man to conceive how or why an improved equilibrium in the use of the hand should involve any corresponding or representative change of equilibrium in the germ-cells.]
Spencer seems to have seen the matter quite clearly. If the physiological units in the germ-cell mould the aggregate organism, the organism modified by incident actions will impress some corresponding modifications on the structures and polarities of its units. And if the physiological units are in any degree so remoulded as to bring their polar forces towards equilibrium with the forces of the modified aggregate, then, when separated in the shape of reproductive centres, these units will tend to build themselves up into an aggregate modified in the same direction.
The drawback to abstract biology based on first principles is that it enables its devotee to develop arguments which seem plausible until they are reduced to the concrete. Why had Herbert Spencer small hands? Because his grandfather and father were schoolmasters who did little from day to day but wield the pen and sharpen the pencil! Through disuse of the sword and the spade their hands were directly equilibrated towards smallness. But since Mr Spencer senior, was "a combination of rhythmically-acting parts in moving equilibrium," the dwindling of the hands and the moulding of the physiological units thereof reverberated through the whole aggregate; a change towards a new state of equilibrium "was propagated throughout the parental system—a change tending to bring the actions of all organs, reproductive included, into harmony with these new actions," or inactions. The modified aggregate impressed some corresponding modification on the structures and polarities of the germ-units. And this was why Herbert Spencer had small hands. At least so he tells us, for the instance is his own.
Practical Conclusion.—It is obvious that we have not in these pages attempted to give an adequate discussion of an extremely difficult problem. We have endeavoured to give a fair statement of Spencer's position in regard to a question which appeared to him of "transcendent importance." "A right answer to the question whether acquired characters are or are not inherited, underlies right beliefs, not only in Biology and Psychology, but also in Education, Ethics, and Politics." "A grave responsibility rests on biologists in respect of the general question; since wrong answers lead, among other effects, to wrong beliefs about social affairs and to disastrous social actions."
It cannot be an easy question this, when we find Spencer on one side and Weismann on the other, Haeckel on one side and Ray Lankester on the other, Turner on one side and His on the other. Therefore while it seems to us that the transmission of acquired characters as strictly defined is non-proven, and while there seems to us to be a strong presumption that they are not transmitted, the scientific position should remain one of active scepticism—leading on to experiment.
And if there is little scientific warrant for our being other than sceptical at present as to the transmission of acquired characters, this scepticism lends greater importance than ever, on the one hand, to a good "nature," to secure which is the business of careful mating; and, on the other hand, to a good "nurture," to secure which for our children is one of our most obvious duties, the hopefulness of the task resting upon the fact that, unlike the beasts that perish, man has a lasting external heritage, capable of endless modification for the better, a heritage of ideas and ideals embodied in prose and verse, in statue and painting, in Cathedral and University, in tradition and convention, and above all in society itself.