CHAPTER V
THE MECHANICAL THEORY OF EVOLUTION: THE DARWIN-WEISMANN EXPLANATION
“Chance guides all things: mind and forethought must call it God alone!”—Menander.
“IN the end,” writes M. Edmond Perrier, “every imaginable theory of evolution must lead up to one or other of two absolute doctrines, essentially antagonistic to each other. Either the inheritance of acquired characteristics must be admitted in its full scope (dans toute sa généralité), or else we must believe in the predestination of protoplasm, developing by virtue of its own internal forces. But in the latter case we pass from the domain of pure science to enter that of metaphysics.”[66]
We have now to consider the most conspicuous attempt made in recent times to escape from this tragic dilemma.
If the acquired and inherited variations of the Lamarckian theory drop out as a contribution to the explanation of evolution, we are reduced to two forces only—innate, or germinal, variability of offspring, and natural selection. Indeed it might be said that we are reduced to variability alone, since natural selection can do nothing until suitable variations are presented to it. The suitable variations do, however, turn up, and the question is, what causes them? The real difficulty for the school of biologists who, like Weismann, “assume the mechanical theory of the world to be correct,” is how to reconcile the aptness and apparent purposefulness of these variations with any mechanical theory.
“We are justified in inquiring,” writes Weismann, “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 connection, ‘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?’”[67]
What loads the dice? There is the great question in which the realms of biology and of philosophy meet each other! Through that borderland no definite frontier has ever been traced, for in thought as in matter the saying is true that natural groupings have nuclei, but no boundaries. It is all the more essential that men of science should understand philosophy and its methods, and that philosophers should understand science. It is to be feared that at present the second of these desiderata is much more fully realized than the first.
However, we have to see now what Weismann, protagonist among contemporary biologists of the mechanical theory of the world, has to answer to the crucial question which he has allowed Osborn to set him.
The problem is to discover how innate, germinal variations can come about, of such a nature as to adapt an organism with striking accuracy to its surroundings and way of life, without our assuming either (1) that the exercise of function had any influence in causing heritable variations, or (2) that they were caused by any non-mechanical power, which, so to speak, had in view the objects which they fulfil. For the variations are to be regarded, on Weismann’s theory of life, as completely fortuitous in respect of the objects they serve. How, then, do they come to serve them, in most cases, so admirably well?
The general nature of Weismann’s explanation may be summed up in a curious illustration given by him in The Evolution Theory.[68] Let us suppose, he says, a snow-field surrounded by precipices on all sides, but with a narrow track leading away from it at one point. Scattered about on the snow-field are a number of persons. A sleigh is now projected among them from some outside point. Each person, when the sleigh comes near him, gives it a push, but he has no object in pushing it anywhere in particular, and simply sends it flying off in whatever direction he chances to be looking. What will happen under these circumstances? After more or less bandying about, the sleigh will, in the vast majority of cases, fall into one of the abysses round the snow-field and be lost But another is then launched on to the snow-field, and then another and another without end; and so, at last, it may happen that a series of pushes will take place which will send the sleigh over the narrow track to its goal.
The goal is supposed to represent some condition to which the organism (the sleigh) has to adapt itself. The random pushes which it receives are the multitude of variations constantly occurring in the reproductive cells. Most of these variations have no decisive tendency, favourable or unfavourable. If a series of unfavourable ones should occur, leading to some development which markedly impairs the chances of the organism for success in life, it, or its line of succession, dies out, and the unfavourable variation is, therefore, not perpetuated. This is illustrated by the sleigh going into the abyss. But if a favourable variation occurs, and is increased till it reaches ‘selection value,’ i.e. till it gives the organisms possessing it a distinct advantage over others in the battle of life, then this favoured type will ultimately, by the action of natural selection, drive out the less favoured, and will establish itself as the sole representative of the species. Having reached this level, of course the same process will go on further indefinitely.
Before criticizing this conception of evolutionary processes, we must inquire into the vital point of how the variations, the random pushes given to the sleighs, ever rise to such intensity as to have selection-value, and to make head against the influence of intercrossing. The explanation is certainly ingenious, but is so purely hypothetical and has an air so fantastic that it has commended itself to very few students of biology. Weismann would have us suppose that the determinants of which the hereditary substance in the reproductive cells is made up are carrying on with each other an incessant struggle for nutriment. If one of them succeeds in getting a little more than its neighbours it thereby grows stronger, and is able to attract still more nutriment to itself, and to impoverish those around it. It is thus launched, as it were, on an ascending scale, and will go on automatically if the variation caused by it proves favourable to the species. If it proves unfavourable (which ex hypothesi it is just as likely to do) its career will be put a stop to by the extinction of the line of descent which inherits this variation. Weismann’s theory of “Germinal Selection” is therefore simply an application to the reproductive cell and its contents of the Darwinian principle of Natural Selection.
The theory is one which plainly makes immense demands upon our faith. As regards the existence of a continual competition among the determinants, there may be reason to accept it, but hardly in the Weismann sense. Suppose two parents to unite, one healthy, well-nourished, full-blooded, the other starved and weakly, it is very likely that, in the resulting offspring, other things being equal, the determinants coming from the well-nourished frame will be seen to have surpassed in potency those from the weakly one. For the determinants are living protoplasm—they depend on nourishment derived from the blood of the organism in which they are lodged, and they are capable, no doubt, of being well-nourished or ill-nourished or possibly over-nourished, according to the constitution and history of that organism. But this is a very different thing from supposing that one determinant can begin to grow in the same cell at the expense of another, when both are absolutely embedded in an ocean of the same nutritive matter. There is not—of course in the nature of things there cannot be—a particle of evidence for the supposition. It is a pure imaginative hypothesis, and on the face of it a most improbable one. It is difficult to believe that it could ever have been adopted save as a desperate attempt to break through the ever-narrowing ring of evidence which is forcing investigation more and more towards a non-mechanical explanation of the processes of life. But even if it were true, what is gained by it? “Appropriate variational tendencies,” writes Weismann, “not only may present themselves, they must do so, if the germ-plasm contains determinants at all by whose fluctuations in a plus or minus direction the appropriate variation is attainable.”[69] But why must they? There is no ‘must’ about Chance, unless one extends its operations to infinity. Why is it so certain that the inequalities of nutriment, on which hereditary variability is supposed to depend, must necessarily run the gamut of all possible variations? There is no ‘must’ in this theory, except that it is the last ditch of the “mechanical conception of the economy of life.” It ‘must’ be true—or that conception must quit the field.
Were evolution to depend on the occurrence, by pure chance, of a few appropriate variations among a vast multitude of indifferent or disadvantageous ones, is it conceivable that we should find in nature anything like the infinite wealth of closely and beautifully adapted structure which is actually present? In particular, how are we to account for the cases in which a number of parts are so modified as to work together in harmonious co-adaptation? Each of these parts, according to Weismann, originates quite independently of the others. Take the case of the Indian leaf-butterfly already referred to.[70] The first beginnings of the midrib on Weismann’s theory had nothing to do with the rest of the rib, nor had any of the veinings with this, or with one another; and the contour of the leaf, sending out a little projection like a stalk exactly where the midrib starts, originated quite independently of that marking, and equally so of the leaf it mimics! To explain co-adaptations like this on Weismann’s theory is really much the same as to suppose that a picture could be painted by simply plastering the scrapings of a palette on a canvas, if only one continued the process long enough. And the marvel in question, the co-adaptation of various parts, has not been attained once or twice but, to a greater or less degree, in every organism possessing any structural complexity.
The difficulty, of course, has not escaped Weismann. His explanation depends on some conception of the potentialities of conjugation and intercrossing which I confess I cannot understand. He finds the key to the mystery in the mingling and constant recombination of determinants from different individuals produced by promiscuous intercrossing. “It is only through amphimixis [conjugation] that simultaneous harmonious adaptation of many parts becomes possible.”[71] But surely this continual mingling and recombination would, primâ facie, be just as likely to break up co-adaptations already forming as to give rise to new ones? Amphimixis, as we have seen, is one of the most potent forces against which the evolution of a new species has to contend. Evolution has to make head against the constant tendency of intercrossing to obliterate individual distinctions. True, if parents exhibiting the same heritable variation unite, their offspring will have that variation in a strongly marked form, and will transmit it further. But this, to be of value for evolution, presupposes the same variation occurring simultaneously in a number of individuals within reach of each other. Weismann had indeed good reason to ascribe to the action of intercrossing “a wealth and diversity of organic architecture otherwise unattainable,” but were it not supplemented by an architectural instinct of nature, the only architecture attainable would be that of the child when it empties its bricks on the floor.
Consider the theory of germinal selection in the light of the following very curious case.[72] Most people have seen an example of the kind of spectacles having what are called bifocal lenses. Each lens is divided across the centre, and the focal lengths of the upper and the lower halves are different. They are intended for persons who see indistinctly both at near and at far distances—the upper half of the lens is used for looking at distant objects and the lower for reading, etc., so as to avoid the inconvenience of having a different pair of glasses for each requirement. Now there is a fish, named Anableps (the Uplooker), living in estuaries on the east coast of South America which actually has its eye-lenses constructed on this principle. The pupil of the eye is divided laterally by prolongations from the iris. The significance of this extraordinary arrangement is that the fish is in the habit of swimming near the surface, and often has its eyes wholly or partly out of water, presumably to look out for attacks from birds of prey. The upper half of the eye has become adapted for vision in the air and the lower for vision in the water.
According to Weismann, the habits and needs of the fish could have had no influence whatever in producing this peculiar adaptation as an inherited characteristic of a species. Any other fish or mammal would have been just as likely as Anableps to begin the development of a bifocal eye. How does it come, then, that from the thousands of species of eyed animals one, and one only, possesses this bifocal eye, and that precisely the one which so greatly needs it? Weismann’s answer would doubtless be that, in the case of other creatures, Natural Selection would not have acted in protecting the individuals which possessed the bifocal eye and penalizing those which did not. But can we imagine that this principle acted very strongly when the bifocal arrangement in Anableps was in a mere rudimentary stage, as it must at first have been? And should we not occasionally see at least traces of the arrangement in the eyes of other creatures, if its full development in Anableps was merely the result of Natural Selection laying hold of and perfecting an originally quite fortuitous variation?
A case still more curious and convincing occurs in connexion with the hermaphroditism exhibited by a whole class of animals belonging to many different orders, but alike in the one respect that it is specially desirable for them to have both sexes comprised in the same individual. These are animals capable only of sluggish movement, the different sexes of which have therefore some difficulty in finding each other out. Terrestrial snails and slugs are an example. All these creatures are double-sexed; any two snails which meet can conjugate, since each can act either as male or as female at will. Oysters are another instance, though in this case the two sexes follow each other at different periods in the life-history of each individual. Clearly, this faculty gives to snails and slugs twice as many opportunities of reproducing their kind as if the sexes were distinct. It is certain from general biological considerations that they were distinct originally. One can easily understand how, if any small group of the original species from which all the present tribes are descended, happened to throw up these bisexual peculiarities, their progeny would multiply faster than the rest and might ultimately exterminate them by the operation of natural selection. But exactly the same might be said of any other tribe of unisexual animals. Any of these might, a priori, on the “mechanical conception of the economy of life,” be just as reasonably expected to develop bisexuality; for no one supposes that there is any physical connexion between sluggishness and hermaphroditism, or swiftness and distinction of the sexes; and the causes which have operated to extend and confirm the type in sluggish and sedentary animals would have the same effect in swift ones. Yet this remarkable adaptation occurs just wherever there is special need for it; there always and there only. What mechanism can account for such a phenomenon as this? No; the dice are loaded. Nature gains her end slowly and not without hesitations and failures, but the phenomena are wholly unlike the results of the play of uncontrolled and fortuitous forces. Imagine a blindfolded archer shooting arrows upwards, downwards, and all around him in every direction as it may take his fancy. There is, unknown to him, a target some distance off. If he went on long enough it is conceivable, though by no means necessary, that some arrow would hit the bull’s eye. But the facts plainly point not to the above analogy, but rather to an aim at a desired object. Some of the arrows miss, some light near the mark, others hit it precisely. The flight, on the whole, is in the right direction, as the immense proportion of complete or partial successes plainly proves.
The two pillars of Weismann’s theory of evolution are germinal variation and natural selection. The one is supposed to originate ceaseless changes of structure, the other to eliminate those changes which are useless[73] or unfavourable and to foster and confirm the favourable. We have seen, if the foregoing considerations are sound, that fortuitous variations do not provide the material with which natural selection can build up a universe of organic life like ours. We have now to turn our attention to the other prop of the system and to inquire whether natural selection can play and does play the part which Darwin and his school assign to it in the economy of nature.
Natural selection is supposed to depend for its efficacy on the existence of a state of strenuous competition for nourishment, or for the avoidance of foes, in the type out of which the favourable variations emerge. But in recent times the fact of any such competition has been gravely doubted. Let us look back to the beginnings of animal life in the world. The first primitive animal organisms found themselves swimming in a boundless sea of nourishment and had no foes at all! Yet they developed into higher and higher grades of life. Competition did not aid in the development of these higher grades—it was they which ultimately created the state of competition. What Nature then achieved without competition she is equally able to perform now. Even now when the earth is swarming with varied life competition plays a much smaller part than was taken for granted in the first flush of Darwinism. Creatures of the same type but on different grades of organization, like the hive-bee and the humble bee, are constantly found side by side, drawing their nourishment from the same sources, but each holding its own without difficulty. Facts like these were not unobserved by Darwin, who met them by the supposition that competition came chiefly into play at exceptional periods, during a drought, an inundation, a severe winter, or the like, in which the less fitted members of the race perished wholesale. But, as Kropotkin, in his interesting work, Mutual Aid among Animals, has remarked,
“If the evolution of the animal world were based exclusively, or even chiefly, upon the survival of the fittest during periods of calamities; if natural selection were limited in its action to periods of exceptional drought, or sudden changes of temperature, or inundations, retrogression would be the rule in the animal world. Those who survive a famine, or a severe epidemic of cholera, or small-pox, or diphtheria, such as we see them in uncivilized countries, are neither the strongest, nor the healthiest, nor the most intelligent. No progress could be based on such survivals—the less so as all survivors usually come out of the ordeal with an impaired health, like the Transbaikalian horses just mentioned, or the Arctic crews, or the garrison of a fortress which has been compelled to live for a few months on half rations, and comes out of its experience with a broken health, and subsequently shows a quite abnormal mortality.”[74]
Kropotkin’s book shows good reason to believe that the principle of mutual aid and support plays at least as great a part in the animal world as does that of mutual competition and extermination.
That the competition of organisms, animal and vegetable, for nourishment and for protection may favour certain types, and depress or even exterminate others, is of course indisputable. We see it when the Japanese worker and the Californian meet in industrial rivalry on the Pacific slopes—we see it when the willows planted by New Zealand rivers destroy the weed which infested them, by absorbing the nourishment from the river-bed on which it lived.[75] What we have to consider, however, is the efficacy of competition in giving predominance and permanence to a type differing but slightly in the initial stages from that of the rest of the species, and differing but in a very few individuals. We have to consider, in fact, whether natural selection is not a consequence rather than a cause of evolution. On no mechanical theory of evolution can we suppose that the first leaf-markings of the butterfly, Kallima paralecta, were either at all pronounced in their mimicry, or that they originated simultaneously in any large group of the original species from which Kallima paralecta sprang. Therefore, with very small advantage in the way of protection from enemies, and with the constant and powerful influence of intercrossing ever tending to obliterate the distinctive leaf-marks, how could natural selection alone enable the new, the mimicking type, to assert and develop itself, as it has done not only in this particular species of butterfly but in hundreds of species of the Lepidoptera and other insects?
“A considerable initial resemblance,” writes Mr. Beddard in his most valuable though somewhat chaotic work on this subject,[76] “may be fairly set down to other causes [than natural selection]; because it is impossible to believe that a slight move in the required direction would be of sufficient importance to serve as material for the action of natural elimination.”
The most convinced Darwinian will hardly deny that the problem involved in this case is a serious one.
Another singular fact to be noted in this connexion is the “conclusion arrived at by the study of mimetic butterflies in all parts of the world—that the females are far more liable to assume this method of defence than the males.”[77] An instance in point, which has been the subject of much discussion, is that of the yellow and black swallow-tailed butterfly, Papilio meriones, found in Madagascar. The island is supposed to be the original home of the species, and here both sexes are much alike. On the mainland of South Africa, however, while the male has undergone the very slight transformations represented by the species P. merope and P. cenea, the females imitate closely three different species of the Danais butterfly which is protected by its disagreeable taste from the usual enemies of the tribe, and which is altogether unlike in shape and coloration to the swallow-tail. “The new forms,” writes Mr. Poulton, “have arisen at so recent a date that many of the intermediate stages can still be seen, while the parent form has been preserved unchanged in a friendly land, where the keener struggle of continental areas is unknown.”[78] The significance of such a fact as this is obvious. If mimicry arose from fortuitous variations of colouring and of form, males alone might show it in some species, females alone in others, and both in yet others, but it is difficult to understand how we could arrive at the actual condition, and find it either common to both sexes or practically confined to the female. If, on the other hand, mimicry and other similar adaptations are ultimately to be interpreted as the common response of the species to the attack of its foes, it is quite natural that the female, as the egg-bearer, the most important factor in the continuance of the species, should be specially protected. It is probable also that she is most in need of protection, as her functions may render her rather more exposed than the male to attack. That natural selection cannot have been the dominant factor in the case we are considering seems clear; for how could it have acted at all without a somewhat vigorous weeding out of unprotected forms? And, in that case, what would have become of the unprotected males of the species?
Difficulties of this kind have, in different cases, been raised again and again since the publication of the Origin of Species, and have had to be answered so often that there seems good prima facie ground for doubting whether they have ever really been answered at all. The strongest advocates of the pure mechanical theory are obliged, as we have seen, to admit that the drift of contemporary scientific opinion is to place little reliance on casual variation and natural selection and to look for the driving force of evolution in other directions.[79] In the introduction to Strasburger’s Text Book of Botany[80] we find this important passage:—
“The tendency is to assume the existence of a development of the organic world due to original, innate capabilities of the living substance and not dependent on selection. The origin of the large subdivisions of the animal and vegetable kingdoms, the ‘Archetypes,’ would be due to this sort of evolution. These archetypes have been, and are still, continually influenced by the environment, and, by their reaction to external conditions, organisms become more or less directly adapted.... The progressive evolution of the archetypes, as well as the direct adaptations to external conditions shown by them, is independent of selection. The latter does, however, exert an influence on the process of evolution of the organic world, though to a much more limited extent than was formerly supposed.”
It is clear that in these original innate capabilities of the living substance we have a power which alone may fully account for the evolution of the organic world, though natural selection can emphasize and hasten its action. Its nature and limits are still undetermined. Biologists are very chary of expressing this power save in terms of chemistry and physics. Men of science are afraid—sometimes I venture to think even morbidly afraid—of opening any door by which the fantastic horde of arbitrary dogmas and superstitions which they have cast out with so much toil and peril might find their way back into the temple of Knowledge. But philosophy must warn them that in shutting out all forces that cannot be weighed and measured in a laboratory they may be shutting out life itself. And those who strenuously insist on reducing nature to a mechanism often find themselves obliged to let in the mysterious life-force by some more or less clandestine entry in order to make their mechanism work. Thus Nägeli, the originator of the theory of heredity which Weismann has developed, attributes the phenomena of growth and evolution not to natural selection but to “internal forces.”[81] He disclaims for these forces any but a physical and chemical significance; but Professor Eimer, in spite of all disclaimers, cannot get rid of the suspicion, well justified in my opinion, that there is in these forces, as conceived by Nägeli, something purposeful and teleological—admit them, he says in effect, and who knows what we shall next be asked to believe?[82] Yet for Eimer himself we find that, as Schopenhauer says, “the lotus of physics is rooted in metaphysics.” Twice in his work on organic evolution, he refers with approval to the view of “our profound philosopher, Oken,”[83] who regarded all existing beings as members or organs of some vast and transcendental organism whose development conditioned theirs. Eimer even makes a somewhat daring application of this principle to a concrete instance in the physical world, one which we have already referred to, the problem of the inheritance of qualities in ants, bees, etc., when these qualities are possessed and exercised only by individuals who cannot transmit them.
“We must regard,” he writes, “the different forms of bees, queens, drones, workers, as discontinuous organs of one whole, which have been evolved from a single indifferent ancestral form.... Only thus can we explain to ourselves the fact that the peculiarities of the workers, notwithstanding that they do not reproduce, are inherited.”[84]
When we are asked to believe in physico-chemical laws of such a nature that they enable the habits of life of a worker-ant or bee to react upon the germ-cells of the queen, just as the exercise of an organ, on Lamarckian principles, affects the reproductive cells of the creature to which it belongs, it becomes plain enough that for modern investigators the so-called mechanical and the so-called psychic conceptions of the universe are really running out at the same point. The gulf between these conceptions, which seemed to yawn so widely after Darwinism, was a mere illusion, arising from a point of view now left behind.
To resume the argument of the foregoing chapters. We have seen that at the basis of all theories of evolution lies the fact of the responsive powers of living protoplasm. But what does it respond to? That is the question of questions. To put it accurately in relation to the process of evolution we must ask, To what do the determinants in the germinal cells of plants and animals respond? To what call did unicellular organisms respond when they first began to interchange chromatin with each other? To what, when they began to divide and form new organisms? To what, when multicellular organisms began to specialize certain cells for reproduction, and these cells to mature themselves for fusion by throwing out half their chromosomes? And when the higher plants and animals came on the scene, reproducing their kind under conditions which make strongly for the fixity of species, how are we to interpret the response of protoplasm when we see organs and structures melt away, and others grow, giving rise to the innumerable types which yield us the existing world with its overwhelming richness and variety of life? Weismann tells us that the response is only to differences in the amount of nutriment obtainable by the various determinants of the germ cell, and has but a fortuitous connexion with the results attained. We have seen the inadequacy of this theory, in the light of the many adaptations such as that of which the fish, Anableps, with its bifocal eyes, and the double sexual organs of terrestrial snails, are types. Lamarck and Darwin, besides the belief in fortuitous variation, held that heritable characters arise from exercise of function. Innumerable cases can be quoted in favour of this explanation, but we have seen instances in which it is absolutely untenable, and yet where the required response takes place just the same. The influence of light and colour tells on the colouring of animals, and impartially protects them when they are preyed upon, or helps them to secure their prey; and this influence is frequently explainable by chemical or electric agencies originating in the environment of the animal, acting on the blood, and thus influencing pigmentation of the skin,[85] but chemistry is helpless to account for the manner in which nature shapes the contour of the wing of a tropical butterfly and paints upon it the veinings of a leaf, or protects a harmless fly by giving it a resemblance to a stinging one, or protects a caterpillar by making it look like a vicious and dangerous reptile. Yet all these protective arrangements are evidently, at bottom, facts of the same order. Protoplasm lives and responds not only discretely in the lowest unit perceptible by the microscope, but collectively in the connected groups of these units called multicellular organisms, and in the disconnected groups of these organisms called species. It really responds not to the exercise of function or to the play of physical forces, but to vital tendencies of the organism. There seems an expansive force in nature which, though working strictly under the dominion of physical laws, is capable of using the combinations brought about by those laws for the preservation and development of life. It is in love with life, it is ever pressing toward action and self-realization, and all roads are one to it if they lead to that end. In it are included the very chemical and physical agencies which it obeys, and also that something beyond which eludes the analysis of the laboratory.
How it acts, under what conditions, what limitations, why here in one way, there in another, are questions of profound interest, the fringe of which philosophy has hardly begun to touch. Nor is philosophy yet in a position to do more, for the scientific conception of nature is but a recent birth of thought; much remains to do in the collection and organization of the facts with which the framework must be filled in, and a philosophy which does not keep closely in touch with scientific fact can have no message for the modern world. But it does seem possible to discern, and it shall now be our endeavour to set forth, in broad outline, certain principles of deep significance from which we may obtain an answer to the question: What can we learn from the physical universe that has a bearing on the spiritual life of man?