CHAPTER IV
THE MECHANICAL THEORY OF EVOLUTION: THE DARWIN-LAMARCK EXPLANATION
“Quelle est donc cette nature sujette à être effacée? La coutume est une seconde nature qui detruit la première. Pourquoi la coutume n’est elle pas naturelle? J’ai bien peur que cette nature ne soit elle-même qu’une première coutume, comme la coutume est une seconde nature.”—Pascal.
WE now approach the arcana of Evolution. The processes we have to deal with in this chapter are not, and probably never will be, the subjects of direct observation. All we can hope to do is to generalize from the results which have risen to the surface of life about the unseen forces from which they spring. The problem is to find (if possible) a generalization which will cover all the facts relating to that modification of natural forms, habits, and instincts which, when it reaches a certain point, means the establishment of a new species. We know that the thing happens, but we shall not understand how it happens until either the mechanism of the process is laid bare, or until it is clear that we are in presence of an agency not entirely definable in terms of mechanical action.
The fixity of species is maintained by a number of conditions, chief among which must be reckoned the law of reproduction by conjugation, with the consequent intermixture of numerous different lines of descent. From one point of view conjugation, as Weismann so often insists, greatly favours the adaptability of the organism to new and varied conditions of life, inasmuch as it results in the mingling together in each individual of a great number of varied determinants. But when the conditions are constant, conjugation has also the obvious effect of constantly reabsorbing, as it were, any heritable abnormalities which may occur in individuals or the species, and bringing them back to type. An individual possessing some abnormality of structure will be most unlikely to find a mate possessing the same abnormality—the mate will be either an ordinary individual or will possess, if any, some quite different variation. Their descendants will, therefore, usually show more resemblance to the normal type than to the one abnormal parent, and in their descendants again, for the same reason, the abnormal feature will be still further reduced, until finally it disappears. It is only by the careful selection of mates extending over many generations that pigeon-fanciers, to take one prominent instance, are able to establish a new type. Left to mate uncontrolled among themselves we should never have had the great variety of breeds which have been produced by the art of the fancier from the original rock-pigeon. The small variations which form the starting points of his operations would, under natural conditions, have soon been resolved into the normal type. What is it in nature, then, that sometimes appears to play the part of the intelligent breeder and to urge the plastic forms of life into new moulds?
The goal of the breeder is some new form which it pleases him to produce, either for its use, or its beauty, or for its mere singularity. The goal of nature, at least the apparent and immediate goal, is the adaptation of each species to the circumstances of its life. And the first thing that strikes the investigator is the way, often indeed not perfect, but usually most impressive in its apparent thoughtfulness and care, in which the organs of plants and animals are fashioned to secure the most favourable results. But all this is the result of development. The whale is a creature excellently adapted for its present mode of life, but we know that it was once a furry land animal with four legs; the legs are all there still, in modified or rudimentary form, and the fur appears at a certain stage of embryonic development. When we ask, How did this extraordinary transformation come about? what we really mean is, How did the determinants composing the chromatin in the reproductive cells of the original land animal so come to alter as to produce the characteristics of the whale? For new species can only be evolved by means of structural modifications capable of being transmitted by inheritance; and nothing can be inherited except through the action of the determinants. A modification which does not affect the reproductive cells has no significance in the evolution of species.
To this question Darwinism has given us our choice of two answers, which may be termed respectively the Darwin-Lamarck and the Darwin-Weismann theories. Lamarck explained the origin of species by the accumulated effect of the inheritance, through many generations, of modifications acquired by the exercise, or the disuse, of the modified organs. Observing that living protoplasm responds to demands upon it (thus, for instance, a muscle when systematically exercised attracts more nourishment from the blood and grows stronger, and callosities form to protect the skin of the hands of a manual worker), he assumed that modifications so acquired might be transmitted by inheritance. Each new generation, then, would start with a slightly better equipment in this particular respect than the former one had when it started; and so, by slow degrees, a new organ, or one markedly differing from the original form, might be built up. The world, since protoplasmic life first appeared upon it, has gone through many changes, and has always presented a vast variety of climatic and other conditions, calling for the most varied types of organic structure. As animal life gradually spread over the earth and sea, the effort to cope with the different conditions it met with would gradually, by the combined action of exercise, of disuse, and of heredity, produce multitudes of different types; and these are what we know as families, orders, genera, and species. When a species is fairly well adapted to its surroundings and way of life it may go on indefinitely without change. But should any members of it be obliged to migrate, from scarcity of food or any other reason, to some new locality where somewhat different conditions prevail, structural alterations would soon begin to appear to suit those new conditions. Thus the giraffe, if we could trace its ancestry back, would probably be found to have originated in some animal not differing from the vast majority of quadrupeds in the relative proportions of its fore and hind quarters. But some members of this original species—or the whole species, owing to some change in their surroundings—found themselves obliged to rely largely for food on leaves growing at a considerable height. They stretched up to reach them, and a prolongation of the bones of the neck (the giraffe has only the usual seven cervical vertebræ) and of the fore-legs would ensue, especially in the young; this prolongation would be handed on by inheritance, and so by degrees the new type of animal would be evolved. The horn of the rhinoceros, the antlers of the stag, the canine teeth of beasts of prey, the flat grinders of ruminants, the flippers of the whale, the proboscis of the honey-feeding butterfly, the jaws of the ant or the beetle, and a host of other adaptations which seem obviously to owe their origin to the exercise of their functions, occur to the mind in confirmation of this theory.
Besides Adaptation, we have what appears the strikingly confirmatory case of what is called Co-adaptation, where the variation of one organ or structure in an animal puts a strain upon other parts, which accordingly respond by auxiliary adaptations. Such co-adaptations are numerous in every animal structure, and, as we cannot suppose them to have all originated simultaneously and by chance, the conclusion drawn by Lamarckians is that one was produced by use, and, in the course of its development, produced the others in the same way. A typical case is that of the Irish elk. The enormous antlers of this beast, sometimes weighing a hundredweight, must have needed (besides other structural changes) a cervical ligament of immense size and power to support them, and from the peculiar structure of the cervical vertebræ it is demonstrable that such a ligament must have existed. What more natural than to suppose that the antlers were developed by fighting wild beasts of prey, combats between male elks, etc., and that then in their gradual growth, as the species was evolved, the ligament and the bony structure associated with it responded to the increasing strain. That is exactly what would happen in an individual. We have only to assume the heritability of modifications acquired by use to understand how these co-adaptations became constant characters in a species.
Not less apparent cogency for the argument for modifications by use have those cases where the modification has been apparently due to disuse. It is well known that living creatures found in the total darkness of great limestone caverns, like those at Kentucky, are blind, through imperfections of one kind or another in the organs of sight. But the rudimentary structures which remain tell us that these creatures had ancestors which were once fully equipped in this respect, and which had wandered into the caverns from the sunlit outer world. Thus the case of a crab has been noted, in which the stalks on which a crab’s eyes are set were preserved, while the eyes had disappeared: it is, as Darwin observes, as if the stand of a telescope had been retained while the telescope itself had gone. Sometimes the eyes of cave-fishes are covered with a horny layer, sometimes the whole structure is atrophied and withered. But never is an animal found under these conditions which has retained its power of sight. The conclusion seems obvious. In individuals, a muscle or other organ is known to strengthen and develop by use and to atrophy by disuse. As use and disuse appear to be invariably accompanied by precisely the same effects in the species as in the individual, and as there seems no way of accounting for this by any known physiological law without assuming that modifications acquired by the individual are transmitted to its progeny, the case for the inheritability of such modifications appears, at first sight, irresistible.54
So matters stood when Darwin’s Origin of Species carried the argument for evolution a long step further. Accepting fully the views of Lamarck, Darwin attempted, by his doctrine of Natural Selection, first to reinforce those views, secondly to explain much that they could not be made to cover. It is plain that if we assume the existence of a severe competition for livelihood among the members of a species, any favourable variations of structure or instinct which may occur among certain members of the species will give their type an advantage over the normal type in the struggle for existence. They will, on the average, live longer and produce more offspring. Ultimately, as the struggle for life is always most severe among nearly related organisms, which seek a living from the same sources, the less perfectly equipped type will be extinguished, and so on, until a species exhibiting the most complete form of adaptation has been evolved. The variations on which Natural Selection has to work are produced, according to Darwin, not only by the exercise of particular organs as in Lamarck’s theory, but also and more potently by “innate variations” originating from unascertained causes in the reproductive cells. Variations, it is indisputable, are always occurring; probably no two members of any species exactly resemble each other. Among low and primitive organisms, such as the Foraminifera, Dr. W. B. Carpenter (I quote from A. R. Wallace’s Darwinism) found, on careful examination, the range of variation so great that characteristics typical not merely of species but of genera and even of orders were liable to vary,55 while at the other end of evolution, in man, to give only one instance, Mr. J. Wood is stated by Darwin to have observed no less than five hundred and fifty-eight variations in the muscular structure of thirty-six subjects examined.56 The cause of these variations is often quite obscure, but it is certain that some kinds of them are capable of arising as the natural response of the organism to changed conditions of food or habitat Conditions such as these, affecting the whole constitution of the organism, have been proved capable of affecting the reproductive cells, and thus of giving rise to hereditary characteristics. Natural Selection, then, by preserving and encouraging the better fitted as opposed to the less fit, acts as a spur to the Lamarckian principles of development by exercise of function, while it also lays hold of and intensifies all kinds of other favourable variations occurring either casually or in consequence of change of habitat, and weeds out the types in which such variations happen to be unfavourable. According to Darwin, therefore, given (1) constant variations of structure arising from use, disuse, or from other known or unknown causes, (2) the capacity to transmit by inheritance these variations whether innate or acquired, (3) a constant struggle for existence among organisms both against each others’ competition and against the general conditions of life57—given these simple data, the secret springs of evolution are laid bare, and the vast complexity of natural forms upon the globe is adequately accounted for without calling in the agency of special creations. But variations are the starting-point in the process: Natural Selection can originate nothing—it can only act on what is presented to it by some quite different force. The relative parts played by the various agencies at work are, with characteristic moderation of statement, thus described by Darwin:—
“On the whole I think we may conclude that habit, use, and disuse, have, in some cases, played a considerable part in the modification of the constitution, and in the structure of various organs; but that the effects of use and disuse have often been largely combined with, and sometimes overmastered by the natural selection of innate variations.”58
To explain evolution, then, we must first explain the occurrence of appropriate variations, strong enough and widespread enough to maintain themselves against the constant reducing influence of promiscuous intercrossing, and they must be variations capable of being transmitted by inheritance. This, we now see, is the true field of the inquiry.
The new factors introduced by Darwin into the process of evolution—Natural Selection and Innate Variations—were destined in our day to have the whole weight of the argument for evolution suddenly thrown upon them. The inheritability of variations acquired by the individual through use and disuse when subjected to fresh investigation by the younger school of biologists has turned out to be open to the gravest doubts, both theoretically, on account of the great difficulty of reconciling it with what has now been ascertained of the nature of the reproductive mechanism in plants and animals, and also on the score of a closer consideration of the facts commonly adduced as evidence for the law. To take these points separately: The reproductive cells in every living creature are now believed to be formed directly from the reproductive cells of its parents. They are not a product of the organism in which they find themselves. They are nourished by its blood, and are therefore liable to be affected by anything which produces a broad general effect on the constitution of the being in whom they are lodged, but it is difficult to see how special modifications of individual parts of that being could affect them so as to influence the determinants in the direction of reproducing that modification. How, for example, could the habit of grubbing for roots in an animal of the pig tribe so affect its reproductive cells as to ensure the birth of an offspring with callosities on their snouts? The physiological mechanism by which such a result could be produced seems hardly conceivable—at any rate no one has yet offered a plausible conception of it. Of course if the fact were indisputably proved one would only have to accept it, and endeavour, if possible, to discover the why and how. But the fact, which once looked so solidly established, is taking on a more and more insubstantial appearance in the light of closer investigation.
The argument against Lamarckism rests on the basis (1) artificial experiment, (2) of observation of nature under normal conditions.
As to the evidence from experiment, opinions fluctuated for some time—Darwin was disposed at one time to deny, at another to admit the alleged proofs it offered. In the present day opinion is overwhelmingly against the validity of these proofs. The cases where artificially produced mutilations are said to have been inherited have, when investigated, turned out to be by no means as clear and trustworthy as was supposed, nor can one place much reliance on a few cases of striking coincidence such as are certain to occur from time to time.59
The adverse instances are very clear indeed. Chinese girls are never born with abnormally small feet. Jews are not born circumcised. Among tribes where tattooing is practised, no traces of this embellishment are ever found to be inherited. If it is a physiological law that the disuse of an organ not only atrophies it in the individual but (by inheritance of the atrophy) eliminates it from the species, there is no apparent reason why this law should not operate in cases where the organ is artificially removed. Yet it rarely or never seems to do so. Experiments upon animals, such as breeding for many generations from mice whose tails have been cut off, have never resulted in producing a clear case of inherited mutilation. A strong presumption is therefore raised that the effects apparently due to use and disuse under natural conditions (as in the eyeless fishes of the Kentucky caves) must be set down to some other cause. The queens in colonies of ants and bees have never exercised the functions of workers for thousands of centuries, yet they transmit these functions unimpaired.
There is, indeed, a case often referred to in this connexion which must be here mentioned. Dr. Brown-Séquard found that by injuring or compressing the sciatic nerve in guinea-pigs epilepsy was produced, and that the descendants of animals so injured had a marked tendency to epileptic fits. This is undoubtedly a very significant and important fact in biology, but it gives no support to the Lamarckian theory. What is inherited by the guinea-pigs is not the injury to the nerve but the pathological condition resulting therefrom. It remains to be discovered how, precisely, this takes place, and the experiment may end in illuminating a very obscure region in physiology, but on Lamarckism it has no bearing at all. A better case is that of atrophy of a toe, which is said to have been inherited in consequence of its original production by severance of the sciatic nerve, but, again, what is inherited is not an actual injury but an effect of it. It is clear, however, that bodily conditions of a large and comprehensive kind produced naturally or artificially in an individual may have an effect on the reproductive cells, especially when the nervous system is affected.
Coming to the observation of what happens under natural conditions, we are struck at the outset by the fact that the inheritance of acquired characteristics, if it works at all, must work under some system of salutary control and not as a blind physiological law. For if each generation starts with some measure at least of what the former generation had acquired, and adds to it by its own activity, then all acquired characteristics would ere long attain a monstrous development, and the species would perish under them. But nothing of the kind is observed to happen. The continual use of the muscles in the labouring classes has not made men stronger than they were thousands of generations ago. The habit of handling the spade and hoe has never produced a peasant child born with callosities on its hands. The horn of the rhinoceros, which on Lamarckian principles we must regard as developed by the gradual increase of a callosity formed by grubbing for roots, does not grow beyond a certain size, however the species may go on grubbing. The Lamarckian law, then, if it has any real effect at all, can only express half the truth about the action of heredity on acquired characteristics. As the column of water in a fountain hovers about a certain height, so the action of heredity in the accumulation of the effects produced by the use of organs seems to have a limit beyond which it cannot pass. May it not be that heredity is really as false an expression for the phenomenon as the popular superstition about ‘water seeking its own level’ is for the upspringing of a fountain?
The cases of co-adaptation, where one organ appears to be developed by use and others by the use of that, as in the case of the Irish elk referred to above, are met by instances just as striking where the elements of modification by use cannot come into play. Weismann mentions the case of the ingenious brush arrangement on the anterior legs of the bee, which the insect uses for cleansing its antennæ. Two adaptations are here developed—a little semicircular notch in the leg, set with small bristles, and a movable projection or flap used for pressing the antenna into the notch as it is drawn through. The bee, no doubt, would naturally try to clean its antennæ with its fore-legs, but how could this process develop the special arrangements referred to in the hard or scaly covering of its limbs? It is not until the shell of the insect has grown quite hard and incapable of further vital changes that the arrangement comes into use. Again, the stridulating noise produced by the legs of the grasshopper is due to serrations occurring on different joints of the limb. Serrations on one joint would in no way tend to develop them on the other, but rather the contrary, yet there they are, in harmonious co-operation. If Nature can obtain these effects, as she does in numberless instances, without the aid of Lamarck’s principle, we cannot help asking whether that principle is ever operative at all.
The three instances which we shall next consider seem to offer very serious obstacles to the Lamarckian theory.
A modification of structure caused by the special use of a certain organ takes place in probably over 90 per cent of the whole human race, male and female. The records of art, of language, and the evidence of actual remains, tend to show that the habitude in question, with the attendant modification, goes back to very ancient, even perhaps to palæolithic times.60 I refer to the preferential use of the right hand and the enlargement of structure thus brought about in the right hand and arm. Every right-handed adult man and woman shows this enlargement of bony and muscular structure. The origin of the habitude does not concern us here. Let us suppose it due, as Dr. D. J. Cunningham suggests, to “a transmitted functional pre-eminence of the left brain,”61 which is larger than the right, and which governs the movements of the right side of the body. However this may be, it is clear that if bodily characteristics acquired by exercise are transmissible by inheritance the new-born child of right-handed ancestry ought to show some appreciable preponderance in weight and size of the right over the left limb. There could hardly be a more crucial test of the validity of the Lamarckian principle. What do the investigations of the dissecting-room reveal? I shall quote the two most recent authorities who have studied this interesting question. Dr. Cunningham, in the lecture already referred to, writes:—
“Although the matter has not been investigated so fully as to place the question outside the region of dispute, the evidence at our disposal distinctly favours the view that at birth the two upper limbs start upon their individual duties equally endowed in so far as strength of muscle and size of bones are concerned. Both in mass and weight the two limbs are to all intents and purposes similar at birth, and the preponderance in bulk and strength which later on distinguishes the right arm is acquired during life, and is caused by the greater amount of work it is called upon to perform.”62
Dr. T. G. Moorhead, Chief Demonstrator in Anatomy in Trinity College, Dublin, after giving the results of the researches of various other inquirers, writes:—
“From this mass of conflicting evidence I am forced to the conclusion that no real differences exist.... After weighing as a whole the limbs of eight foetuses I was unable to detect any constant difference.”63
These results appear to conflict most seriously with the theory of the transmissibility of acquired modifications.
Fig. 3.
Kallima paralecta, as it appears at rest, with wings closed.
From Weismann’s The Evolution Theory.
K, the head; B, the limbs.
Every one is familiar with the fact that species of animals which are preyed on by others, or which require to be inconspicuous for the purpose of preying, are very apt to take the colour of their habitual surroundings. Individuals of the same species will even differ according to their special habitat. Perhaps the most marvellous instances of this kind of adaptation are to be found in certain tropical butterflies, such as the Indian butterfly, Kallima paralecta, here illustrated. We have here, painted on the butterfly’s wing, the picture of a leaf belonging to a shrub which it frequents—a picture, when seen under natural conditions, capable of baffling all but the closest inspection. The different parts—the midrib, the lateral veinings, the little blotches and spots which represent patches of mould or drops of water, even the outer contour of the wing itself—all form an harmonious whole composed of related parts which have separately no meaning or use. They certainly did not all appear in full development at the same time. Nor could any one of them, if it appeared first, have exercised the smallest influence on the appearance of the others, as the antlers of the elk were supposed to have influenced the development of the ligamentum nuchæ. The early stages must have been anticipatory of the later ones, but exercise could have had nothing to do with the result from first to last. The butterfly never practised looking like a leaf. Nor can any large chemical and elemental influences have been at work. If nature is capable of producing such effects as this without the agency of Lamarck’s principle, are there not excellent grounds for seeking for some other agency which will cover all the phenomena alike?
Finally let us take the case of the slave-owning ‘Amazon’ ants, Polyergus rufescens. Here we have a case which at the first blush looks like a perfect picture of an evolutionary process conducted on the principles of Lamarck’s theory. These ants, it may be supposed, were originally of the ordinary type of that industrious and respectable insect, but they were led by the weakness of some of their neighbours of another species to make occasional attacks on them for the purpose of carrying off their immature brood, the pupæ, as food. Some of these pupæ, near maturity at the period of their capture, would come out while stored-up in the nest of the conquerors, and when they did so would immediately set about doing the household work of the hive as if they were at home. Polyergus rufescens ultimately became aware that a life of aristocratic leisure awaited him if he only captured enough pupæ of another species of ant to do his work. He accordingly confined himself entirely to piratical expeditions of this nature, and in the course of time underwent a moral and physical transformation of a most remarkable kind. The ordinary ant instincts have disappeared in this variety. They do not make their nests, they do not gather stores, they do not mind their young, they do not even feed themselves—an Amazon ant will perish of starvation in the presence of food if there is not a slave ant to put it into his mouth. But they fight ferociously in their slave-raids, and the form of their mandible has changed to suit their mode of life. It has become a pair of sabre-like nippers, excellent for slaying a foe, but ill-adapted for carrying objects and other industrial occupations. Corresponding changes have taken place in the head and in the chitinous and muscular structure.
We have before us, then, what would seem to an uninformed observer, a striking picture of the acquirement of a certain bodily form and a certain set of instincts by use, and the total loss of other traits by disuse, and of the fixing of these characters in a species by heredity. Yet the picture is altogether an illusion. However we are to explain the facts—of which more anon—we cannot do so by Lamarckism, for the simple reason that the peculiar instincts and bodily structure of the Amazon ants are confined to the so-called ‘worker,’ or in this case ‘soldier,’ caste, which are sexless, and incapable of reproducing their kind. If these were the individuals which originally started the slave system among the species, they could not possibly have transmitted the modifications, moral and physical, which they acquired. The queen-ants, which normally are the only fertile ants, transmit them, but do not possess them, and neither do the drones.
The case of these mysterious communities of insects, composed largely of neuters which do the work of the community but do not reproduce their kind, was one of the difficulties in the way of Darwin’s theory of evolution which, he said, staggered him every time he reflected on it.64 It is not surprising, therefore, that this difficulty came to be the battlefield, or a main position thereof, in a most interesting and illuminating controversy on Natural Selection versus Lamarckism, waged between Mr. Herbert Spencer and Dr. Weismann in the years 1893-4.65 Spencer considered the inheritance of acquired characteristics a factor in evolution of the very first importance; and so, indeed, from his point of view it is. “Either,” he declared, “there has been inheritance of acquired characteristics, or there has been no evolution.” Met by the case, among others, of the slave-making ants, his explanation is substantially as follows: It was not the workers (soldiers) which originally acquired military traits, but the queens, the fully developed females, which lost them. There was once, as every one admits, a time when all ants, bees, etc. were sexually mature. There were only males and females. At this stage, possibly, the Amazon ants were already predatory. It was then that they may have acquired the military habits and structure, which they were then able to perpetuate by inheritance.
How, then, did the queens lose these traits?” From the queens,” replies Spencer, “they have slowly disappeared by inheritance of the effects of disuse.” The obvious and unanswerable rejoinder made by Weismann and his followers was that Spencer had only shifted the difficulty to another ground—from the workers to the queens. If the queens (and drones) lost the military characteristics by disuse, how do they come to transmit them unimpaired to the workers? It is the very essence of Lamarckism that whatever modifications are produced by use or by disuse shall be transmissible by inheritance.
In this controversy, however, there was another string to the Lamarckian bow. Worker-ants, bees, etc. are imperfectly developed females. They have four or five egg-tubes where the queen has two hundred, but they cannot be fertilized by the drones. It occasionally happens, however, that these neuter insects do lay a few eggs. These unfertilized eggs always develop into drones. One of these drones might, it was suggested, now and then fertilize a genuine queen, and thus hand on the traits of the worker from which it sprang. But apart from the fact that an occasional occurrence of this sort would hardly suffice to maintain the worker-characteristics unimpaired throughout the ages, there is the decisive answer, as Weismann points out, that we know at least one species of ant in which the evolution of a neuter caste is absolutely complete, for the workers of Tetramorium caespitum possess no egg-tubes at all. Yet the transmission of characteristics from queens and drones who never exercise them to workers who cannot pass them on, goes forward in this species of any ant just as in any other.
Nature, therefore, while doing in the case of these insect communities exactly what she appears to be doing elsewhere by the accumulation of acquired characteristics, must, in reality, have been working on entirely different lines. If we can discover what those lines were, they will cover the apparently Lamarckian cases as well, but the Lamarckian principle certainly will not cover these.
In the next chapter we shall review the alternative explanation offered by Darwinism, the explanation of Weismann; and we shall see whether Spencer was not as successful in demolishing it as Weismann was in showing that, if evolution exists at all, some other basis must be found for it than that on which it was so largely rested by Herbert Spencer.
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 useless73 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 Botany80 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?