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 life[57]—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.