Nägeli’s Perfecting Principle
Nägeli used the term completing principle (“Vervollkommungsprincip”) to express a tendency toward perfection and specialization. Short-sighted writers, he says, have pretended to see in the use of this principle something mystical, but on the contrary it is intended that the term shall be employed in a purely physical sense. It represents the law of inertia in the organic realm. Once set in motion, the developmental process cannot stand still, but must advance in its own direction. Perfection, or completion, means nothing else than the advance to complicated structure, “but since persons are likely to attach more meaning to the word perfection than is intended, it would perhaps be better to replace it with the less objectionable word progression.”
Nägeli says that Darwin, having in view only the condition of adaptation, designates that as more complete which gives its possessor an advantage in the battle for existence. Nägeli claims that this is not the only criterion that applies to organisms, and it leaves out the most important part of the phenomenon. There are two kinds of completeness which we should keep distinctly apart: (1) the completeness of organization characterized by the complication of the structure and the most far-reaching specialization of the parts; (2) the completeness of the adaptation, present at each stage in the organization, which consists in the most advantageous development of the organism (under existing conditions) that is possible with a given complication of structure and a given division of functions.
The first of these conceptions Nägeli always calls “completeness” (Vollkommenheit), for want of a simpler and better expression; the second he calls adaptation. By way of illustrating the difference between the two, the following examples may be given. The unicellular plants and the moulds are excellently adapted each to its conditions of life, but they are much less complete in structure than an apple tree, or a grape vine. The rotifers and the leeches are well adapted to their station, but in completeness of structure they are much simpler than the vertebrates.
If we consider only organization and division of labor as the work of the completing principle, and leave for the moment adaptation out of account, we may form the following picture of the rise of the organic world. From the inorganic world there arose the simplest organic being thinkable, being little more than a drop of substance. If this underwent any change at all, it would have been necessarily in the direction of greater complication of structure; and this would constitute the first step in the upward direction. In this way Nägeli imagines the process once begun would continue. When the movement has reached a certain point, it must continue in the same direction. The organic kingdom consists, therefore, of many treelike branches, which have had a common starting-point. Not only does he suppose that organisms were once spontaneously generated, and began their first upward course of development, but the process has been repeated over and over again, and each time new series have been started on the upward course. The organic kingdom is made up, therefore, of all degrees of organization, and all these have had their origins in the series of past forms that arose and began their upward course at different times in the past. Those that are the highest forms at the present time represent the oldest series that successfully developed; the lowest forms living at the present time are the last that have appeared on the scene of action.
Organisms, as has been said, are distinguished from one another, not only in that one is simpler and another more complicated, but also in that those standing at the same stage of organization are unequally differentiated in their functions and in their structure, which is connected primarily with certain external relations which Nägeli calls adaptations.
Adaptation appears at each stage of the organization, which stage is, for a given environment, the most advantageous expression of the main type that was itself produced by internal causes. For this condition of adaptation, a sufficient cause is demanded, and this is, as Nägeli tries to show later, the result of the inherited response to the environment. In many cases this cause will continue to act until complete adaptation is gained; in other cases, the external conditions give a direction only, and the organism itself continues the movement to its more perfect condition.
The difference between the conception of the organic kingdom as the outcome of mechanical causes on the one hand, or of competition and extermination on the other hand, can be best brought out, Nägeli thinks, by the following comparison of the two respective methods of action. There might have been no competition, and no consequent extermination in the plant kingdom, if from the beginning the surface of the earth had continually grown larger in proportion as living things increased in numbers, and if animals had not appeared to destroy the plants. Under these conditions each germ could then have found room and food, and have unfolded itself without hinderance. If now, as is assumed to be the case on the Darwinian theory, individual variations had been in all directions, the developmental movement could not have gone beyond its own beginnings, and the first-formed plants would have remained swinging now on one side and now on another of the point first reached. The whole plant kingdom would have remained in its entirety at its first stage of evolution, that is, it would never have advanced beyond the stage of a naked drop of plasma with or without a membrane. But, according to the further Darwinian conception, competition, leading to extermination, is capable of bringing such a condition to a higher stage of development, since it is assumed that those individuals which vary in a beneficial direction would have an advantage over those that have not taken such a step, or have made a step backward.
If, on the other hand, under the above-mentioned conditions of unrestricted development, without competition, variations were determined by “mechanical principles,” then, according to Nägeli’s view, all plant forms that now exist would still have evolved, and would be found living at the present time, but along with all those that now exist there would be still other forms in countless numbers. These would represent those forms which have been suppressed. On Nägeli’s view competition and suppression do not produce new forms, but only weed out the intermediate forms. He says without competition the plant kingdom would be like the Milky Way; in consequence of competition the plant kingdom is like the firmament studded with bright stars.
The plant kingdom may also be compared to a branched tree, the ends of whose branches represent living species. This tree has an inordinate power of growth, and if left to itself it would produce an impenetrable tangle of interwoven branches. The gardener prevents this crowding by cutting away some of the parts, and thus gives to the tree distinct branches and twigs. The tree would be the same without the watchful trimming of the gardener, but without definite form.
Nägeli states: “From my earlier researches I believe that the external influences are small in comparison to the internal ones. I shall speak here only of the influences of climate and of food, which are generally described as the causes of change, without however any one’s having really determined whether or not a definite result can be brought about by these factors. Later I shall speak of a special class of external influences which, according to my view, bring forth beyond a doubt adaptive changes.”
The external influence of climate and of food act only as transitory factors. A rich food supply produces fat, lack of food leads to leanness, a warm summer makes a plant more aromatic, and its fruit sweeter; a cold year means less odor and sour fruit. Of two similar seeds the one sown in rich soil will produce a plant with many branches and abundance of flowers; the other, planted in sandy soil, will produce a plant without branches, with few flowers, and with small leaves. The seeds from these two plants will behave in exactly the same way; they have inherited none of the differences of their parents. Influences of this sort, even if extending over many generations, have no permanent effect. Alpine plants that have lived since the ice age under the same conditions, and have the characters of true high-mountain plants, lose these characters completely during the first summer, if transplanted to the plains. Moreover, it makes no difference whether the seed or the whole plant itself be transferred. In place of the dwarfed, unbranched growth, and the reduced number of organs, the plant when transferred to the plains shoots up in height, branches strongly, and produces numerous leaves and flowers. The plants retain their new characters as long as they live in the plain without any other new variation being observed in them.
Other characteristics also, which arise from different kinds of external influences due to different localities, such as dampness and shade, a swampy region, or different geological substrata, last only so long as the external conditions last.
These transient peculiarities make up the characters of local varieties. That they have no permanency is intelligible, since they exhibit no new characters, but the change consists mainly in the over- or under-development of those peculiarities that are dependent on external influences. The effect of these influences may be compared to an elastic rod, which, however much it may be distorted by external circumstances, returns again to its original form as soon as released.
Besides these temporary changes, due to external influences, there are many cases known in which the same plant lives under very diverse conditions and yet remains exactly the same. For example, the species of Rhododendron ferragineum lives on archæan mountains and especially where the soil is poor in calcium. Another species, Rhododendron hirsutum is found especially on soil rich in calcium. The difference in the two species has been supposed to depend on differences in the soil, and if so, we would imagine that, if transplanted for a long time, the one should change in the direction of the other. Yet it is known that the rusty rhododendron may be found in all sorts of localities, even on dry, sunny, calcareous rocks of the Apennines and of the Jura, and despite its residence in these localities, since the glacial epoch, no change whatever has taken place.
Single varieties of the large and variable genus of Hieracium have lived since the glacial period in the high regions of the Alps, Carpathians, and in the far north, and also in the plains of different geological formations, but these varieties have remained exactly the same, although on all sides there are transitional forms leading from these to other varieties.
Some parasitic species also furnish excellent illustrations of the same principle. Besides the several species of Orobanchia and of the parasitic moulds, the mistletoe deserves special mention. It lives on both birch and apple trees and on both presents exactly the same appearance; and even if it is true that mistletoe growing on conifers presents certain small deviations in its character, it is still doubtful whether, if transferred to the birch or apple tree, it would not lose these differences, thus indicating that they are not permanent.
It is a fact of general observation that, on the one hand, the same variety occurs in different localities and under different surroundings, and, on the other hand, that slightly different varieties live together in the same place and therefore under the same external conditions. It is evident, then, that food conditions have neither originated the differences nor kept them up. The rarer cases in which in different localities different varieties exist show nothing, because competition and suppression keep certain varieties from developing where it would be possible otherwise for them to exist.
Nägeli says his conclusion may be tested from another point of view. If food conditions, as is generally supposed, have a definite, i.e. a permanent, effect on the organism, then all organisms living under the same conditions should show the same characters. Indeed, it has been claimed in some instances that this is actually the case. Thus it is stated that dry localities cause plants to become hairy, and that absence of hairiness is met with in shady localities. This may apply to certain species, but in other cases exactly the reverse is true, and even the same species behaves differently in different regions, as in Hieracium. And so it is with all characteristics which are ascribed to external influences. As soon as it is supposed a discovery has been made in this direction, we may rest assured that in other cases the reverse will be found to hold. We have had, in respect to the influence of the outer world on organisms, the same experience as with the rules for the weather,—when we come to examine the facts critically there are found to be as many exceptions as confirmations of the rule.
If climatic influence has a definite effect, the entire flora of a special locality ought to have the same peculiarities, but this stands in contradiction to all the results of experience. The character of the vegetation is not determined by the environment of the plants but by their prehistoric origin, and as the result of competition. Nägeli concludes his discussion with the statement that all of our experience goes to show that the effects of external influences (climate and food) appear at once, and their results last only as long as the influences themselves last, and are then lost, leaving nothing permanent behind. This is true even when the external influences have lasted for a long time,—since the glacial epoch, for instance. We find, he claims, nothing that supports the view that such influences are inherited.
If we next examine the question of changes from internal causes, Nägeli claims that here also observation and research fail to show the origin of a new species, or even of a new variety from external causes. In the organic world little change has taken place, he believes, since the glacial epoch. Many varieties have even remained the same throughout the whole intervening time; and while it cannot be doubted that new varieties have also been formed, yet the cause of their origin cannot be empirically demonstrated. The permanent, hereditary characters, of whose origin we know something from experience, belong to the individual changes which have appeared under cultivation in the formation of domestic races. These are for the most part the result of crossing. So far as we have any definite information as to the origin of the changes, they are the result of inner, and never of external, causes. We recognize that this must be the case, since under the same external conditions individuals behave differently—in the same flower-bud some seeds give rise to plants like the parent, others to altered ones. The strawberry with a single leaflet, instead of three, arose in the last century in a single individual amongst many other ordinary plants. From the ten seeds of a pear Van Mons obtained as many different kinds of pears. The most conclusive proof of the action of inner causes is most clearly seen when the branches of the same plant differ. In Geneva a horse-chestnut bore a branch with “filled” flowers, and from this branch, by means of cuttings, this variation has been carried over all Europe. In the Botanic Garden at Munich there is a beech with small divided leaves; but one of its branches produces the common broad undivided leaves. Many such examples have been recorded which can only be explained by assuming that a cell, or a group of cells, like those from which the other branches arose, have become changed in some unknown way as the result of inner causes. The properties that are permanent and inherited are contained in the idioplasm, which the parent transmits to its offspring. A cause that permanently transforms the organism must also transform the idioplasm. How powerless, in comparison to internal causes, the external causes are is shown most conclusively in grafting. The graft, although it receives its nourishment through the stock, which may be another species, remains itself unchanged.
Nägeli makes the following interesting comparison between the development of the individual from an egg, and the evolution, or development, of the phylum. No one will doubt that the egg during the entire time of its process of transformation is guided by internal factors. Each successive stage follows with mechanical necessity from the preceding. If an animal can develop from inner causes from a drop of plasma, why should not the entire evolutionary process have also been the outcome of developmental inner causes? He admits that there is a difference in the two cases in that the plasma that forms the egg has come from another animal, and contains all the properties of the individual in a primordial condition. In the other case we must suppose that the original drop of plasma did not contain at first the primordium of definite structures, but only the ability to form such. Logically the difference is unimportant. The main point is that in the primordium of the germ a special peculiarity of the substance is present which by forming new substances grows, and changes as it grows, and the one change of necessity excites the next until finally a highly organized being is the result.
Nägeli discusses a question in this connection, which, he says, has been unnecessarily confused in the descent theory. Since we are entirely in the dark as to how much time has been required for the formation of phyla, so also are we ignorant as to how long it may have taken for each step in advance. We may err equally in ascribing too much and too little time to the process. It is, moreover, not necessary that for every step the same amount of time should have been required. On the contrary, the probability is that recognizable changes may at times follow each other rapidly, and then for a time come to a standstill,—just as in the development of the individual there are periods of more rapid and others of less rapid change.
A more difficult problem than that relating to the sort of changes the external influences bring about in the organism, is the question as to how they effect the organism, or how they act on it mechanically. This, as is well known, was answered by Darwin, who regards all organization as a problem of adaptation: only those chance variations surviving which are capable of existence, the others being destroyed. On this theory external influences have only a negative or a passive action, namely, in setting aside the unadapted individuals. Nägeli, on the other hand, looks upon some kinds of external conditions as directly giving rise to the adaptive characters of the organism. This is accomplished, he supposes, in the following ways: two kinds of influence are recognized; the direct action, which, as in inorganic nature, comes to an end when the external influences come to an end, as when cold diminishes the chemical actions in the plant; and the indirect action, generally known as a stimulus, which starts a series of molecular motions, invisible to us, but which we recognize only in their effects. Very often the stimulus starts only a reflex action, usually at the place of application.
A stimulus acting for but a short time produces no lasting effect on the idioplasm. A person stung by a wasp suffers no permanent effect from the injury. But if a stimulus acts for a long time, and through a large number of generations, then it may, even if of small strength, so change the idioplasm, that a tendency or disposition capable of being seen may be the result. This appears to be the case in regard to the action of light, which causes certain parts of the plant to turn toward it and others away from it; also for the action of gravity, which determines the downward direction of the roots. It may be claimed, perhaps, that these are the results of direct influence and not of an internal response, but this is not the case; for some plants act in exactly the opposite way, and send a stem downward, as in the case of the cleistogamous flowers of Cardamine chenopodifolia; and other plants turn away from the light. This means that the idioplasm behaves differently in different plants in response to the same stimulus.
Concerning the more visible effects of adaptation, Nägeli states that in regard to some of them there can be no question as to how they must have arisen. Protection against cold, by the formation of a thick coat of hair, is the direct result of the action of the cold on the skin of the animal. The different weapons of offence and of defence, horns, spurs, tusks, etc., have arisen, he maintains, through stimulus to those parts of the body where these structures arise.
The causes of the other adaptations, especially of those occurring in plants, are less obvious. Land plants protect themselves from drying by forming a layer of cork over the surface. The most primitive plants were water plants, which acclimated themselves little by little to moist, and then to dry, air. When they first emerged from the water the drying acted as a stimulus on the surface, and caused it to harden in the same way as a drop of glue hardens. This hardening in turn acted as a stimulus, causing a chemical transformation of the surface into a corky substance. This effect was inherited, and in this way the power to form cork originated.
Land plants have, in addition to the soft parts, the hard bast and wood which serves the mechanical purpose of supporting the soft tissues and protecting them from being injured. The arrangement of the hard parts is such as to suggest that they are the result of the action of pressures and tensions on the plant, for the strongest cells are found where there is most need for them. It is easy to imagine, Nägeli adds, that this important arrangement of the tissues is the result of external forces which brought about the result in these parts.
Nägeli accounts for the origin of twining plants as follows. Being overshadowed by other plants, the stem will grow rapidly in the damp air. Coming in contact with the stems of other plants, the delicate stem is stimulated on one side, and grows around the point of contact. This tendency becomes inherited, and the habit to twine is ultimately established.
The difference in the two sides of leaves is explained by Nägeli as the result of the difference in the illumination of the two sides. This influence of light on the leaf has been inherited. The formation of the tubular corolla that is seen in many plants visited by insects is explained as the result of the stimulus produced by the insects in looking for the pollen. The increase in the length of the proboscis of the insect is the result of the animal straining to reach the bottom of the ever elongating tube of the corolla. “The tubular corolla and the proboscis of the insect appear as though made for each other. Both have slowly developed to their present condition, the long tube from a short tube and the long proboscis from a short one.” Thus, by purely Lamarckian principles, Nägeli attempts to account for many of the adaptations between the organism and the outer world. But if this takes place, where is there left any room for the action for his so-called perfecting principle? Nägeli proceeds to show how he supposes that the two work together.
As a result of inner causes the organism would pass through a series of perfectly definite stages, J, J1, J2. But if, at any stage, external influences produced an effect on the organism so that the arrangement of the idioplasm changes in response, a new adaptation is produced. In this way new characters, not inherent in the idioplasm, may be added, and old ones be changed or lost. “In order not to be misunderstood in regard to the completing or perfecting principle I will add, that I ascribe to it no determinate action in the organism, neither in producing the long neck of the giraffe, nor the prehensile tail of the ape, neither the claws of the crab, nor the decoration of the bird of paradise. These structures are the outcome of both factors. I cannot picture to myself how external causes alone, and just as little how internal causes alone, could have changed a monad into a man.” But Nägeli goes on to say, that if at any stage of organization one of the two causes should cease to act, the other could only produce certain limited results. Thus, if external causes alone acted, the organization would remain at the same stage of completeness, but might become adapted to all kinds of external conditions—a worm, for instance, would not develop into a fish, but would remain a worm forever, although it might change its worm structure in many ways in response to external stimuli. If, on the other hand, only the completing principle acted, then without changing its adaptations the number of the cells and the size of the organs might be increased, and functions that were formerly united might become separated. Thus, without altering the character of the organism, a more highly developed (in the sense of being more specialized) organism would appear.
Nägeli, as we have just seen, has attempted to build up a conception of nature based on two assumptions, neither of which has been demonstrated to be an actual principle of development. His hypothesis appears, therefore, entirely arbitrary and speculative to a high degree. Even if it were conceivable that two such principles as these control the evolution of organisms, it still requires a good deal of imagination to conceive how the two go on working together. Moreover, it is highly probable that whole groups have evolved in the direction of greater simplification, as seen especially in the case of those groups that have become degenerate. To what principle can we refer processes of this sort?
It is certainly a strange conclusion this, at which Nägeli finally arrives, for, after strenuously combating the idea that the external factors of climate and of food have influence in producing new species, he does not hesitate to ascribe all sorts of imaginary influences to other external causes. The apparent contradiction is due, perhaps, to the fact that his experience with actual species led him to deny that the direct action of the environment produces permanent changes, while in theory he saw the necessity of adding to his perfecting principle some other factor to explain the adaptations of the new forms produced by inner causes. Nägeli seems to have felt strongly the impossibility of explaining the process of evolution and of adaptation as the outcome of the selection of chance variations, now in this direction, now in that. He seems to have felt that there must be something within the organism that is driving it ever upward, and he attempts to avoid the teleological element, which such a conception is almost certain to introduce, by postulating the inheritance of the effects of long-continued action of the environment, in so far as certain factors in the environment produce a response in the organism. Nevertheless, this combination is not one that is likely to commend itself, aside from the fact that the assumptions have no evidence to support them. Despite Nägeli’s protest that his principles are purely physical, and that there is nothing mystical in his point of view, it must be admitted that his conception, as a whole, is so vague and difficult in its application that it probably deserves the neglect which it generally receives.
Nägeli’s wide experience with living plants convinced him that there is something in the organism over and beyond the influence of the external world that causes organisms to change; and we cannot afford, I think, to despise his judgment on this point, although we need not follow him to the length of supposing that this internal influence is a “force” driving the organism forward in the direction of ever greater complexity. A more moderate estimate would be that the organism often changes through influences that appear to us to be internal, and while some of the changes are merely fluctuating or chance variations, there are others that appear to be more limited in number, but perfectly definite and permanent in character. It is the latter, which, I believe, we can safely accredit to internal factors, and which may be compared to Nägeli’s internal causes, but this is far from assuming that these changes are in the direction of greater completeness or perfection, or that evolution would take place independently of the action of external agencies.
CHAPTER X
THE ORIGIN OF THE DIFFERENT KINDS OF ADAPTATIONS
In the present chapter we may first consider, from the point of view of discontinuous variations as contrasted with the theory of the selection of individual variations, the structural adaptations of animals and plants, i.e. those cases in which the organism has a definite form that adapts it to live in a particular environment. In the second place, we may consider those adaptations that are the result of the adjustment of each individual to its surroundings. In subsequent chapters the adaptations connected with the responses of the nervous system and with the process of sexual reproduction will be considered.
It should be stated here, at the outset, that the term mutation will be used in the following chapters in a very general way, and it is not intended that the word shall convey only the idea which De Vries attaches to it; it is used rather as synonymous with discontinuous and also definite variation of all kinds. The term will be used to include “the single variations” of Darwin, “sports,” and even orthogenic variation, if this has been definite or discontinuous.