Popular Science Monthly/Volume 79/September 1911/The Purpose and Some Principles of Systematic Zoology
|THE PURPOSE AND SOME PRINCIPLES OF SYSTEMATIC ZOOLOGY|
By HUBERT LYMAN CLARK
MUSEUM OF COMPARATIVE ZOOLOGY, HARVARD UNIVERSITY
SOME months ago, I had the opportunity of examining at my leisure an unpublished manuscript, dealing with a group of animals with which my own studies have made me familiar. At the same time, I had occasion to consult in connection with my work two publications, by different authors, concerning related, though not identical groups of animals. The contrasts offered by the three writers were so remarkable that numerous questions were raised in my mind, as to the motives that led to the investigations, and the principles that had governed them. So far as I can see, each paper may properly be classed as a contribution to systematic zoology and yet the three are totally unlike. In one of the published papers, the writer is wholly occupied with questions of names. He produces evidence to show that a given name is of earlier date than hitherto supposed, another is preoccupied, another was never properly defined, and still others have been erroneously used. Even though the results are disturbing, the facts brought out are interesting and the methods used are clever, but the questions arose in my mind—Is this zoology? Or is it history? Or what is it?—The other publication was utterly different. The author eschewed books entirely. He gave a series of descriptions of a number of what he designated as new species. The names he had given them were above reproach. The descriptions appeared to be lucid. The measurements seemed to be accurate. The locality from which each species came was given with more or less exactitude. But that was all! Not one species was commented on in any way. Not one was compared with any other. There was no more apparent connection between them than that between the books of a dealer's catalogue. And again the question forced itself on me—Is this zoology?—The manuscript which I examined was strikingly different from either of the published articles and yet it certainly had features in common with each of them. There were frequent references to books and names, there were descriptions of new species, and in neither respect did the writer show greater learning or skill than the authors mentioned. Yet the significance of structures and the interrelationships of the species were so illuminatingly treated that I never felt any doubt that the work was really zoology, or that any zoologist would fail to knowledge its value. But the point of view and the methods of the writer seemed so different from those of the other authors, that this question arose in my mind—Is it fair to call such work systematic zoology? And so, naturally, I fell to considering the purpose of systematic zoology and the principles which should govern it.
A century ago any one who was a zoologist was, almost of necessity, a systematist. Although Lamarck is now perhaps best known for his contribution to philosophical zoology, he was preeminent in his day as a systematic zoologist, and in some groups of invertebrates his work may be regarded as forming in large part, if not wholly, the foundation of our present classification. So too Cuvier, famed as a comparative anatomist, left as his chief monument, the great systematic work, "Le Règne Animal," in which the results of his anatomical studies were fittingly summarized. But in the days of Lamarck and Cuvier, as Agassiz pointed out in his "Essay on Classification," the fauna of Europe, so far as the larger animals were concerned, became so well known that men with a love for zoology, but without means of securing collections from foreign lands and not attracted by the minute forms of life, turned from the describing and classifying of animals to a more intensive study of those already well known. Not only did anatomy receive more adequate attention, but the habits of animals and their relation to their environment became subjects of investigation, while under the inspiring leadership of Dollinger and von Baer, embryology, virtually a new field, was opened to investigators. Then came the days of Lyell, Agassiz, Darwin, Wallace and their many illustrious contemporaries, and zoologists began to realize the magnitude of their field and the multiplicity of its problems. No such sudden enlargement of the field for zoological research had ever occurred before and probably never will again. It was natural, therefore, that many zoologists working on the frontiers of the new territory should not merely lose sight of their fellows, who had not traveled so far, or who had journeyed in different directions, but should also lose sympathy with them. Although the lazy, the easy-going, the incompetent, do as a rule lag behind when a new country is opened, those who remain in the old fields do not usually do so from laziness or incompetence, Charlatans and self-seekers are conspicuous in frontier communities, but, of course, the majority of those at the front are not such. A man's motives may be the highest and the quality of his work the best, regardless of whether he remains in the old fields or seeks to push the frontier further on. In any case, however, he should know what he is trying to do and why he is trying to do it. He should be ready and willing to give a reason for the faith that is in him—faith that the work he is doing ought to be done and that it is his work. It is eminently fitting, therefore, that we who are still busied with systematic zoology should make clear our central purpose and formulate some of the principles that guide us in our work. In attempting to do this, I am obviously speaking only for myself. I could not, even if I would, express another worker's motives or principles. The excuse for publishing my own is two-fold; the thought that they may be in some way suggestive to other systematists, and the larger hope that they may serve to increase the mutual sympathy between all classes of zoologists.
There seem to be at least three current opinions as to systematic zoology. One is that it is engaged in the vast undertaking of cataloguing the animal kingdom. Every species of animal must be listed and hence must have a name, or, at least, a number. That the names may be attached to the right animals, descriptions and figures must be published. As there are still an unknown number of unnamed species, the describing of new species is the most important part of systematic work. The remainder consists of arranging in some sort of comprehensible system the thousands of names already in use. This type of systematic work is well shown in Linné's "Systema Naturæ," the purpose of which is, frankly, to give a complete catalogue of all natural objects. The confidence that such a catalogue would be immensely useful for many purposes was a sufficient incentive to undertake the labor involved.
A second opinion of systematic zoology, which has found expression several times since the opening of the twentieth century, is at the other extreme from the preceding. This very modern view is that systematic zoology covers the whole field—morphology, physiology, embryology, histology, paleozoology, even cytology are but assistants in systematic work. As taxonomic characters occur obviously on the exterior, so they occur no less really, though obscurely, in the internal structure, in the performance of functions, in the tissues, in the development, even in the mitotic figures in the cells. As comparative study of recent animals is essential to a proper understanding of character values, so the careful study of fossils and the revelations of the geological record are supremely important for systematic work. According to this view, the systematist is not the assistant erecting the scaffolding with the aid of which the real building is to be done by the other specialists, but he is himself the master-builder and the others are the carriers of material with which he may build.
The third opinion of systematic work, held in some degree by many zoologists, though often more or less unconsciously and seldom openly avowed, is that it forms an elementary sort of study which has a certain educational value in training the eye and the judgment of those who are to become zoologists. It bears much the same relation to zoology proper that arithmetic does to what we call higher mathematics, and the really able man will not delay in it, after he has secured the training it affords. It is preeminently the field for the amateur and the untrained worker. According to this view, a piece of systematic work may well be a part of the training of all professional zoologists, but capable men will naturally go on into the supposedly more fertile fields of physiology, cytology and experimental zoology.
As is usually the case where diverse opinions clash, a measure of truth is to be found in each one of the above-mentioned three, and doubtless there are other views more or less equally true, to which I have given no expression. As a systematist I should be very glad if I could bring myself to believe that the second view, formulated above, is essentially correct but the more I have thought on the subject the more strongly I have felt that it claims too much. It juggles with words and distorts some inescapable facts. On the other hand, every systematist whose zoological horizon is not hopelessly limited must reject the first view as utterly inadequate, while he will very naturally resent the implications of the third. There can be no question that so far as it goes the first view is true; naming species and cataloguing them, and even determining the correct names to use, are a conspicuous part of systematic work still. If, however, the systematist goes no further, he can not expect high rank as a zoologist. He ought not to ignore the significance of his observed facts; he ought to welcome, even if he can not seek for, information from each and every part of the field of zoology. If he neglects or refuses to listen to the suggestions of physiology, embryology or paleontology, he is not worthy of his task. And this, it seems to me, is the very real truth in the second view mentioned. In the third view, the vital fact is that good systematic work requires more than ordinary training of the abilities to see, and to estimate the relative worth-of the facts observed. It errs in assuming that a mind so trained can not find an adequate field of usefulness in systematic zoology.
Combining the above-given truths, we find we are still far from expressing the central purpose of systematic zoology. We have only brought together a statement of certain means to be used, of certain sources of material and of certain abilities required. The end in view is hardly suggested. As the purpose of zoology is something beyond the mere knowledge of all the phenomena of animal life, seeking further the true interpretation of those phenomena and even further to the ultimate interpretation of life itself, so it seems to me, the purpose of systematic zoology reaches beyond the mere increasing of our knowledge of animal forms and seeks a true interpretation of the resemblances and differences which we find among them. Primarily, however, it deals with results and is only indirectly concerned with the methods by which those results have been attained. It deals with the travelers, the routes traveled and the destinations reached in the animal kingdom, but it leaves for other zoologists to determine the means of transportation and the causes of the traveling. Briefly we may express the motive thus:
The purpose of systematic zoology is to determine the racial characteristics, and to set forth clearly the mutual interrelationships of animals.
The validity of this statement will not be affected, I think, by either the size of the group with which the systematist may deal, or the phase of the subject which specially interests him. It applies just as well to the man who specializes in a single genus as to him who attempts to comprehend a whole order or class. The only difference is that the smaller the group, the more the worker may hope to attain his purpose, at least to some degree. The larger the group the less is it possible for the purpose to be attained. Nor does it matter whether the systematist is especially interested in new species, or in the morphology or life histories of old ones, or in the geographical or geological distribution of animals. If his purpose is to determine more accurately the racial characteristics, or to make more clear the interrelationships, of the animals with which he deals, the value of his work as systematic zoology, at least so far as it is reliable, can not be questioned. And should the day ever dawn when it can be fairly said that the purpose of systematic zoology, here formulated, has been attained, we shall have, not merely a complete catalogue, but a complete history of the animal kingdom.
Having thus defined what seems to me the purpose of systematic zoology, I hope I may be pardoned if I attempt to formulate some of the principles which it seems to me ought to govern such work. And I may say at the outset that few inexperienced workers appreciate the difficulties involved. It is no uncommon thing to hear systematic work and workers severely criticized for the instability and uncertainty of their results. Such critics forget that nature is essentially unstable and that the fundamental difficulty of the systematist is the continual variation of the material with which he deals. No doubt much descriptive work has been poorly done and unfortunately it is true that in the past some systematists have ignored their predecessors and their colleagues. But at the present day descriptive work is, as a rule, well done and is often accompanied by most accurate figures, while ignoring the work of others is remarkably uncommon and is very rarely intentional. Nature, however, is as variable as ever and the best of descriptions and figures can not deal adequately with her marvelous diversity. Moreover, in addition to the inherent difficulty of variation in his material, the systematist has to face the even more exasperating difficulty of variation in human judgment. Not only do the judgments of his non-systematic colleagues differ from his own, but on any given point the best trained students of his particular group are quite likely to differ from him and from each other. Still worse is the indubitable fact that his own judgment varies. What seems to him well-established at one time may a few months later be incredible, not so much because of new facts bearing on the matter, but because his horizon has enlarged or his point of view become fundamentally changed. As long as nature and human judgment are what they are, it is hopeless to look for perfect stability even in as artificial a thing as nomenclature, except by arbitrary decisions adopted by practically unanimous consent. No code can be devised which will meet all the needs of the case, and most zoologists will continue to call a holothurian a holothurian in spite of the codes, until it is arbitrarily agreed to call it something else. I can not see that the proposed substitution of numbers for names would tend to either greater simplicity, intelligibility or stability. The difficulty is not with the names we have given, but with the objects we have named and the judgments which interpret our definitions. Recognizing then these two fundamental difficulties at the very base of systematic work, I venture to suggest a few principles which would, I think, if universally adopted, increase the clarity and stability of our results. They are more or less generally accepted even now, and I claim no originality in setting them forth. I only hope their formulation may lead to more extended recognition. The first one may be expressed thus:
Naming and describing new species and correcting nomenclatural errors, while valuable and indeed essential, is frankly the most elementary and hence the lowest form of zoology.
The zoologist who, like myself, enjoys collections and finds an unceasing interest in the diversity of animal forms, very easily overlooks this principle, though he will rarely question it when fairly stated. He will grant that after all, names are but handles by which the ever-increasing number of animal units may be shifted and turned and made use of in zoological building, and he will probably admit that the handle is not so important as the unit. It would be well if we went further and acted on the principle that the smoother a handle is and the more perfectly it is adapted to its object, the better and more usable it is. A clumsy name or a meaningless name or a name that has no natural and inherent association with the object is not a desirable name, for it is not a well-formed handle. This is why, in my judgment, the use of the names of persons for zoological units is to be deplored. They rarely have any inherent association with the object, and after a short time they have no significance whatever. Any one looking at the animal can see why a certain oddly shaped sand-dollar was called Rotula dentata, but how many of us know who Rumphius was that the same animal should have been called by another writer Rotula rumphii? The use of place-names has more justification, especially for insular or local forms, but it has led to many meaningless or misleading names and is seldom to be recommended. There can be no doubt that badly formed and inappropriate names have been a just cause of much of the criticism of systematic work. The correcting of errors is always an ungracious task, especially where the errors are apparently trivial and obviously unintentional, and this is particularly true in systematic zoology. No reasonable person will, however, question the necessity of it or refuse to accept the correction if it is justified. But in such work, accuracy of statement, soundness of judgment, clearness of reasoning and perfect courtesy are required to an unusual degree. These qualities are, of course, essential for the best scientific work of any kind, but they are particularly so in systematic zoology, where so much work is of an elementary nature. The lack of one or more of them has depreciated the value of many an elaborate monograph. On the other hand, their presence does not alone guarantee the worth of a piece of zoological research. And this suggests the second principle I desire to emphasize.
To be of real worth and permanent value, the systematic study of any group of animals must take into account, so far as they are known, the pre- and post-natal development, the geological history and the geographical distribution of the species which compose it.
A systematic study of any group of animals which considers only the adults, even if the morphology, physiology and habits are all taken into account, can not be regarded as complete. Nature would indeed be a puzzle and interrelationships a hopeless snarl, if the stages of development were not discoverable or were meaningless. It is incredible to me that any zoologist, who has examined the evidence at all, can deny the existence of stages in both pre-and post-natal development or question the fact that those stages have some meaning. And I am unable to believe that we can even approximate the true history of any group of animals so long as those stages are ignored. Equally important is the paleontological evidence and to ignore it when it exists in any appreciable amount is indefensible and may result in deplorable error. For some reason the relationship of systematic zoology to geographical distribution has been more generally recognized than its relation to paleontology, but it seems to me obvious that in the diversification of the animal kingdom, the element of time has been fully as important as that of space. In neither case, however, is it justifiable to assume that quantity (of either time or space) is necessarily correlated with specific qualities. Discontinuous distribution, either geologically or geographically, ought never to be regarded by itself as a differential character. Either may be used as an additional character for a group otherwise structurally distinguishable, but no species (or other group) ought ever to be recognized whose identity can not be determined without knowing the locality or the geological horizon from which it came. To act on any other principle can not fail to lead to serious confusion. In view of all the evidence then which must be taken into account, it is clear that the final decision on the validity of any group must be rendered by the systematic workers in the larger group to which it belongs. It is not to be expected that physiologists or experimental zoologists or even entomologists can decide as to either the validity of a species of sea-urchin or the desirability of a genus of birds. And this leads to a third principle often overlooked.
While genera and larger groups in our systems of classification ought to be based on relationship, their delimitation is often of necessity artificial and is purely a matter of expediency or convenience.
The English zoologist Duncan maintained that it is "impossible to admit genera which are not differentiated by characters which have a decided and important physiological value." Other zoologists have ignored or distinctly repudiated this view. The difference of opinion seems to be based on a difference in the approach to the question and it is to this same difference of approach that all discussion as to the relative merits of large and small genera is due. The zoologist who is particularly occupied with the diversity of species and who has examined large numbers of individuals in the attempt to establish specific limits becomes impressed with the great importance of constancy in any given character and, when he finds a group of species which possess in common a character, or certain characters, constantly maintained, he finds it desirable to designate them by a common name and the group is to him a genus. Such a process naturally leads to numerous small genera. On the other hand, the zoologist who studies, from the morphologist's view point, relatively few specimens, representing perhaps many species, is naturally most impressed by the resemblances, and he finds that as regards characters which to him seem of physiological importance, his material divides into comparatively few groups. These he designates as genera and since the minor characters reveal a more or less marked differentiation into species his genera are naturally large. It seems to me that the difficulty is the same as would arise were one trying to decide what is a branchlet on a tree. In certain old, nearly dead trees, or in very young ones, there might be little difficulty, main branches, secondary branches, branchlets and twigs would all be sufficiently few to be distinguishable and there would be little disagreement as to the limits of the branchlets. But in most trees this would not be the case and much would depend on whether one began at the twigs and worked downward or at the branch and worked upward. In other words, while genera are, or at least ought to be, natural groups, their limits are often necessarily artificial and arbitrary. We recognize them by name for convenience and their limitation is largely to be determined by expediency. To say, as Duncan does, that it is "impossible" to recognize certain genera is an exaggeration, however inexpedient we may deem such genera to be. Wherever the number of species in a genus is, by the extension of our knowledge, so increased as to make the group unwieldy and more or less heterogeneous, it may for convenience be divided. "Whether such divisions are called genera or subgenera is again only a matter of expediency, but for my part I have never been able to see the merit of subgenera. If a group of species is not marked by sufficiently constant characters to make it recognizable, it is not entitled to a name, and if it is so marked, I fail to see why it should not be called a genus. I do not think the description of a tree would be made more lucid by an attempt to recognize sub-branchlets. It is true that the extensive use of subgenera would permit a corresponding decrease in generic names, but I do not think this would be any real advantage. We may as well face the fact that it is no longer possible for any one man, unless he be rarely gifted with the right sort of a memory, to know the principal genera of all classes of animals. Our knowledge of the animal kingdom has so expanded in the last thirty years, that even if a man specializes in the most elementary form of systematic zoology, he can not hope to have a comprehensive view of all known genera. No one will deny that this is to be regretted, but while the fault may be in part due to our systems of nomenclature, the chief blame must rest on nature and the curiosity of zoologists. The diversity of animal life and the zoologist's insatiable desire to continually increase our knowledge thereof are at the root of the trouble. It has been said that the general lack of interest in natural history among the people at large is due to our complex nomenclatural system. I doubt if interest in natural history is any less now than it has been in the past, but if it is I do not believe the fault is with our nomenclature. Granting, however, that it is, I do not see how the difficulty can be avoided. When we are told that the would-be naturalist knows a robin as Turdus migratorius and it is unfair to him to so divide Turdus that the robin has to be called by some other name, we can only reply that the inconvenience and annoyance of giving up the old name are obvious, but the progress of our knowledge of thrushes in their specific diversity has shown that the robin, when compared with the original species of the genus, is not a Turdus, and it is therefore inaccurate, to say the least, to continue to use that name. And in scientific work to be inaccurate is a more serious fault than to be annoying. The importance of accuracy in systematic work suggests a fourth principle which may be expressed as follows:
The value of a character for distinguishing species or higher groups depends chiefly on its constancy, and for indicating relationships within a group on its significance; in neither case is its conspicuousness anything more than a matter of convenience.
Few zoologists realize that the usefulness to the systematist of any characteristic of an animal depends on the end in view. Since speciesare the units with which our work is chiefly done, their most usable characters are those which will readily distinguish them from each other. But when we try to arrange them in genera and show the natural interrelationships of any group, the characters in which they are alike are of far more importance than those in which they differ. For defining species then we are often justified in using characters which seem to us trivial and of no significance. Not infrequently newly proposed species are condemned because the characters on which they are based are said to be "so trivial." When this criticism is examined, however, it will be found that it really confuses two very different things which may be illustrated as follows. A newly described bird is said to differ from another species in having the iris white instead of brown and the tail feathers tipped with cream-color instead of yellow. Such differences are certainly trivial, but experience has taught us that the color of the iris rarely, if ever, shows such a degree of diversity within a single species, and if the examination of a fair amount of material shows the two characters given to be constant, the validity of the species can hardly be questioned. If, however, the new bird is said to be characterized by a longer bill, a more markedly yellow tinge below and a greater amount of yellow on the tail feathers, these characters would not only be considered trivial but we should be justified in being skeptical of the validity of the species because experience has taught us that such characters are subject to very great diversity. If new species are to be condemned, then accuracy demands that it shall not be because the characters assigned are trivial, but because they are inconstant and hence unreliable. When we come to genera, however, while it is of course desirable that the characters should not be trivial in any sense, the essential point is that they should have some significance, either historical or physiological. These two terms are not synonymous, for such a structure as the vermiform appendix in man has a historical, though apparently not a physiological, significance. Usually, significant characters are more or less-conspicuous, and I am strongly inclined to believe that where they are internal, or otherwise difficult to ascertain, they are associated with external, or in some way obvious, characters. The formation of genera based on larval characters, or those of some other special stage of development, is greatly to be deplored, and I have no doubt that if such groups really represent natural relationships, differential characters will be found in the adults. If they are not, it seems to me clear that, notwithstanding the possibilities of what we call "parallelism" and "convergence," the characters of the special stage are temporary adaptations of variable significance. It is impossible to designate any character or group of characters which may be relied on for systematic purposes in all groups of animals. Certain characters of remarkable constancy or obvious significance in some groups are quite unreliable in others. Color is a conspicuous illustration of this important fact, for in certain groups it is very constant, or has some very evident significance, while in others it is so inconstant and diversified that it is of no use to the systematist. And this remarkable difference in the value of color may occur within the limits of a single genus. Only by extended knowledge of a group can it be determined what characters are of value for either the differentiation or the grouping of species. It is perfectly proper to speak of questioning the validity of a species and its validity must ultimately be determined by the constancy of the character or group of characters supposed to distinguish it. But it is not correct to speak of questioning the "validity" of a genus. Its desirability and the accuracy of its definition may either or both be questioned. One genus is as "valid" as another but the desirability of naming any group is a matter of opinion. And this brings us to the last of the principles I wish to formulate.
In all systematic work, the line between facts of nature and opinions of the worker should be sharply drawn; the value of the work often depends on the clearness of this line.
One of America's greatest zoologists was wont to repeat over and over again to his students these words: "The assertion that outstrips the evidence is a crime." Like most aphorisms of the kind this sentence needs some qualifications, but zoologists will hardly question the modified form that "the assertion as a fact, of what is really only an opinion, is a crime." The more unqualified the assertion, or the hazier the opinion, the greater is the crime. The opinions of a writer, particularly if based on careful observations and long experience, may be as valuable as his facts, but it ought never to be possible to confuse the two. It should be the aim of every systematic zoologist to set forth his facts so distinctly and so unmixed with opinions that any qualified worker may form accurate opinions for himself, and to so express his opinions that the justification for them in the facts of nature may be clear to all.
Having thus set forth what seem to me five of the most important principles of systematic zoology and realizing the possibility of varied shortcomings of my own with reference to them, I can only add in conclusion, as indicative of my repentance, Peccavi.