Such being the fundamental conception of zoology, it remained only for the investigator to study each individual species with an eye to its affinities with other species, that each might be assigned by a scientific classification to the particular place in the original scheme of creation which it was destined to occupy. Once such affinities had been correctly determined and interpreted for all species, the zoological classification would be complete for all time. A survey of the completed schedule of classification would then show at a glance the details of the preconceived system in accordance with which the members of the animal kingdom were created, and zoology would be a "finished" science.

In the application of this relatively simple scheme, to be sure, no end of difficulties were encountered. Each higher animal is composed of so many members and organs, of such diverse variations, that naturalists could never agree among themselves as to just where a balance of affinities between resemblances and differences should be struck; whether, for example, a given species varied so much from the type species of a genus—say the genus Gothic house—as to belong properly to an independent genus—say Romanesque house; or whether, on the other hand, its divergencies were still so outweighed by its resemblances as to permit of its retention as an aberrant member of genus number one. Perpetual quibbling over these matters was quite the order of the day, no two authorities ever agreeing as to details of classification. The sole point of agreement was that preconceived types were in question—if only the zoologists could ever determine just what these types were. Meantime, the student who supposed classifications to be matters of moment, and who laboriously learned to label the animals and birds of his acquaintance with an authoritative Latin name, was perpetually obliged to unlearn what he had acquired, as a new classifier brought new resources of hair-splitting pursuit of a supposed type or ideal to bear on the subject. Where, for example, our great ornithologists of the early part of the century, such as Wilson and Audubon, had classed all our numerous hawks in a genus falco, later students split the group up into numerous genera—just how many it is impossible to say, as no two authorities agreed on that point. Wilson, could he have come back a generation after his death, would have found himself quite at a loss to converse with his successors about the birds he knew and loved so well, using their technical names—though the birds themselves had not changed.

Notwithstanding all the differences of opinion about matters of detail, however, there was, nevertheless, substantial agreement about the broader outlines of classifications, and it might fairly enough have been hoped that some day, when longer study had led to finer discrimination, the mysteries of all the types of creation would be fathomed. But then, while this hope still seemed far enough from realization, Charles Darwin came forward with his revolutionizing doctrine—and the whole time-honored myth of "types" of creation vanished in thin air. It became clear that the zoologists had been attempting a task utterly Sisyphean. They had sought to establish "natural groups" where groups do not exist in nature. They were eagerly peering after an ideal that had no existence outside their imagination. Their barriers of words could not be made to conform to barriers of nature, because in nature there are no barriers.

What, then, was to be done? Should the whole fabric of classification be abandoned? Clearly not, since there can be no science without classification of facts about labelled groupings, however arbitrary. Classifications then must be retained, perfected; only in future it must be remembered that any classification must be more or less arbitrary, and in a sense false; that it is at best only a verbal convenience, not the embodiment of a final ideal. If, for example, we consider the very "natural" group of birds commonly called hawks, we are quite justified in dividing this group into several genera or minor groups, each composed of several species more like one another than like the members of other groups of species—that is, of other genera. But in so doing we must remember that if we could trace the ancestry of our various species of hawks we should find that in the remote past the differences that now separate the groups had been less and less marked, and originally quite non-existent, all the various species having sprung from a common ancestor. The genera of to-day are cousin-groups, let us say; but the parents of the existing species were of one brood, brothers and sisters. And what applies to the minor groups called genera applies also, going farther into the past, to all larger groups as well, so that in the last analysis, all existing creatures being really the evolved and modified descendants of one primordial type, it may be said that all animate creation is but a single kind. In this broadened view the details of classification ceased to have the importance once ascribed to them, and the quibblings of the classifiers seem amusing rather than serious. Yet the changed point of view left the subject by no means barren of interest. For if the multitudinous creatures of the living world are but diversified twig-lets of a great tree of ascent, spread by branching from a common root, at least it is worth knowing what larger branches each group of twiglets—representing a genus, let us say—has sprung from. In particular, since the topmost twig of the tree is represented by man himself and his nearest relatives, is it of human interest to inquire just what branches and main stems will be come upon in tracing back the lineage of this particular offshoot. This attempt had, perhaps, no vast, vital importance in the utilitarian sense in which these terms are oftenest used, but at least it had human interest. Important or otherwise, it was the task that lay open to zoology, and apparently its only task, so soon as the Darwinian hypothesis had made good its status. The man who first took this task in hand, and who has most persistently and wisely followed it, and hence the man who became the recognized leader in the field of the new zoology, was, as I have already intimated, Professor Haeckel. His hypothetical tree of man's lineage, tracing the ancestry of the human family back to the earliest geological times and the lowest orders of beings, has been familiar now for just a third of a century. It was at first confessedly only a tentative genealogy, with many weak limbs and untraced branches. It was perfected from time to time, as new data came to hand, through studies of paleontology, of embryology, and of comparative anatomy. It will be of interest, then, to inquire just what is its status today and to examine briefly Professor Haeckel's own most recent pronouncement regarding it.

Perhaps it is not worth our while here to go too far down towards the root of the genealogical tree to begin our inquiry. So long as it is admitted that the remote ancestry is grounded in the lowest forms of organisms, it perhaps does not greatly matter to the average reader that there are dark places in the lineage during the period when our ancestor had not yet developed a spinal column—when, in other words, he had not attained the dignity of the lowest fish. Neither, perhaps, need we mourn greatly that the exact branch by which our reptilian or amphibian non-mammalian ancestor became the first and most primitive of mammals is still hidden in unexplored recesses of early strata. The most patrician monarch of to-day would not be greatly disturbed as to just who were his ancestors of the days of the cave-dweller. It is when we come a little nearer home that the question begins to take on its seemingly personal significance. Questions of grandparents and great-grandparents concern the patrician very closely. And so all along, the question that has interested the average casual investigator of the Darwinian theory has been the question as to man's immediate ancestor—the parents and grandparents of our race, so to speak. Hence the linking of the word "monkey" with the phrase "Darwinian theory" in the popular mind; and hence, also, the interpretation of the phrase "missing link" in relation to man's ancestry, as applying only to our ancestor and not to any other of the gaps in the genealogical chain.

What, then, is the present status of Haeckel's genealogical tree regarding man's most direct ancestor? Prom what non-human parent did the human race directly spring? That is a question that has proved itself of lasting, vital human interest. It is a question that long was answered only with an hypothesis, but which Professor Haeckel to-day professes to be able to answer with a decisive and affirmative citation not of theories but of facts. In a word, it is claimed that man's immediate ancestor is now actually upon record, that the much-heralded "missing link" is missing no longer. The principal single document, so to speak, on which this claim is based consists of the now famous skull and thigh-bone which the Dutch surgeon, Dr. Eugene Dubois, discovered in the year 1891 in the tertiary strata of the island of Java. Tertiary strata, it should be explained, had never hitherto yielded any fossils bordering on the human type, but this now famous skeleton was unmistakably akin to the human. The thigh in particular, taken by itself, would have been pronounced by any competent anatomist to be of human origin. Unquestionably the individual who bore it had been accustomed to take an erect attitude in walking. And yet the skull was far inferior in size and shape to that of any existing tribe of man—was, indeed, rather of a simian type, though, on the other hand, of about twice the capacity of any existing ape. In a word, it seemed clear that the creature whose part skeleton had been found by Dr. Dubois was of a type intermediate between the lowest existing man and the highest existing man-apes. It was, in short, the actual prototype of that hypothetical creature which Haeckel, in his genealogical tree, had christened pithecanthropus, the ape-man. As such it was christened Pithecanthropus erectus, the erect ape-man.

Now the discovery of this remarkable form did not make Professor Haeckel any more certain that some such form had existed than he was thirty years before when he christened a hypothetical subject with the title now taken by a tangible claimant. But, after all, there is something very taking about a prophecy fulfilled, and so the appearance of Pithecanthropus erectus created no small sensation in the zoological world. He was hailed by Haeckel and his followers as the veritable "missing link," and as such gained immediate notoriety. But, on the other hand, a reactionary party at once attacked him with the most bitter animadversions, denouncing him as no true ancestor of man with a bitterness that is hard to understand, considering that the origin of man from some lower form has long ceased to be matter of controversy. "Pithecanthropus is at least half an ape," they cried, with the clear implication of "anything but an ape for an ancestor!"

I confess I have always found it hard to understand just why this peculiar aversion should always be held against the unoffending ape tribe. Why it would not be quite as satisfactory to find one's ancestor in an ape as in the alternative lines of, for example, the cow, or the hippopotamus, or the whale, or the dog has always been a mystery. Yet the fact of this prejudice holds. Probably we dislike the ape because of the very patency of his human affinities. The poor relation is objectionable not so much because he is poor as because he is a relation. So, perhaps, it is not the apeness, so to speak, of the ape that is objectionable, but rather the human-ness. In any event, the aversion has been matter of common notoriety ever since the Darwinian theory became fully accepted; it showed itself now with renewed force against poor pithecanthropus. A half-score of objections were launched against him. It is needless to rehearse them now, since they were all met valiantly, and the final verdict saw the new-comer triumphantly ensconced in man's ancestral halls as the oldest sojourner there who has any title to be spoken of as "human." He is only half human, to be sure—a veritable ape-man, as his name implies—but exactly therein lies his altogether unique distinction. He is the embodiment of that "missing link" whose nonappearance had hitherto given so much comfort to the sceptical.

Perhaps some crumbs of comfort may be found by the reactionists in the fact that it is not held by Professor Haeckel, or by any other competent authority, that the link which pithecanthropus supplies welds man directly with any existing man-ape—with gorilla, chimpanzee, or orang. It is held that these highest existing apes are side branches, so to say, of the ancestral tree, who developed, in their several ways, contemporaneously with our direct ancestors, but are not themselves directly of the royal line. The existing ape that has clung closest to the direct ancestral type of our own race, it appears, is the gibbon—a creature far less objectionable in that role because of the very paucity of his human characteristics, as revealed to the casual observer. Gibbon-like fossil apes are known, in strata representing a time some millions of years antecedent to the epoch of pithecanthropus even, which are held to be directly of the royal line through which pithecanthropus, and the hypothetical Homo stupidus, and the known Homo neanderthalensis, and, lastly, proud Homo sapiens himself have descended. Thus Professor Haeckel is able to make the affirmation, as he did recently before the International Zoological Congress in Cambridge, that man's line of descent is now clearly traced, from a stage back in the Eocene time when our ancestor was not yet more than half arrived to the ape's estate, down to the time of true human development. "There no longer exists," he says, "a 'missing link.' The phyletic continuity of the primate stem, from the oldest lemurs down to man himself, is an historical fact."

It should, perhaps, be added that the force of this rather startling conclusion rests by no means exclusively upon the finding of pithecanthropus and the other fossils, nor indeed upon any paleontological evidence whatever. These, of course, furnish data of a very tangible and convincing kind; but the evidence in its totality includes also a host of data from the realms of embryology and comparative anatomy—data which, as already suggested, enabled Professor Haeckel to predicate the existence of pithecanthropus long in advance of his actual discovery. Whether the more remote gaps in the chain of man's ancestry will be bridged in a manner similarly in accord with Professor Haeckel's predications, it remains for future discoveries of zoologist and paleontologist to determine. In any event, the recent findings have added an increment of glory to that philosophical zoology of which Professor Haeckel is the greatest living exponent.

This tracing of genealogies is doubtless the most spectacular feature of the new zoology, yet it must be clear that the establishment of lines of evolution is at best merely a preparation for the all-important question, Why have these creatures, man included, evolved at all? That question goes to the heart of the new zoological philosophy. A partial answer was, of course, given by Darwin in his great doctrine of natural selection. But this doctrine, while explaining the preservation of favorable variations, made no attempt to account for the variations themselves. Professor Haeckel's contribution to the subject consisted in the revival of the doctrine of Lamarck, that individual variations, in response to environmental influences, are transmitted to the offspring, and thus furnish the material upon which, applying Darwin's principle, evolution may proceed. This Lamarck-Haeckel doctrine was under a cloud for a recent decade, during the brief passing of the Weismannian myth, but it has now emerged, and stands as the one recognized factor in the origin of those variations whose cumulative preservation through natural selection has resulted in the evolution of organic forms.

But may there not be other factors, as yet unrecognized, that supplement the Lamarckian and Darwinian principles in bringing about this marvellous evolution of beings? That, it would seem, is the most vital question that the philosophical zoology of our generation must hand on to the twentieth century. For today not even Professor Haeckel himself can give it answer.



VII. SOME MEDICAL LABORATORIES AND MEDICAL PROBLEMS

THE PASTEUR INSTITUTE

THE national egotism that characterizes the French mind is not without its compensations. It leads, for example, to the tangible recognition of the merits of the great men of the nation and to the promulgation of their names in many public ways. Thus it would be hard to mention a truly distinguished Frenchman of the older generations whose name has not been given to a street in Paris. Of the men of science thus honored, one recalls off-hand the names of Buffon, Cuvier, Geoffroy Saint-Hilaire, Pinel, Esquirol, Lamarck, Laplace, Lavoisier, Arago, Claude Bernard, Broca—indeed, one could readily extend the list to tiresome dimensions. Moreover, it is a list that is periodically increased by the addition of new names, as occasion offers, for the Parisian authorities never hesitate to rechristen a street or a portion of a street, regardless of former associations.

One of the most recent additions to this roll of fame is the name of Pasteur. The boulevard that bears that famous name is situated in a somewhat out-of-the-way corner of the city, though to reach it one has but to traverse the relatively short course of the Avenue de Breteuil from so central a position as the tomb of Napoleon. The Boulevard Pasteur itself is a not long but very spacious thoroughfare, which will some day be very beautiful, when the character of its environing buildings has somewhat changed and its quadruple rows of trees have had time for development. At present its chief distinction, in the eyes of most observers, would probably be found in the fact that it is the location of the famous fete forain at one of the annually recurring stages of the endless itinerary of that noted function. During the period of this distinction, which falls in the month of May, the boulevard becomes transformed into a veritable Coney Island of merry-go-rounds, shooting-galleries, ginger-bread booths, and clap-trap side-shows, to the endless delight of throngs of pleasure-seekers. There is no sight in all Paris worthier inspection for the foreigner than the Boulevard Pasteur offers at this season, for one gains a deep insight into the psychology of a people through observation of the infantile delight with which the adult population here throws itself into the spirit of amusements which with other nations are for the most part reserved for school-children. Only a race either in childhood or senescence, it would seem, could thus give itself over with undisguised delight to the enchantments of wooden horses, cattle, cats, and pigs; to the catching of wooden fish with hooks; to the shooting at targets that one could almost touch with the gun-muzzle, and to the grave observation of sideshow performances that would excite the risibilities of the most unsophisticated audience that could be found in the Mississippi Valley.

As we move among this light-hearted and lightheaded throng we shall scarcely escape a feeling of good-humored contempt for what seems an inferior race. It will be wholesome, therefore, for us to turn aside from the boulevard into the Rue Dotot, which leads from it near its centre, and walk a few hundred yards away from the pleasure-seekers, where an evidence of a quite different and a no less characteristic phase of the national psychology will be before us. For here, within easy sound of the jangling discords of the organs that keep time for the march of the cheveaux de bois, rises up a building that is in a sense the monument of a man who was brother in blood and in sentiment to the revellers we have just left in the boulevard, yet whose career stamped him as one of the greatest men of genius of any race or any time. That man was Louis Pasteur. The building before us is the famous institute that bears his name.

In itself this building is a simple and unimposing structure, yet of pleasing contour. It is as well placed as the surroundings permit, on a grassed terrace, a little back from the street, where a high iron fence guards it and gives it a degree of seclusion. There are other buildings visible in the rear, which, as one learns on entering, are laboratories and the like, where the rabbits and guinea-pigs and dogs that are so essential to the work of the laboratory are kept. On the terrace in front is a bronze statue of a boy struggling with a rabid dog—a reminder of the particular labor of the master-worker which led directly to the foundation of the institution. It will be remembered that it was primarily to give Pasteur a wider opportunity to apply his newly discovered treatment for the prevention of rabies that the subscription was undertaken which led finally to the erection of the buildings before us and brought the Pasteur Institute in its present form into being. Of the other aims and objects of the institution I shall speak more at length in a moment.

I have just said that the building before us is in effect the monument of the great savant. This is true in a somewhat more literal sense than might be supposed, for the body of Pasteur rests in a crypt at its base. The personal labors of the great discoverer were practically ended at the time when the institute was opened in 1888, on which occasion, as will be remembered, the scientific representatives of all nations gathered in Paris to do honor to the greatest Frenchman of his generation. He was spared to the world, however, for seven years more, during which time he fully organized the work of the institution along the lines it has since followed, and was, of course, the animating spirit of all the labors undertaken there by his devoted students and assistants. He is the animating spirit of the institution still, and it is fitting that his body should rest in the worthy mausoleum within the walls of that building whose erection was the tangible culmination of his life labors. The sarcophagus is a shrine within this temple of science which will serve to stimulate generations of workers here to walk worthily in the footsteps of the great founder of the institution. For he must be an unimaginative person indeed who, passing beneath that arch bearing the simple inscription "Ici Repose Pasteur," could descend into the simple but impressive mausoleum and stand beside the massive granite sarcophagus without feeling the same kind of mental uplift which comes from contact with a great and noble personality. The pretentious tomb of Galileo in the nave of Santa Croce at Florence, and the crowded resting-place of Newton and Darwin in Westminster Abbey, have no such impressiveness as this solitary vault where rests the body of Pasteur, isolated in death as the mightier spirits must always be in life.

AIMS AND OBJECTS OF THE PASTEUR INSTITUTE

If one chances to come to the institute in the later hours of the morning he will perhaps be surprised to find a motley company of men, women, and children, apparently of many nationalities and from varied walks of life, gathered about one of the entrances or sauntering near by. These are the most direct beneficiaries of the institution, the unfortunate victims of the bites of rabid dogs, who have come here to take the treatment which alone can give them immunity from the terrible consequences of that mishap. Rabies, or hydrophobia as it is more commonly termed with us, is well known to be an absolutely fatal malady, there being no case on record of recovery from the disease once fully established. Even the treatment which Pasteur developed and which is here carried out cannot avail to save the victim in whom the active symptoms of the malady are actually present. But, fortunately, the disease is peculiarly slow in its onset, sometimes not manifesting itself for weeks or months after the inoculation; and this delay, which formerly was to the patient a period of fearful doubt and anxiety, now suffices, happily, for the application of the protective inoculations which enable the person otherwise doomed to resist the poison and go unscathed. Thus it is that the persons who gather here each day to the number of fifty, or even one hundred, have the appearance of and the feelings of average health, though a large proportion of them bear in their systems, on arrival, the germs of a disease that would bring them speedily to a terrible end were it not that the genius of Pasteur had found a way to give them immunity. The number of persons who have been given the anti-rabic treatment here is more than twenty-five thousand. To have given safety to such an army of unfortunates is, indeed, enough merit for any single institution; but it must not be supposed that this record is by any manner of means the full measure of the benefits which the Institut Pasteur has conferred upon humanity. In point of fact, the preparation and use of the anti-rabic serum is only one of many aims of the institution, whose full scope is as wide as the entire domain of contagious diseases. Pasteur's personal discoveries had demonstrated the relation of certain lower organisms, notably the bacteria, to the contagious diseases, and had shown the possibility of giving immunity from certain of these diseases through the use of cultures of the noxious bacteria themselves. He believed that these methods could be extended and developed until all the contagious diseases, which hitherto have accounted for so startling a proportion of all deaths, were brought within the control of medical science. His deepest thought in founding the institute was to supply a tangible seat of operations for this attempted conquest, where the brilliant assistants he had gathered about him, and their successors in turn, might take a share in this great struggle, unhampered by the material drawbacks which so often confront the would-be worker in science.

He desired also that the institution should be a centre of education along the lines of its work, adding thus an indirect influence to the score of its direct achievements. In both these regards the institution has been and continues to be worthy of its founder. The Pasteur Institute is in effect a school of bacteriology, where each of the professors is at once a teacher and a brilliant investigator. The chief courses of instruction consist of two series each year of lectures and laboratory demonstrations on topics within the field of bacteriology. These courses, at which all the regular staff of the institution assist more or less, are open to physicians and other competent students regardless of nationality, and they suffice to inculcate the principles of bacteriology to a large band of seekers each year.

But more important, perhaps, than this form of educational influence is the impetus given by the institute to the researches of a small, select band of investigators who have taken up bacteriology for a life work, and who come here to perfect themselves in the final niceties of the technique of a most difficult profession. Thus such men as Calmette, the discoverer of the serum treatment of serpent-poisoning, and Yersin, famous for his researches in the prevention and cure of cholera by inoculation, are "graduates" of the Pasteur Institute. Indeed, almost all the chief laborers in this field in the world to-day, including the directors of practically all the daughter institutes bearing the same name that are now scattered all over the world, have had at least a share of their training in the mother institute here in Paris.

Of the work of the men who form the regular staff of the Pasteur Institute only a few words need be said here. Doctors Roux, Grancher, Metchnikoff, and Chamberland all had the privilege of sharing Pasteur's labors during the later years of the master's life, and each of them is a worthy follower of the beloved leader and at the same time a brilliant original investigator.*1* Roux is known everywhere in connection with the serum treatment of diphtheria, which he was so largely instrumental in developing. Grancher directs the anti-rabic department and allied fields. Metchnikoff, a Russian by birth and Parisian by adoption, is famous as the author of the theory that the white blood-corpuscles of the blood are the efficient agents in combating bacteria. Chamberland directs the field of practical bacteriology in its applications to hygiene, including the department in which protective serums are developed for the prevention of various diseases of domesticated animals, notably swine fever and anthrax. About one million sheep and half as many cattle are annually given immunity from anthrax by the serum here produced.

Of the patient and unremitting toil demanded of the investigator in this realm of the infinitely little; of the skill in manipulation, the fertility of resource, the scrupulous exactness of experiment that are absolutely prerequisite to success; of the dangers that attend investigations which deal with noxious germs, every one who knows anything of the subject has some conception, but those alone can have full comprehension who have themselves attempted to follow the devious and delicate pathways of bacteriology. But the goals to which these pathways lead have a tangibility that give them a vital interest for all the world. The hopes and expectations of bacteriology halt at nothing short of the ultimate extirpation of contagious diseases. The way to that goal is long and hard, yet in time it will be made passable. And in our generation there is no company of men who are doing more towards that end than the staff of that most famous of bacteriological laboratories the Pasteur Institute.

THE VIRCHOW INSTITUTE OF PATHOLOGY

Even were the contagious diseases well in hand, there would still remain a sufficient coterie of maladies whose origin is not due to the influence of living germs. There are, for example, many diseases of the digestive, nutritive, and excretory systems, of the heart and arteries, of the brain and nerves, and various less clearly localized abnormal conditions, that owe their origin to inherent defects of the organism, or to various indiscretions of food or drink, to unhygienic surroundings, to material injuries, or to other forms of environmental stress quite dissociated from the action of bacteria. It is true that one would need to use extreme care nowadays in defining more exactly the diseases that thus lie without the field of the bacteriologist, as that prying individual seems prone to claim almost everything within sight, and to justify his claim with the microscope; but after that instrument has done its best or worst, there will still remain a fair contingent of maladies that cannot fairly be brought within the domain of the ever-present "germ." On the other hand, all germ diseases have of course their particular effects upon the system, bringing their results within the scope of the pathologist. Thus while the bacteriologist has no concern directly with any disease that is not of bacterial origin, the pathologist has a direct interest in every form of disease whatever; in other words, bacteriology, properly considered, is only a special department of pathology, just as pathology itself is only a special department of general medicine.

Whichever way one turns in science, subjects are always found thus dovetailing into one another and refusing to be sharply outlined. Nevertheless, here as elsewhere, there are theoretical bounds that suffice for purposes of definition, if not very rigidly lived up to in practice; and we are justified in thinking of the pathologist (perhaps I should say the pathological anatomist) as the investigator of disease who is directly concerned with effects rather than with causes, who aims directly at the diseased tissue itself and reasons only secondarily to the causes. His problem is: given a certain disease (if I may be permitted this personified form of expression), to find what tissues of the body are changed by it from the normal and in what manner changed.

It requires but a moment's reflection to make it clear that a certain crude insight into the solution of this problem, as regards all common diseases, must have been the common knowledge of medical men since the earliest times. Thus not even medical knowledge was needed to demonstrate that the tissues of an in: flamed part become red and swollen; and numerous other changes of diseased tissues are almost equally patent. But this species of knowledge, based on microscopic inspection, was very vague and untrustworthy, and it was only after the advent of the perfected microscope, some three-quarters of a century ago, that pathological anatomy began to have any proper claim to scientific rank. Indeed, it was not until about the year 1865 that the real clew was discovered which gave the same impetus to pathology that the demonstration of the germ theory of disease gave at about the same time to etiology, or the study of causes of disease. This clew consisted of the final demonstration that all organic action is in the last resort a question of cellular activities, and, specifically, that all abnormal changes in any tissues of the body, due to whatever disease, can consist of nothing more than the destruction, or the proliferation, or the alteration of the cells that compose that tissue.

That seems a simple enough proposition nowadays, but it was at once revolutionary and inspiring in the day of its original enunciation some forty years ago. The man who had made the discovery was a young German physician, professor in the University of Freiburg, by name Rudolph Virchow. The discovery made him famous, and from that day to this the name of Virchow has held somewhat the same position in the world of pathology that the name of Pasteur occupied in the realm of bacteriology. Virchow was called presently to a professorship in the University of Berlin. In connection with this chair he established his famous Institute of Pathology, which has been the Mecca of all students of pathology ever since. He did a host of other notable things as well, among others, entering the field of politics, and becoming a recognized leader there no less than in science. Indeed, it seemed during the later decades of his life as if one encountered Virchow in whatever direction one turned in Berlin, and one feels that it was not without reason that his compatriots spoke of him as "the man who knows everything." To the end he retained all the alertness of intellect and the energy of body that had made him what he was. One found him at an early hour in the morning attending to the routine of his hospital duties, his lectures, and clinical demonstrations. These finished, he rushed off, perhaps to his parliamentary duties; thence to a meeting of the Academy of Sciences, or to preside at the Academy of Medicine or at some other scientific gathering. And in intervals of these diversified pursuits he was besieged ever by a host of private callers, who sought his opinion, his advice, his influence in some matter of practical politics, of statecraft, or of science, or who, perhaps, had merely come the length of the continent that they might grasp the hand of the "father of pathology."

In whatever capacity one sought him out, provided the seeking were not too presumptuous, one was sure to find the great savant approachable, courteous, even cordial. A man of multifarious affairs, he impressed one as having abundance of time for them all, and to spare. There is a leisureliness about the seeming habit of existence on the Continent that does not pertain in America, and one felt the flavor of it quite as much in the presence of this great worker as among those people who from our stand-point seem never really to work at all. This is to a certain extent explained if one visited Virchow in his home, and found to his astonishment that the world-renowned physician, statesman, pathologist, anthropologist was domiciled in a little apartment of the most modest equipment, up two flights, in a house of most unpretentious character. Everything was entirely respectable, altogether comfortable, to be sure; but it was a grade of living which a man of corresponding position in America could not hold to without finding himself quite out of step with his confreres and the subject of endless comment. But in this city of universal apartment-house occupancy and relatively low average of display in living it is quite otherwise. Virchow lived on the same plane, generally speaking, with the other scientists of Europe; it is only from the American standpoint that there is any seeming disparity between his fame and his material station in life; nor do I claim this as a merit of the American stand-point.

Be that as it may, however, our present concern lies not with these matters, but with Virchow the pathologist and teacher. To see the great scientist at his best in this role, it was necessary to visit the Institute of Pathology on a Thursday morning at the hour of nine. On the morning of our visit we found the students already assembled and gathered in clusters all about the room, examining specimens of morbid anatomy, under guidance of various laboratory assistants. This was to give them a general familiarity with the appearances of the disease-products that would be described to them in the ensuing lecture. But what is most striking about the room was the very unique method of arrangement of the desk or table on which the specimens rested. It was virtually a long-drawn-out series of desks winding back and forth throughout the entire room, but all united into one, so that a specimen passed along the table from end to end will make a zigzag tour of the room, passing finally before each person in the entire audience. To facilitate such transit, there was a little iron railway all along the centre of the table, with miniature turn-tables at the corners, along which microscopes, with adjusted specimens for examination, might be conveyed without danger of maladjustment or injury. This may seem a small detail, but it is really an important auxiliary in the teaching by demonstration with specimens for which this room was peculiarly intended. The ordinary lectures of Professor Virchow were held in a neighboring amphitheatre of conventional type.

Of a sudden there was a hush in the hum of voices, as a little, thin, frail-seeming man entered and stepped briskly to the front of the room and upon the low platform before the blackboard in the corner. A moment's pause for the students to take their places, and the lecturer, who of course was Virchow himself, began, in a clear, conversational voice, to discourse on the topic of the day, which chanced to be the formation of clots in blood-vessels. There was no particular attempt at oratory; rather the lecturer proceeded as if talking man to man, with no thought but to make his meaning perfectly clear. He began at once putting specimens in circulation, as supplied on his demand by his assistants from a rather grewsome-looking collection before him. Now he paused to chaff the assistant who was making the labels, poking good-humored jokes at his awkwardness, but with no trace of sting. Again he became animated, his voice raised a little, his speech more vehement, as he advanced his own views on some contested theory or refuted the objections that some opponent had urged against him, always, however, with a smile lurking about his eyes or openly showing on his lips.

Constantly the lecturer turned to the blackboard to illustrate with colored, crayons such points of his discourse as the actual specimens in circulation might leave obscure. Everything must be made plain to every hearer or he would not be satisfied. One can but contrast such teaching as this with the lectures of the average German professor, who seems not to concern himself in the least as to whether anything is understood by any one. But Virchow had the spirit of the true teacher. He had the air of loving his task, old story as it was to him. Most of his auditors were mere students, yet he appealed to them as earnestly as if they were associates and equals. He seemed to try to put himself on their level—to make his thought near to them. Physically he was near to them as he talked, the platform on which he stood being but a few inches in height, and such physical nearness conduces to a familiarity of discourse that is best fitted for placing lecturer and hearers en rapport. All in all, appealing as it does almost equally to ear and eye, it is a type of what a lecturer should be. Not a student there but went away with an added fund of information, which is far more than can be said of most of the lectures in a German university.

Needless to say, there are other departments to the Institute of Pathology. There are collections of beautifully preserved specimens for examination; rooms for practical experimentation in all phases of the subject, the chemical side included; but these are not very different from the similar departments of similar institutions everywhere. What was unique and characteristic about this institution was the personality of the director. Now he is gone, but his influence will not soon be forgotten. The pupils of a great teacher are sure to carry forward the work somewhat in the spirit of the master for at least a generation.

THE BERLIN INSTITUTE OP HYGIENE

I purposely refrain from entering into any details as to the character of the technical work done at the Virchow Institute, because the subject of pathology, despite its directly practical bearings, is in itself necessarily somewhat removed from the knowledge of the general reader. One cannot well understand the details of changes in tissues under abnormal conditions unless one first understands the normal conditions of the tissues themselves, and such knowledge is reserved for the special students of anatomy. For the nonprofessional observer the interest of the Virchow Institute must lie in its general scope rather than in the details of the subjects there brought under investigation, which latter have, indeed, of necessity, a somewhat grewsome character despite the beneficent results that spring from them. It is quite otherwise, however, with the work of the allied institution of which I now come to speak. The Institute of Hygiene deals with topics not very remote from those studied in the Virchow Institute, part of its work, indeed, falling clearly within the scope of pathology; but it differs in being clearly comprehensible to the general public and of immediate and tangible interest from the most strictly utilitarian stand-point, hygiene being, in effect, the tangible link between the more abstract medical sciences and the affairs of every-day life.

The Institute of Hygiene has also the interest that always attaches to association with a famous name, for it was here that Professor Koch made the greater part of those investigations which made his name the best known, next to that of Pasteur, of any in the field of bacteriology. In particular, the researches on the cholera germ, and those even more widely heralded researches that led to the discovery of the bacillus of tuberculosis, and the development of the remedy tuberculin, of which so much was at first expected, were made by Professor Koch in the laboratories of the antiquated building which was then and is still the seat of the Institute of Hygiene. More recently Professor Koch has severed his connection with the institution after presiding over it for many years, having now a semi-private laboratory just across from the Virchow Institute, in connection with the Charite Hospital; but one still thinks of the Institute of Hygiene as peculiarly the "Koch Institute" without injustice, so fully does its work follow the lines laid out for it by the great leader.

But however much the stamp of any individual personality may rest upon the institute, it is officially a department of the university, just as is the Virchow Institute. Like the latter, also, its local habitation is an antiquated building, strangely at variance, according to American ideas, with its reputation, though by no means noteworthy in this regard in the case of a German institution. It is situated in a part of the city distant from any other department of the university, and there is nothing about it exteriorly to distinguish it from other houses of the solid block in which it stands. Interiorly, it reminds one rather of a converted dwelling than a laboratory proper. Its rooms are well enough adapted to their purpose, but they give one the impression of a makeshift. The smallest American college would be ill-satisfied with such an equipment for any department of its work. Yet in these dingy quarters has been accomplished some of the best work in the new science of bacteriology that our century will have to boast.

The actual equipment of the bacteriological laboratory here is not, indeed, quite as meagre as it seems at first, there being numerous rooms, scattered here and there, which in the aggregate give opportunity for work to a large number of investigators, though no single room makes an impressive appearance. There is one room, however, large enough to give audience to a considerable class, and here lectures were given by Professor Koch and continue to be given by his successors to the special students of bacteriology who come from all over the world, as well as to the university students who take the course as a part of their regular medical curriculum. In regard to this feature of its work, the Institute of Hygiene differs in no essential respect from the Pasteur Institute and other laboratories of bacteriology. The same general routine of work pertains: the patient cultivation of the minute organisms in various mediums, their careful staining by special processes, and their investigation under the microscope mark the work of the bacteriologist everywhere. Many details of the special methods of culture or treatment originated here with Professor Koch, but such matters are never kept secret in science, so one may see them practised quite as generally and as efficiently in other laboratories as in this one. Indeed, it may frankly be admitted that, aside from its historical associations with the pioneer work in bacteriology, which will always make it memorable, there is nothing about the bacteriological laboratory here to give it distinction over hundreds of similar ones elsewhere; while in point of technical equipment, as already noted, it is remarkable rather for what it lacks than for what it presents.

The department of bacteriology, however, is only one of several important features of the institute. One has but to ascend another flight of stairs to pass out of the sphere of the microbe and enter a department where attention is directed to quite another field. We have now come to what may be considered the laboratory of hygiene proper, since here the investigations have to do directly with the functionings of the human body in their relations to the every-day environment. Here again one is struck with the meagre equipment with which important results may be attained by patient and skilled investigators. In only one room does one find a really elaborate piece of apparatus. This exceptional mechanism consists essentially of a cabinet large enough to give comfortable lodgment to a human subject—a cabinet with walls of peculiar structure, partly of glass, and connected by various pipes with sundry mysterious-seeming retorts. This single apparatus, however, is susceptible of being employed for the investigation of an almost endless variety of questions pertaining to the functionings of the human body considered as a working mechanism.

Thus, for example, a human subject to be experimented upon may remain for an indefinite period within this cabinet, occupied in various ways, taking physical exercise, reading, engaged in creative mental labor, or sleeping. Meantime, air is supplied for respiration in measured quantities, and of a precisely determined composition, as regards chemical impurities, moisture, and temperature. The air after passing through the chamber being again analyzed, the exact constituents added to it as waste products of the human machine in action under varying conditions are determined. It will readily be seen that by indefinitely varying the conditions of such experiments a great variety of data may be secured as to the exact physiological accompaniments of various bodily and mental activities. Such data are of manifest importance to the physiologist and pathologist on the one hand, while at the same time having a direct bearing on such eminently practical topics as the construction of shops, auditoriums, and dwellings in reference to light, heat, and ventilation. It remains only for practical architecture to take advantage of the unequivocal data thus placed at its disposal—an opportunity of which practical architecture, in Germany as elsewhere on the Continent, has hitherto been very slow to avail itself.

THE MUSEUM OF HYGIENE

The practical lessons thus given in the laboratory are supplemented in an even more tangible manner, because in a way more accessible to the public, in another department of the institution which occupies a contiguous building, and is known as the Museum of Hygiene. This, unlike the other departments of the institute, is open to the general public on certain days of each week, and it offers a variety of exhibits of distinctly novel character and of high educational value. The general character of the exhibits may be inferred from the name, but perhaps the scope is even wider than might be expected. In a word, it may be said that scarcely anything having to do with practical hygiene has been overlooked. Thus one finds here numberless models of dwelling-houses, showing details of lighting, heating, and ventilation; models not merely of individual dwellings, but also of school-buildings, hospitals, asylums, and even prisons. Sometimes the models represent merely ideal buildings, but more generally they reproduce in miniature actual habitations. In the case of the public buildings, the model usually includes not merely the structures themselves but the surroundings—lawns, drives, trees, out-buildings—so that one can get a very good idea of the more important hospitals, asylums, and prisons of Germany by making a tour of the Museum of Hygiene. Regarding the details of structure, one can actually gain a fuller knowledge in many cases than he could obtain by actual visits to the original institutions themselves.

The same thing is true of various other features of the subjects represented. Thus there is a very elaborate model here exhibited of the famous Berlin system of sewage-disposal. As is well known, the essential features of this system consist of the drainage of sewage into local reservoirs, from which it is forced by pumps, natural drainage not sufficing, to distant fields, where it is distributed through tile pipes laid in a network about a yard beneath the surface of the soil. The fields themselves, thus rendered fertile by the waste products of the city, are cultivated, and yield a rich harvest of vegetables and grains of every variety suitable to the climate. The visitor to this field sees only rich farms and market-gardens under ordinary process of cultivation. The system of pipes by which the land is fertilized is as fully hidden from his view as are, for example, the tributary sewage-pipes beneath the city pavements. The average visitor to Berlin knows nothing, of course, about one or the other, and goes away, as he came, ignorant of the important fact that Berlin has reached a better solution of the great sewage problem than has been attained by any other large city. Such, at least, is likely to be the case unless the sight-seer chance to pay a visit to the Museum of Hygiene, in which case a few minutes' inspection of the model there will make the matter entirely clear to him. It is to be regretted that the authorities of other large cities do not make special visits to Berlin for this purpose; though it should be added that some of them have done so, and that the Berlin system of "canalization" has been adopted in various places in America. But many others might wisely follow their example, notably the Parisians, whose sewerage system, despite the boasted exhibition canal-sewer, is, like so many other things Parisian, of the most primitive character and a reproach to present-day civilization.

It may be added that there are plenty of things exhibited in this museum which the Germans themselves might study to advantage, for it must be understood that the other hygienic conditions pertaining to Berlin are by no means all on a par with the high modern standard of the sewerage system. In the matter of ventilation, for example, one may find admirable models in the museum, showing just how the dwelling and shop and school-room should make provision for a proper supply of pure air for their occupants. But if one goes out from the museum and searches in the actual dwelling or shop or school-room for the counterparts of these models, one will be sorely puzzled where to find them. The general impression which a casual inspection will leave in his mind is that the word ventilation must be as meaningless to the German mind as it is, for example, to the mind of a Frenchman or an Italian. This probably is not quite just, since the German has at least reached the stage of having museum models of ventilated houses, thus proving that the idea does exist, even though latent, in his mental equipment, whereas the other continental nationalities seem not to have reached even this incipient stage of progress. All over Europe the people fear a current of air as if veritable miasm must lurk in it. They seem quite oblivious to any systematic necessity for replenishing the oxygen supply among large assemblies, as any one can testify who has, for example, visited their theatres or schools. And as to the private dwellings, after making them as nearly air-tight as practicable, they endeavor to preserve the status quo as regards air supply seemingly from season to season. They even seem to have passed beyond a mere negative regard for the subject of fresh air, inasmuch as they will bravely assure you that to sleep in a room with an open window will surely subject you to the penalty of inflamed eyes.

In a country like France, where the open fireplace is the usual means employed to modify the temperature (I will not say warm the room), the dwellings do of necessity get a certain amount of ventilation, particularly since the windows are not usually of the best construction. But the German, with his nearly air-tight double windows and his even more nearly sealed tile stove, spends the winter in an atmosphere suggestive of the descriptions that arctic travellers give us of the air in the hut of an Eskimo. It is clear, then, that the models in the Museum of Hygiene have thus far failed of the proselyting purpose for which they were presumably intended. How it has chanced that the inhabitants of the country maintain so high an average of robust health after this open defiance is a subject which the physiological department of the Institute of Hygiene might well investigate.

Even though the implied precepts of the Museum of Hygiene are so largely disregarded, however, it must be admitted that the existence of the museum is a hopeful sign. It is a valuable educational institution, and if its salutary lessons are but slowly accepted by the people, they cannot be altogether without effect. At least the museum proves that there are leaders in science here who have got beyond the range of eighteenth-century thought in matters of practical living, and the sign is hopeful for the future, though its promise will perhaps not be fulfilled in our generation.



VII. SOME UNSOLVED SCIENTIFIC PROBLEMS

IN recent chapters we have witnessed a marvellous development in many branches of pure science. In viewing so wonderfully diversified a field, it has of course been impossible to dwell upon details, or even to glance at every minor discovery. At best one could but summarize the broad sweep of progress somewhat as a battle might be described by a distant eye-witness, telling of the general direction of action, of the movements of large masses, the names of leaders of brigades and divisions, but necessarily ignoring the lesser fluctuations of advance or recession and the individual gallantry of the rank and file. In particular, interest has centred upon the storming of the various special strongholds of ignorant or prejudiced opposition, which at last have been triumphantly occupied by the band of progress. In each case where such a stronghold has fallen, the victory has been achieved solely through the destructive agency of newly discovered or newly marshalled facts—the only weapons which the warrior of science seeks or cares for. Facts must be marshalled, of course, about the guidon of a hypothesis, but that guidon can lead on to victory only when the facts themselves support it. Once planted victoriously on the conquered ramparts the hypothesis becomes a theory—a generalization of science—marking a fresh coign of vantage, which can never be successfully assailed unless by a new host of antagonistic facts. Such generalizations, with the events leading directly up to them, have chiefly occupied our attention.

But a moment's reflection makes it clear that the battle of science, thus considered, is ever shifting ground and never ended. Thus at any given period there are many unsettled skirmishes under way; many hypotheses are yet only struggling towards the stronghold of theory, perhaps never to attain it; in many directions the hosts of antagonistic facts seem so evenly matched that the hazard of war appears uncertain; or, again, so few facts are available that as yet no attack worthy the name is possible. Such unsettled controversies as these have, for the most part, been ignored in our survey of the field. But it would not be fair to conclude our story without adverting to them, at least in brief; for some of them have to do with the most comprehensive and important questions with which science deals, and the aggregate number of facts involved in these unfinished battles is often great, even though as yet the marshalling has not led to final victory for any faction. In some cases, doubtless, the right hypothesis is actually in the field, but its supremacy not yet conclusively proved—perhaps not to be proved for many years or decades to come. Some of the chief scientific results of the nineteenth century have been but the gaining of supremacy for hypotheses that were mere forlorn hopes, looked on with general contempt, if at all heeded, when the eighteenth century came to a close—witness the doctrines of the great age of the earth, of the immateriality of heat, of the undulatory character of light, of chemical atomicity, of organic evolution. Contrariwise, the opposite ideas to all of these had seemingly a safe supremacy until the new facts drove them from the field. Who shall say, then, what forlorn hope of to-day's science may not be the conquering host of to-morrow? All that one dare attempt is to cite the pretensions of a few hypotheses that are struggling over the still contested ground.

SOLAR AND TELLURIC PROBLEMS

Our sun being only a minor atom of the stellar pebble, solar problems in general are of course stellar problems also. But there are certain special questions regarding which we are able to interrogate the sun because of his proximity, and which have, furthermore, a peculiar interest for the residents of our little globe because of our dependence upon this particular star. One of the most far-reaching of these is as to where the sun gets the heat that he gives off in such liberal quantities. We have already seen that Dr. Mayer, of conservation-of-energy fame, was the first to ask this question. As soon as the doctrine of the persistence and convertibility of energy was grasped, about the middle of the century, it became clear that this was one of the most puzzling of questions. It did not at all suffice to answer that the sun is a ball of fire, for computation showed that, at the present rate of heat-giving, if the sun were a solid mass of coal, he would be totally consumed in about five thousand years. As no such decrease in size as this implies had taken place within historic times, it was clear that some other explanation must be sought.

Dr. Mayer himself hit upon what seemed a tenable solution at the very outset. Starting from the observed fact that myriads of tiny meteorites are hurled into the earth's atmosphere daily, he argued that the sun must receive these visitants in really enormous quantities—sufficient, probably, to maintain his temperature at the observed limits. There was nothing at all unreasonable about this assumption, for the amount of energy in a swiftly moving body capable of being transformed into heat if the body be arrested is relatively enormous. Thus it is calculated that a pound of coal dropped into the sun from the mathematician's favorite starting-point, infinity, would produce some six thousand times the heat it could engender if merely burned at the sun's surface. In other words, if a little over two pounds of material from infinity were to fall into each square yard of the sun's surface each hour, his observed heat would be accounted for; whereas almost seven tons per square yard of stationary fuel would be required each hour to produce the same effect.

In view of the pelting which our little earth receives, it seemed not an excessive requisition upon the meteoric supply to suppose that the requisite amount of matter may fall into the sun, and for a time this explanation of his incandescence was pretty generally accepted. But soon astronomers began to make calculations as to the amount of matter which this assumption added to our solar system, particularly as it aggregated near the sun in the converging radii, and then it was clear that no such mass of matter could be there without interfering demonstrably with the observed course of the interior planets. So another source of the sun's energy had to be sought. It was found forthwith by that other great German, Helmholtz, who pointed out that the falling matter through which heat may be generated might just as well be within the substance of the sun as without—in other words, that contraction of the sun's heated body is quite sufficient to account for a long-sustained heat-supply which the mere burning of any known substance could not approach. Moreover the amount of matter thus falling towards the sun's centre being enormous—namely, the total substance of the sun—a relatively small amount of contraction would be theoretically sufficient to keep the sun's furnace at par, so to speak.

At first sight this explanation seemed a little puzzling to many laymen and some experts, for it seemed to imply, as Lord Kelvin pointed out, that the sun contracts because it is getting cooler, and gains heat because it contracts. But this feat is not really as paradoxical as it seems, for it is not implied that there is any real gain of heat in the sun's mass as a whole, but quite the reverse. All that is sought is an explanation of a maintenance of heat-giving capacity relatively unchanged for a long, but not an interminable, period. Indeed, exactly here comes in the novel and startling feature of. Helmholtz's calculation. According to Mayer's meteoric hypothesis, there were no data at hand for any estimate whatever as to the sun's permanency, since no one could surmise what might be the limits of the meteoric supply. But Helmholtz's estimate implied an incandescent body cooling—keeping up a somewhat equable temperature through contraction for a time, but for a limited time only; destined ultimately to become liquid, solid; to cool below the temperature of incandescence—to die. Not only so, but it became possible to calculate the limits of time within which this culmination would probably occur. It was only necessary to calculate the total amount of heat which could be generated by the total mass of our solar system in falling together to the sun's centre from "infinity" to find the total heat-supply to be drawn upon. Assuming, then, that the present observed rate of heat-giving has been the average maintained in the past, a simple division gives the number of years for which the original supply is adequate. The supply will be exhausted, it will be observed, when the mass comes into stable equilibrium as a solid body, no longer subject to contraction, about the sun's centre—such a body, in short, as our earth is at present.

This calculation was made by Lord Kelvin, Professor Tait, and others, and the result was one of the most truly dynamitic surprises of the century. For it transpired that, according to mathematics, the entire limit of the sun's heat-giving life could not exceed something like twenty-five millions of years. The publication of that estimate, with the appearance of authority, brought a veritable storm about the heads of the physicists. The entire geological and biological worlds were up in arms in a trice. Two or three generations before, they hurled brickbats at any one who even hinted that the solar system might be more than six thousand years old; now they jeered in derision at the attempt to limit the life-bearing period of our globe to a paltry fifteen or twenty millions.

The controversy as to solar time thus raised proved one of the most curious and interesting scientific disputations of the century. The scene soon shifted from the sun to the earth; for a little reflection made it clear that the data regarding the sun alone were not sufficiently definite. Thus Dr. Croll contended that if the parent bodies of the sun had chanced to be "flying stars" before collision, a vastly greater supply of heat would have been engendered than if the matter merely fell together. Again, it could not be overlooked that a host of meteors are falling into the sun, and that this source of energy, though not in itself sufficient to account for all the heat in question, might be sufficient to vitiate utterly any exact calculations. Yet again, Professor Lockyer called attention to another source of variation, in the fact that the chemical combination of elements hitherto existing separately must produce large quantities of heat, it being even suggested that this source alone might possibly account for all the present output. On the whole, then, it became clear that the contraction theory of the sun's heat must itself await the demonstration of observed shrinkage of the solar disk, as viewed by future generations of observers, before taking rank as an incontestable theory, and that computations as to time based solely on this hypothesis must in the mean time be viewed askance.

But the time controversy having taken root, new methods were naturally found for testing it. The geologists sought to estimate the period of time that must have been required for the deposit of the sedimentary rocks now observed to make up the outer crust of the earth. The amount of sediment carried through the mouth of a great river furnishes a clew to the rate of denudation of the area drained by that river. Thus the studies of Messrs. Humphreys and Abbot, made for a different purpose, show that the average level of the territory drained by the Mississippi is being reduced by about one foot in six thousand years. The sediment is, of course, being piled up out in the Gulf at a proportionate rate. If, then, this be assumed to be an average rate of denudation and deposit in the past, and if the total thickness of sedimentary deposits of past ages were known, a simple calculation would show the age of the earth's crust since the first continents were formed. But unfortunately these "ifs" stand mountain-high here, all the essential factors being indeterminate. Nevertheless, the geologists contended that they could easily make out a case proving that the constructive and destructive work still in evidence, to say nothing of anterior revolutions, could not have been accomplished in less than from twenty-five to fifty millions of years.

This computation would have carried little weight with the physicists had it not chanced that another computation of their own was soon made which had even more startling results. This computation, made by Lord Kelvin, was based on the rate of loss of heat by the earth. It thus resembled the previous solar estimate in method. But the result was very different, for the new estimate seemed to prove that a period of from one hundred to two hundred millions of years has elapsed since the final crust of the earth formed.

With this all controversy ceased, for the most grasping geologist or biologist would content himself with a fraction of that time. But the case for the geologist was to receive yet another prop from the studies of radio-activity, which seem to prove that the atom of matter has in store a tremendous, supply of potential energy which may be drawn on in a way to vitiate utterly all the computations to which I have just referred. Thus a particle of radium is giving out heat incessantly in sufficient quantity to raise its own weight of water to the boiling-point in an hour. The demonstrated wide distribution of radio-active matter—making it at least an open question whether all matter does not possess this property in some degree—has led to the suggestion that the total heat of the sun may be due to radio-active matter in its substance. Obviously, then, all estimates of the sun's age based on the heat-supply must for the present be held quite in abeyance. What is more to the point, however, is the fact, which these varying estimates have made patent, that computations of the age of the earth based on any data at hand are little better than rough guesses. Long before the definite estimates were undertaken, geologists had proved that the earth is very, very old, and it can hardly be said that the attempted computations have added much of definiteness to that proposition. They have, indeed, proved that the period of time to be drawn upon is not infinite; but the nebular hypothesis, to say nothing of common-sense, carried us as far as that long ago.

If the computations in question have failed of their direct purpose, however, they have been by no means lacking in important collateral results. To mention but one of these, Lord Kelvin was led by this controversy over the earth's age to make his famous computation in which he proved that the telluric structure, as a whole, must have at least the rigidity of steel in order to resist the moon's tidal pull as it does. Hopkins had, indeed, made a somewhat similar estimate as early as 1839, proving that the earth's crust must be at least eight hundred or a thousand miles in thickness; but geologists had utterly ignored this computation, and the idea of a thin crust on a fluid interior had continued to be the orthodox geological doctrine. Since Lord Kelvin's estimate was made, his claim that the final crust of the earth could not have formed until the mass was solid throughout, or at least until a honeycomb of solid matter had been bridged up from centre to circumference, has gained pretty general acceptance. It still remains an open question, however, as to what proportion the lacunas of molten matter bear at the present day to the solidified portions, and therefore to what extent the earth will be subject to further shrinkage and attendant surface contortions. That some such lacunae do exist is demonstrated daily by the phenomena of volcanoes. So, after all, the crust theory has been supplanted by a compromise theory rather than completely overthrown, and our knowledge of the condition of the telluric depths is still far from definite. If so much uncertainty attends these fundamental questions as to the earth's past and present, it is not strange that open problems as to her future are still more numerous. We have seen how, according to Professor Darwin's computations, the moon threatens to come back to earth with destructive force some day. Yet Professor Darwin himself urges that there are elements of fallibility in the data involved that rob the computation of all certainty. Much the same thing is true of perhaps all the estimates that have been made as to the earth's ultimate fate. Thus it has been suggested that, even should the sun's heat not forsake us, our day will become month-long, and then year-long; that all the water of the globe must ultimately filter into its depths, and all the air fly off into space, leaving our earth as dry and as devoid of atmosphere as the moon; and, finally, that ether-friction, if it exist, or, in default of that, meteoric friction, must ultimately bring the earth back to the sun. But in all these prognostications there are possible compensating factors that vitiate the estimates and leave the exact results in doubt. The last word of the cosmic science of our generation is a prophecy of evil—if annihilation be an evil. But it is left for the science of another generation to point out more clearly the exact terms in which the prophecy is most likely to be fulfilled.

PHYSICAL PROBLEMS

In regard to all these cosmic and telluric problems, it will be seen, there is always the same appeal to one central rule of action—the law of gravitation. When we turn from macrocosm to microcosm it would appear as if new forces of interaction were introduced in the powers of cohesion and of chemical action of molecules and atoms. But Lord Kelvin has argued that it is possible to form such a conception of the forms and space relations of the ultimate particles of matter that their mutual attractions may be explained by invoking that same law of gravitation which holds the stars and planets in their course. What, then, is this all-compassing power of gravitation which occupies so central a position in the scheme of mechanical things?

The simple answer is that no man knows. The wisest physicist of to-day will assure you that he knows absolutely nothing of the why of gravitation—that he can no more explain why a stone tossed into the air falls back to earth than can the boy who tosses the stone. But while this statement puts in a nutshell the scientific status of explanations of gravitation, yet it is not in human nature that speculative scientists should refrain from the effort to explain it. Such efforts have been made; yet, on the whole, they are surprisingly few in number; indeed, there are but two that need claim our attention here, and one of these has hardly more than historical interest. One of these is the so-called ultramundane-corpuscle hypothesis of Le Sage; the other is based on the vortex theory of matter.

The theory of Le Sage assumes that the entire universe is filled with infinitely minute particles flying in right lines in every direction with inconceivable rapidity. Every mass of tangible matter in the universe is incessantly bombarded by these particles, but any two non-contiguous masses (whether separated by an infinitesimal space or by the limits of the universe) are mutually shielded by one another from a certain number of the particles, and thus impelled towards one another by the excess of bombardment on their opposite sides. What applies to two masses applies also, of course, to any number of masses—in short, to all the matter in the universe. To make the hypothesis workable, so to say, it is necessary to assume that the "ultramundane" particles are possessed of absolute elasticity, so that they rebound from one another on collision without loss of speed. It is also necessary to assume that all tangible matter has to an almost unthinkable degree a sievelike texture, so that the vast proportion of the coercive particles pass entirely through the body of any mass they encounter—a star or world, for example—without really touching any part of its actual substance. This assumption is necessary because gravitation takes no account of mere corporeal bulk, but only of mass or ultimate solidarity. Thus a very bulky object may be so closely meshed that it retards relatively few of the corpuscles, and hence gravitates with relative feebleness—or, to adopt a more familiar mode of expression, is light in weight.

This is certainly heaping hypotheses together in a reckless way, and it is perhaps not surprising that Le Sage's conception did not at first arouse any very great amount of interest. It was put forward about a century ago, but for two or three generations remained practically unnoticed. The philosophers of the first half of our century seem to have despaired of explaining gravitation, though Faraday long experimented in the hope of establishing a relation between gravitation and electricity or magnetism. But not long after the middle of the century, when a new science of dynamics was claiming paramount importance, and physicists were striving to express all tangible phenomena intenus of matter in motion, the theory of Le Sage was revived and given a large measure of attention. It seemed to have at least the merit of explaining the facts without conflicting with any known mechanical law, which was more than could be said of any other guess at the question that had ever been made.

More recently, however, another explanation has been found which also meets this condition. It is a conception based, like most other physical speculations of the last generation, upon the hypothesis of the vortex atom, and was suggested, no doubt, by those speculations which consider electricity and magnetism to be conditions of strain or twist in the substance of the universal ether. In a word, it supposes that gravitation also is a form of strain in this ether—a strain that may be likened to a suction which the vortex atom is supposed to exert on the ether in which it lies. According to this view, gravitation is not a push from without, but a pull from within; not due to exterior influences, but an inherent and indissoluble property of matter itself. The conception has the further merit of correlating gravitation with electricity, magnetism, and light, as a condition of that strange ethereal ocean of which modern physics takes so much account. But here, again, clearly, we are but heaping hypothesis upon hypothesis, as before. Still, an hypothesis that violates no known law and has the warrant of philosophical probability is always worthy of a hearing. But we must not forget that it is hypothesis only, not conclusive theory.

The same caution applies, manifestly, to all the other speculations which have the vortex atom, so to say, for their foundation-stone. Thus Professors Stewart and Tait's inferences as to the destructibility of matter, based on the supposition that the ether is not quite frictionless; Professor Dolbear's suggestions as to the creation of matter through the development of new ether ripples, and the same thinker's speculations as to an upper limit of temperature, based on the mechanical conception of a limit to the possible vibrations of a vortex ring, not to mention other more or less fascinating speculations based on the vortex hypothesis, must be regarded, whatever their intrinsic interest, as insecurely grounded, until such time as new experimental methods shall give them another footing. Lord Kelvin himself holds all such speculations utterly in abeyance. "The vortex theory," he says, "is only a dream. Itself unproven, it can prove nothing, and any speculations founded upon it are mere dreams about a dream."*1*

That certainly must be considered an unduly modest pronouncement regarding the only workable hypothesis of the constitution of matter that has ever been imagined; yet the fact certainly holds that the vortex theory, the great contribution of the nineteenth century towards the solution of a world-old problem, has not been carried beyond the stage of hypothesis, and must be passed on, with its burden of interesting corollaries, to another generation for the experimental evidence that will lead to its acceptance or its refutation. Our century has given experimental proof of the existence of the atom, but has not been able to fathom in the same way the exact form or nature of this ultimate particle of matter.

Equally in the dark are we as to the explanation of that strange affinity for its neighbors which every atom manifests in some degree. If we assume that the power which holds one atom to another is the same which in the case of larger bodies we term gravitation, that answer carries us but a little way, since, as we have seen, gravitation itself is the greatest of mysteries. But again, how chances it that different atoms attract one another in such varying degrees, so that, for example, fluorine unites with everything it touches, argon with nothing? And how is it that different kinds of atoms can hold to themselves such varying numbers of fellow-atoms—oxygen one, hydrogen two, and so on? These are questions for the future. The wisest chemist does not know why the simplest chemical experiment results as it does. Take, for example, a water-like solution of nitrate of silver, and let fall into it a few drops of another water-like solution of hydrochloric acid; a white insoluble precipitate of chloride of silver is formed. Any tyro in chemistry could have predicted the result with absolute certainty. But the prediction would have been based purely upon previous empirical knowledge—solely upon the fact that the thing had been done before over and over, always with the same result. Why the silver forsook the nitrogen atom and grappled the atom of oxygen no one knows. Nor can any one as yet explain just why it is that the new compound is an insoluble, colored, opaque substance, whereas the antecedent ones were soluble, colorless, and transparent. More than that, no one can explain with certainty just what is meant by the familiar word soluble itself. That is to say, no one knows just what happens when one drops a lump of salt or sugar into a bowl of water. We may believe with Professor Ostwald and his followers that the molecules of sugar merely glide everywhere between the molecules of water, without chemical action; or, on the other hand, dismissing this mechanical explanation, we may say with Mendeleef that the process of solution is the most active of chemical phenomena, involving that incessant interplay of atoms known as dissociation. But these two explanations are mutually exclusive, and nobody can say positively which one, if either, is right. Nor is either theory at best more than a half explanation, for the why of the strange mechanical or chemical activities postulated is quite ignored. How is it, for example, that the molecules of water are able to loosen the intermolecular bonds of the sugar particles, enabling them to scamper apart?