Popular Science Monthly/Volume 57/June 1900/The Physical Geography of the Lands
| THE PHYSICAL GEOGRAPHY OF THE LANDS. |
By Professor W. M. DAVIS.
HARVARD UNIVERSITY.
THE most important principles established in physical geography during the ninetenth century are that the description of the earth's surface features must be accompanied by explanation, and that the surface features must be correlated with their inhabitants. During the establishment of these evolutionary principles, exploration at home and abroad has greatly increased the store of recorded facts; the more civilized countries have been in large part measured and mapped; the coasts of the world have been charted; the less civilized continents have been penetrated to their centers. This harvest of fact has been an indispensable stimulus to the study of physical geography; yet it can not be doubted that the spirit which has given life to the letter of the subject is the principle of evolution—inorganic and organic. This is especially true of the geography of the lands.
The century has seen the measurement of higher peaks in the Himalayas than had been previously measured in the Andes. The Nile has been traced to its source in the lakes of equatorial Africa, verifying the traditions of the ancients; and the Kongo has been found to cross the equator twice on its way to the sea. Facts without number have been added to the previous sum of knowledge. But at the same time, it has been discovered that the valleys of mountain ranges are the work of erosion; that the product of valley erosion is often seen in extensive piedmont fluviatile plains; that waterfalls are retrogressively worn away until they are reduced to the smooth grade of a maturely established river; and that interior basins are slowly filling with the waste that is washed in from their rims upon their floors. Here are explanatory generalizations, involving, yet going far beyond matter of direct observation. Such generalizations in geography correspond to the recognition in astronomy that planetary movements exemplify the law of gravitation; they are the Newton as against the Kepler of the subject.
The sufficient justification of the demand that has now arisen for explanation and correlation in the study of land forms is found in the repeated experience that until an explanatory description of a region can be given, one may be sure that some of its significant elements pass unnoticed; and until the controls that it exerts on living forms are studied, one may be confident that its geographical value is but half measured. A sentence from Guyot's Earth and Man may here be taken as a guide: "To describe, without rising to the causes, or descending to the consequences, is no more science than merely and simply to relate a fact of which one has been a witness." There could hardly be devised a more concise and searching test of good work than this quotation suggests. The causes, in so far as the physical geography of the lands is concerned, have been learned chiefly through the study of geology; yet it does not by any means follow that all geologists are possessed of such knowledge of these causes as will constitute them geographers. The consequences have been learned through the study of evolutionary biology; yet a distinct addition to the usual discipline of biology is required in order to apprehend its geographical correlations. The limited space allowed to this article will require that further consideration of the consequences be excluded, in order to give due consideration to the causes.
One of the preparatory steps in the century's advance was taken by the German geographer, Ritter, who, near the beginning of the century, advocated a new principle that may be illustrated by the change in the definition of geography from "the description of the earth and its inhabitants" to "the study of the earth in relation to its inhabitants;" but advance beyond this beginning was for a long time obstructed by certain ancient beliefs. Theological preconceptions as to the age of the earth and the associated geological doctrine of catastrophism, although attacked by the rising school of uniformitarianism, were then dominant. They gave to the geographer a ready-made earth, on which the existing processes of change were unimportant. Furthermore, the belief in the separate creation of every organic species led to the doctrine of teleology, which maintained the predetermined fitness of the earth for its inhabitants, and of its inhabitants for their lifework. All this had to be outgrown before geographers could understand the slow development of land forms and the progressive adaptation of all living beings to their environments. Yet the beginning that Ritter made was of great importance, and it would have led further had it not happened that for many decades professors of geography in Europe brought chiefly a historical training to their chairs, to the almost entire neglect of physical geography. In the last thirty years there has been a reaction from this condition in Germany and France, but Italy, with many professors of geography in her universities, still for the most part follows historical methods.
Id the victory of the uniformitarians over the catastrophists began the fortunate alliance of geography with geology, which was long afterwards happily phrased by Mackinder: "Geology considers the past in the light of the present; geography considers the present in the light of the past." Instead of believing in cataclysmic upheavals and in overwhelming floods, Playfair and other exponents of the Huttonian school taught that mountains were slowly upheaved and slowly worn down. The simplicity of Playfair's argument finds excellent illustration in the often quoted passage regarding the origin of valleys: "Every river appears to consist of a main trunk, fed from a variety of branches, each running in a valley proportioned to its size, and all of them together 'forming a system of vallies, communicating with one another, and having such a nice adjustment of their declivities that none of them join the principal valley either on too high or too low a level; a circumstance which would be infinitely improbable if each of these vallies were not the work of the stream that flows in it." Descriptions of valleys should always recognize the share that rivers have had in eroding them, or else the "nice adjustment of their declivities" may pass unnoticed.
It should be noted, however, that to this day explanation is not always allowed an undisputed place in the treatment of the lands, however fully it is accepted as appropriate to the presentation of other divisions of physical geography. But the manner in which explanation is extending over a larger and larger part of the subject gives assurance that the geographers of the coming century will insist upon a uniformly rational treatment of all divisions of their science. The active phenomena of the earth's surface first secured explanation; it has long been considered essential to explain as well as to describe such phenomena as the winds of the air and the currents of the ocean; indeed, this is now so habitual that many geographers who may object to the explanation of a peculiar kind of a valley as a trespass upon geology, will nevertheless demand an explanation of rainfall and tides, although these truly geographical subjects are manifestly shared with physics and astronomy. Land forms of very elementary character, like deltas, or of rapid production, like volcanoes, have had to give some account of themselves all through the century; but it was not for many years after the announcement of Playfair's law, that the erosion of valleys by the rivers that drain them came to be regarded as a subject appropriate to a geographical treatise. Only in the later years of the century has the fuller treatment of this beautiful subject been attempted; even now much of it remains to be developed in the century to come.
The treatment of physical geography will be much more even, to the great advantage of its students, when explanatory description is applied to all its parts. The alluvial fans at the base of arid mountains should be accounted for as well as the dunes of deserts. The fault cliffs of broken plateau blocks and the weathered cliffs of retreating escarpments deserve to be considered as carefully as the wave-cut cliffs of coasts; the essential differences of these forms are reached most' easily through their explanation. The varied sculpturing of a mountain slope may, in time, come to be as well understood as is now the erosion of a simple valley in a low plain.
One of the most notable elements of the century's progress is the increasing breadth of view gained as explanatory descriptions are extended further and further over the geographical field. At first explanation was given to various individual features, item by item; now it is recognized that an appropriate place must be provided for all kinds of land forms in a comprehensive scheme of physiographic classification. Many instances of the earlier stage might be given, beginning with examples from the works of Humboldt, the acknowledged leader of scientific explorers in the opening decades of the century. His attempts, more or less completely successful, to explain the facts that he observed, as well as to correlate life with environment, may be traced all through his writings; but his 'Cosmos' (1845) did not reach a careful discussion of land forms, although it entered so far into an explanatory treatment as to consider the formation of mountain ranges.
Innumerable examples of isolated facts and special explanations, unrelated to a comprehensive scheme of physiographic classification, might be taken from the reports of exploring expeditions and of geological surveys; from books of travel and from geographical and geological journals with which the nineteenth century has filled so many library shelves; but lack of space will prevent mention of all sources, save a few treatises in which the accumulated knowledge of their time is summarized. Such a work as Mrs. Somerville's 'Physical Geography' (1848) gives in the early pages a brief general consideration of land forms, and then enters at once upon the areal description of the continents; later pages present a short outline of the features of rivers, and then the rivers of the world are taken up. This is as if a text-book of botany should pass rapidly over the structure and classification of plants, and devote most of its pages to the flora of different regions. Again, Klöden's compendious geography includes a volume on 'Physical Geography,' in which much material is gathered (3d ed., 1873); but the treatment is very uneven, as is natural in the absence of a good scheme of classification. Glaciers receive much attention, but valleys are rather curtly dismissed; deltas are elaborately described, but little space is given to other forms assumed by the waste of the land on the way to the sea. Ansted's 'Physical Geography' (5th ed., 1871) contains abundant fact, but much of it is a kind that is better presented on a map than in verbal form. Many pages are devoted to statistical statements, from which no student can gain inspiration for further study, for example: "The Danube receives a large number of tributaries, of which the most important are, on the right, the Isar, Inn, Raab, Drave, Save, Morave, and Isker. On the left are the Altmühl, Regen, Waag, Gran, Theiss, Temes, Aluta, Sereth, and Pruth. Many of these are large streams with other important tributaries. The Danube drains upwards of 300.000 square miles of country."
A decided advance over earlier books in the way of rational or explanatory treatment is found in the works of Peschel and Eeclus; it is to the former that a reaction against the historical treatment of geography in Germany is largely dne; while the latter is to be credited with an enlarged attention to the detail of land forms; but the books of neither of these authors recognize the systematic evolution of land forms. The same may be said of various other treatises which approach, but do not yet reach, the ideal that seems to be in sight. One of the chief responsibilities of the geographer—the description of landscape—can not be fully met by students who accept the principles set forth in these books as their guides; for in spite of the increasing attention given to the lands in modern books, and in spite of the greater number of forms recognized, the combination of all forms in a well-organized whole is not yet accomplished.
It seems to have been against the empirical method of such books as Ansted's that Huxley protested in his 'Physiography,' urging its replacement by a more educative method. He wrote:
"I do not think that a description of the earth, which commences by telling a child that it is an oblate spheriod, moving around the sun in an elliptical orbit, and ends without giving him the slightest hint towards an understanding of the ordnance map of his own country, or any suggestion as to the meaning of the phenomena offered by the brook which runs through his village, or of the gravel pit whence the roads are mended, is calculated either to interest or to instruct. . . . Physiography has very little to do with this sort of Physical Geography. My hearers were not troubled with much about latitudes and longitudes, the heights of mountains, depths of seas, or the geographical distribution of kangaroos or Compositæ. . . . I endeavored to give them. . . . a view of the 'place in nature' of a particular district of England—the basin of the Thames—and to leave upon their minds the impression that the muddy waters of our metropolitan river, the hills between which it flows, the breezes which blow over it, are not isolated phenomena, to be taken as understood because they are familiar. On the contrary, I endeavored to show that the application of the plainest and simplest processes of reasoning to any one of these phenomena suffices to show, lying behind it, a cause, which again suggests another; until, step by step, the conviction dawns upon the learner that, to attain to even an elementary conception of what goes on in his own parish, he must know something about the universe; that the pebble he kicks aside would not be what it is and where it is, unless a particular chapter of the earth's history, finished untold ages ago, had been exactly what it was. . . . Many highly valuable compendia of Physical Geography, for the use of scientific students of that subject, are extant; but in my judgment most of the elementary works I have seen begin at the wrong end, and too often terminate in an ominum gatherum of scraps of all sorts of undigested and unconnected information; thereby entirely destroying the educational value of that study which Kant justly termed the 'propædeutic of natural knowledge.'" (Preface to 'Physiography,' 1878).
Here we find clear recognition of the need of introducing a consideration of causes, just as was urged by Guyot; and furthermore a recognition of the need of linking together in their natural relations all the items which together constitute the content of the subject. It may, however, be contended that the attempt to combine in a single course of study the elementary principles of chemistry and physics, of geology and astronomy, along with those of physical geography, is not practicable from an educational point of view; such a combination will not secure either the clear knowledge or the strong discipline that can be derived from systematic courses in two or three of these subjects, presented separately. Text-books like Hinman's 'Eclectic Physical Geography' and Mill's 'Realm of Nature,' in both of which a broad range of other than geographical subjects is covered, do not seem to-day to be in so much favor as those books which attend more closely to the true content of our subject. Indeed, with respect to physical geography, considered from the scientific and educational point of view, a report on College Entrance Requirements, recently published by our National Educational Association,[1] presents the best definition and outline of the subject that has yet appeared. It advises the omission of irrelevant matter, however interesting such matter may be in itself. The principles of physics and the succession of geological formations with their fossils, the classification and distribution of plants and animals must be taught elsewhere; but much profit may be had from terrestrial phenomena by which the principles of physics are illustrated, and from the consequences of past geological changes in determining present geographical conditions, and especially from the physiographic controls by which the distribution of organic forms is determined.
The general scheme under which all land forms may receive explanatory description must consider chiefly the movement and erosion of the earth's crust. Deformation offers a part of the earth's crust to be worked upon. Various destructive processes of erosion work upon the offered mass, and the streams, with their transported waste, follow the depressions in the carved surface. So important is the element of erosion, and so leading is the part played by rivers in erosive work, that McGee would gather all land forms under a classification determined by their drainage systems.[2] Others have preferred a classification based, first on peculiarities of structure as determined by accumulation and deformation; and, secondly, on the progress of erosion; but in either scheme, the erosive work of rivers is so important that a sketch of the progress of the physical geography of the lands towards a systematic classification of its items may well follow the order in which valleys have been explained, branching off, as occasion may require, from the leading theme of rivers that flow under a normal humid climate to special conditions of erosion under an arid or a frigid climate. The progress which has made the physical geography of the lands what it is to-day is more the work of geologists than of geographers; and the chief reason for this is the indifference of many geographers to the physical side of their subject; an indifference that was undoubtedly favored by the cultivation of historical geography in continental Europe, and by the acceptance of the traveler or explorer as a full-fledged geographer in Great Britain. In the United States, it is only in the latter part of the century that the physical geography of the lands has gained a scientific standing, and the advantages that it now enjoys are geographical grafts upon a geological stock.
The emancipation of geology from the doctrine of catastrophism was a necessary step before progress could be made towards an understanding of the lands. The slow movements of elevation and depression of certain coasts in historic time were of great importance in this connection. Studies of geological structures at last overcame the belief in the sudden and violent upheaval of mountain chains, which, under the able and authoritative advocacy of Elie de Beaumont, held a place even into the second half of the century. But even when it came to be understood that mountains and plateaus have been slowly upheaved, it still remained to be proved that the valleys and canyons by which they are drained were produced by erosion, and not by fractures and unequal movements of elevation. Advance was here made on two lines. Along one, a better understanding was gained of the forms producible by deformation alone; along the other, sea currents, floods and earthquake waves, to which the earlier observers trusted as a means of modifying the forms of uplift, were gradually replaced by the slow action of weather and water. Processes of deformation were found to act in a large way, producing massive forms without detail—broad plains and plateaus, extensive domes, straight cliffs and rolling corrugations; and thus it was learned that the varied and detailed forms of lofty mountain ranges and dissected plateaus must be ascribed almost entirely to the processes of erosion. But it should be noted that in exceptional instances land forms initiated by deformation, so recently as to have suffered as yet only insignificant sculpture, may exhibit much irregularity. The most striking example of this kind, an example of the very highest value in the systematic study of land forms, is that afforded by the diversely tilted lava blocks of Southern Oregon, as described by Russell.[3]
Turning now to the second line of advance, it is noteworthy that so keen an observer as Lesley insisted, as late as 1856, that the peculiar topographical features of Pennsylvania, which he knew and described so well, could have been produced only by a great flood. But the principles of the uniformitarians were constantly gaining ground against these older ideas; and after the appearance in England of Scrope's studies in Central France and of Greenwood's polemic little work on 'Rain and Rivers' (1857), victory may be said to have been declared for the principles long before announced by Hutton and Playfair, which, since then, have obtained general acceptance and application.
Yet even the most ardent uniformitarians would, in the middle of the century, go no further than to admit that rain and rivers could roughen a region by carving valleys in it; no consideration was then given to the possibility that, with longer and longer time, the hills must be more and more consumed, the valleys must grow wider and wider open, until, however high and uneven the initial surface may have been, it must at last be reduced to a lowland of small relief. The surface of such a lowland would truncate the underground structures indifferently; but when such truncating surfaces were noticed (usually now at considerable altitudes above sea level, as if elevated after having been planed, and therefore more or less consumed by the erosion of a new system of valleys), they were called plains of marine denudation by Ramsay (1847), or plains of marine abrasion by Richthofen (1882). To-day it is recognized that both subaërial erosion and marine abrasion are theoretically competent to produce lowlands of denudation; the real question here at issue concerns the criteria by which the work of either agency can be recognized in particular instances. In the middle of the century, not only every plain of denudation, but every line of escarpments was held by the marinists to be the work of sea waves; and it was not till after a sharp debate that the bluffs of the chalk downs which enclose the Weald of southeastern England were acepted as the product of ordinary atmospheric weathering, instead of as the work of the sea. Whitaker's admirable essay on 'Subaërial Denudation,' which may be regarded as having given the victory in this discussion to the subaerialists, was considered so heterodox that it was not acceptable for publication in the Quarterly Journal of the Geological Society, of London, but had to find a place in the more modest Geological Magazine (1867), whose pages it now honors. So signal indeed was this victory that, in later years, the destructive work of the sea has been not infrequently underrated in the almost exclusive attention given to land sculpture by subaërial agencies. Truly, the sea does not erode valleys; it does not wear out narrow lowlands of irregular form between enclosing uplands, as was maintained by some of the most pronounced marinists in the middle of the century; but it certainly does attack continental borders in a most vigorous fashion, and many are the littoral forms that must be ascribed to its work, as may be learned from Richthofen's admirable Führer fur Forschungsreisende' (1886). As this problem can not be further considered here, the reader may be at once referred to the most general discussion of the subject that has yet appeared, in an essay on 'Shoreline Topography* recently published by F. P. Gulliver.[4]
At about the time when the subaerial origin of valleys and escarpments was being established in England, the explorations and surveys of our western territories were undertaken, and a flood of physiographic light came from them. One of the earliest and most important of the many lessons of the West was that Playfair's law obtained even in the case of the Grand canyon of the Colorado, which was visited by the Ives expedition in 1858. Newberry, the geologist of the expedition, concluded that both the deep and fissure-like canyon and the broader valleys enclosed by cliff-like walls "belong to a vast system of erosion, and are wholly due to the action of water." Although he bore the possibility of fractures constantly in mind and examined the structure of the canyons with all possible care, he "everywhere found evidence of the exclusive action of water in their formation." This conclusion has, since then, been amply confirmed by Powell and Button, although these later observers might attribute a significant share of the recession of cliffs in arid regions to wind action. In a later decade, Heim demonstrated that the valleys of the Alps were not explicable as the result of mountain deformation, and that they found explanation only in river erosion. By such studies as these, of which many examples could be given, the competence of rivers to carve even the deepest valleys has been fully established; yet so difficult is it to dislodge old-fashioned belief that Sir A. Geikie felt it necessary to devote two chapters in his admirable 'Scenery of Scotland' (1887) to prove that the bens of the Highlands were not so many individual upheavals, but that the glens were so many separate valleys of erosion; and as able an observer as Prestwich, a warm advocate of the erosion of ordinary valleys by their rivers, maintained (1886), with the results of our western surveys before him, that fissures were probably responsible for the origin of the deep and narrow canyons of the Colorado plateau.
The tumultuous forms of lofty mountains 'tossed up' as they seem to be when viewed from some commanding height, are, in by far the greater number of examples yet studied, undoubtedly the result of the slow erosion of the valleys between them; but it should not be forgotten that regions of very recent disturbances—as the earth counts time--may possess strong inequalities directly due to deformation. The tilted lava blocks of Oregon have already been mentioned. The bold forms of the St. Elias Alps, also described by Russell, are regarded by him as chiefly produced by the tilting of huge crustal blocks on which erosion has as yet done relatively little work. An altogether exceptional case is described by Button, who says that on the margin of one of the "high plateaus of Utah a huge block seems to have cracked off and rolled over, the beds opening with a V and forming a valley of grand dimensions." 'Rift valleys,' or trough-like depressions produced by the down-faulting of long, narrow, crustal blocks with respect to the bordering masses, are occasionally found, as in eastern Africa, where the 'Great Rift valley' has been described by Gregory. Trough-like depressions of similar origin, but much more affected by the degradation of their borders and the aggradation of their floors, are known to European geographers in the valleys of the Saône and of the middle Rhine. But no rift valley, no depression between the tilted lava blocks, resembles the branching valleys that are produced by the erosive action of running water.
Thus far, while much attention had been given to the work of rivers, little or no attention had been given to the arrangement of their courses. It seems to have been tacitly assumed that the courses of all streams were consequent upon the slope of the initial land surface. The explicit recognition of this origin, indicated by the provision of a special name, 'consequent streams,' was an important step in advance due to our western geologists. The discovery soon followed that rivers have held their courses through mountain ridges that slowly rose across their path; the rivers, concentrating the drainage of a large headwater region upon a narrow line, cut down their channels as the land was raised. This idea first came into prominence through Powell's report on the Colorado River of the West (1875), in which he gave the name, 'antecedent,' to rivers of this class. He believed that the Green river, in its passage through the Uinta mountains, was to be explained as an antecedent stream. Much doubt has, however, been thrown upon this interpretation. Other accounts of antecedent rivers have been published, and to-day the Green is not so safe a type of antecedence as the Rhine below Bingen, the Meuse in the Ardennes, or several of the Himalayan rivers in the gorges that they have cut through the youngest marginal ridges of the range.
Rapidly following the establishment of these two important classes of valleys came the recognition of the very antithesis of antecedent rivers in those streams which have grown by headward erosion along belts of weak structure, without relation to the initial trough lines. To these the term 'subsequent' has been applied. It is frequently in association with streams of this class that drainage areas are rearranged by the migration of divides, and that the upper waters of one river are captured by the headward growth of another. This is accomplished by a most beautiful process of inorganic natural selection, which leads to a survival of the fittest and thus brings about a most intimate adjustment of form to structure, whereby the more resistent rock masses come to constitute the divides, and the less resistent are cbosen for the excavation of valleys. Many workers have contributed to the solution of problems of this class; notably Heim, in his studies of the northern Alps (1876), and Löwl, who showed that, in folded mountain structures of great age, the original courses of streams might be greatly altered through the development of new lines of drainage (1882). A valuable summary of this subject is given by Philippson in his 'Studien über Wasserscheiden' (1886). The extraordinary depredations committed by the waxing Severn on the waning Thames have recently been set forth by Buckman. The turning of side branches from the slender trunk of the Meuse has been recognized in France. Many remarkable instances of stream captures have been found in the Appalachians, where the opportunity for the adjustment of streams to structures has been exceptionally good. Hayes and Campbell have, on the other hand, emphasized the importance of drainage modifications independent of the growth of subsequent streams on weak structures, but governed by a slight tilting of the region, whereby some streams are accelerated and their opponents are retarded. It should be noted that the proof of the adjustment or rearrangement of drainage marks a victory for the uniformitarian school that is even more significant than that gained in the case of the antecedent rivers; for in one case a growing mountain range is subdued by the concentrated discharge of a large drainage area; but in the other case, the mountain slowly melts away under the attacks of the weather alone on the headwater slopes of the growing valleys.
The reason why all these studies of land carving are of importance to the geographer is that they greatly enlarge the number of type forms that he may use in descriptions, and that they recognize the natural correlations among various forms which must otherwise be set forth in successive itemized statements. The brief terminology learned in early school days, somewhat enlarged by a more mature variety of adjectives, is usually the stock of words with which the explorer tries to reproduce the features of the landscapes that he crosses, and as a result his descriptions are often unintelligible; the region has to be explored again before it can become known to those who do not see it. The longitudinal relief of certain well-dissected coastal plains, or the half-buried ranges of certain interior aggraded basins, may be taken as examples of forms which are easily brought home and familiarized by explanation, but which commonly remain remote and unknown under empirical description.
It may be urged that in many geological discussions from which geography has taken profit, consideration is given to form-producing processes rather than to the forms produced. This was natural enough while the subject was in the hands of geologists; but geographers should take heed that they do not preserve the geological habit. The past history of land forms and the action upon them of various processes by which existing forms have been developed, are pertinent to geography only in so far as they aid the observation and description of the forms of to-day.
Further illustration of the growing recognition of form as the chief object of the physiographic study of the lands is seen in the use of the term, 'geomorphology' by some American writers; but more important than the term is the principle which underlies it. This is the acceptance of theorizing as an essential part of investigation in geography, just as in other sciences. All explanation involves theorizing. When theory is taken piecemeal and applied only to elementary problems, such as the origin of deltas, it does not excite unfavorable comment among geographers. But when the explanation of more complicated features is attempted, and when a comprehensive scheme of classification and treatment, in which theorizing is fully and frankly recognized, is evolved for all land forms, then the conservatives recoil, as if so bold a proposition would set them adrift on the dangerous sea of unrestrained imagination. They forget that the harbor of explanation can only be reached by crossing the seas of theory. They are willing to cruise, like the early navigators, the empirical explorers, only close along shore; not venturing to trust themselves out of sight of the land of existing fact; but they have not learned to embark upon the open ocean of investigation, trusting to the compass of logical deduction and the rudder of critical judgment to lead them to the desired haven of understanding of facts of the past.
One of the bolder explorers of the high seas of theory is Powell, who defined in the term 'baselevel' an idea that had long been more or less consciously present in the minds of geologists, and which has been since then of the greatest service to physiographers. Powell and his followers, especially Gilbert, Dutton and McGee, have consistently carried the consequences of subaërial erosion to their legitimate end in a featureless lowland, and have recognized the controlling influence of the baselevel during all the sequence of changes from the initial to the ultimate form. It is not here essential whether such a featureless lowland exists or ever has existed, but it is absolutely essential to follow the lead of deduction until all the consequences of the theory of erosion are found; and then to accept as true those theoretical deductions which successfully confront the appropriate facts of observation. Only in this way can the error of regarding geography as a purely observational natural science be corrected. Following the acceptance of the doctrine of baselevels came the method of reconstituting the original form initiated by deformation, as a means of more fully understanding the existing form; for only by beginning at the initial form can the systematic sequence of the changes wrought by destructive processes be fully traced and the existing form appreciated. This had often been done before in individual cases, but it now became a habit, an essential step in geomorphological study. Naturally enough, the terms of organic growth, such as young, mature, old, revived, and so on, came to be applied to stages in the development of inorganic forms; and thus gradually the idea of the systematic physiographic development of land forms has taken shape. This idea is to-day the most serviceable and compact summation of all the work of the century on the physical geography of the lands. It recognizes the results of deformation in providing the broader initial forms on which details are to be carved. It gives special attention to the work of destructive processes on these forms, and especially to the orderly sequence of various stages of development, recognizing that certain features are associated with youth, and others with maturity and old age. It gives due consideration to the renewed movements of deformation that may occur at any stage in the cycle of change, whereby a new sequence of change is introduced. It gives appropriate place, not only to the forms produced by the ordinary erosive action of rain and rivers, but to the forms produced by ice and by wind action as well; and it co-ordinates the changes that are produced by the sea on the margin of the land with the changes that are produced by other agencies upon its surface. It considers not only the various forms assumed by the water of the land, such as torrents, rapids, falls and lakes, appropriately arranged in a river system as to time and place, but also the forms assumed by the waste of the land, which, like the water, is on its way to the sea. In a word, it lengthens our own life, so that we may, in imagination, picture the life of a geographical area as clearly as we now witness the life of a quick-growing plant, and thus as readily conceive and as little confuse the orderly development of the many parts of a land form, its divides, cliffs, slopes and water courses, as we now distinguish the cotyledons, stem, buds, leaves, flowers and fruit of a rapidly-maturing annual that produces all these forms in appropriate order and position in the brief course of a single summer.
The time is ripe for the introduction of these ideas. The spirit of evolution has. been breathed by the students of the generation now mature all through their growing years and its application to all lines of study is demanded. It is true that the acceptance of inorganic as well as of organic evolution is often implied rather than outspoken; yet evolution. is favorably regarded, as is proved by the eagerness with which even school boards and school teachers, conservatives among conservatives, hail the appearance of books in which the new spirit of geography is revealed. In the last years of the century, the school books most widely used in this country have made great advance in the explanatory treatment of land forms. Tarr's Physical Geographies and Russell's monographic volumes on the 'Lakes,' 'Glaciers,' 'Volcanoes' and 'Rivers' of North America, all presenting land forms in an explanatory rather than an empirical manner, have been warmly welcomed in this country. Penck's 'Morphologie der Erdoberfläche' (1894), although largely concerned with the historical development of the subject, presents all forms as the result of process. De Lapparent's 'Leçons de géographie physique' (1886) treats land forms generically; and a second edition of the book is called for soon after the first. 'Earth Sculpture,' by James Geikie (1899), and Marr's 'Scientific Study of Scenery' (1900), carry modern ideas to British readers. There can be little doubt that the books of the coming century will extend the habit of explanation even further than it has yet reached.
This review of the advance of the century in the study of land forms, the habitations of all the higher forms of life, might have been concerned wholly with the concrete results of exploration, as was implied in an earlier paragraph. Travels in the Far East of the Old World, or in the Far West of the New, have yielded fact enough to fill volumes. But such a view of the century has been here replaced by another; not because the first is unimportant, for it is absolutely essential, but because the second includes the first and goes beyond it. Not the facts alone, but the principles that the facts exemplify, demand our attention. These principles, founded upon a multitude of observations, are the greater contribution of the closing to the opening century in the study of the Forms of the Land.