Chapter I. the Foundation of Ecology

THE NEED OF A SYSTEM

=1. The scope of ecology.= The clue to the field of ecology is found in the Greek word, οἲκος, home. The point of view in the following treatise is constantly that which is inherent in the term itself. Ecology is therefore considered the dominant theme in the study of plants, indeed, as the central and vital part of botany. This statement may at first appear startling, if not unfounded, but mature reflection will show that all the questions of botanical science lead sooner or later to the two ultimate facts: plant and habitat. The essential truth of this has been much obscured by detached methods of study in physiology, morphology, and histology, which are too often treated as independent fields. These have suffered incomplete and unsymmetric development in consequence of extreme specialistic tendencies. Analytic methods have dominated research to the exclusion of synthetic ones, which, in a greatly diversified field, must be final, if botanical knowledge is something to be systematized and not merely catalogued. Physiology in particular has suffered at the hands of detached specialists. Originally conceived as an inquiry into the origin and nature of plants, it has been developed strictly as a study of plant activities. It all but ignores the physical factors that control function, and the organs and tissues that reflect it.

=2. Ecology and physiology.= There can be little question in regard to the essential identity of physiology and ecology. This is evident when it is clearly seen that the present difference between the two fields is superficial. Ecology has been largely the descriptive study of vegetation; physiology has concerned itself with function; but, when carefully analyzed, both are seen to rest upon the same foundation. In each, the development is incomplete: ecology has so far been merely superficial, physiology too highly specialized. The one is chaotic and unsystematized, the other too often a minute study of function under abnormal circumstances. The greatest need of the former is the introduction of method and system, of the latter, a broadening of scope and new objectives. The growing recognition of the identity of the two makes it desirable to anticipate their final merging, and to formulate a system that will combine the good in each, and at the same time eliminate superficial and extreme tendencies. In this connection, it becomes necessary to point out to ecologist and physiologist alike that, while they have been working on the confines of the same great field, each must familiarize himself with the work and methods of the other, before his preparation is complete. Both must broaden their horizons, and rearrange their views. The ecologist is sadly in need of the more intimate and exact methods of the physiologist: the latter must take his experiments into the field, and must recognize more fully that function is but the middleman between habitat and plant. It seems probable that the final name for the whole field will be physiology, although the term ecology has distinct advantages of brevity and of meaning. In this event, however, it should be clearly recognized that, although the name remains the same, the field has become greatly broadened by new viewpoints and new methods.

HISTORICAL DEVELOPMENT

=3. Geographical distribution.= The systematic analysis of the great field of ecology is essential to its proper development in the future. A glance at its history shows that, while a number of essential points of attack have been discovered, only one or two of these have been organized, and that there is still an almost entire lack of correlation and coordination between these. The earliest and simplest development of the subject was concerned with the distribution of plants. This was at first merely an offshoot of taxonomy, and, in spite of the work of Humboldt and Schouw, has persisted in much of its primitive form to the present time, where it is represented by innumerable lists and catalogues. Geographical distribution was grounded upon the species, a fact which early caused it to become stereotyped as a statistical study of little value. This tendency was emphasized by the general practice of determining distribution for more or less artificial areas, and of instituting comparisons between regions or continents too little known or too widely remote. The fixed character of the subject is conclusively shown by the fact that it still persists in almost the original form more than a half century after Grisebach pointed out that the formation was the real unit of vegetation, and hence of distribution.

=4. The plant formation.= The corner-stone of ecology was laid by Grisebach in 1838 by his recognition of the plant formation as the fundamental feature of vegetation. Earlier writers, notably Linné (1737, 1751), Biberg (1749), and Hedenberg (1754), had perceived this relation more or less clearly, but failed to reduce it to a definite guiding principle. This was a natural result of the dominance of descriptive botany in the 18th century, by virtue of which all other lines of botanical inquiry languished. This tendency had spent itself to a certain degree by the opening of the 19th century, and both plant distribution and plant physiology began to take form. The stimulus given the former by Humboldt (1807) turned the attention of botanists more critically to the study of vegetation as a field in itself, and the growing feeling for structure in the latter led to Grisebach’s concept of the formation, which he defined as follows: “I would term a group of plants which bears a definite physiognomic character, such as a meadow, a forest, etc., a phytogeographic formation. The latter may be characterized by a single social species, by a complex of dominant species belonging to one family, or, finally, it may show an aggregate of species, which, though of various taxonomic character, have a common peculiarity; thus, the alpine meadows consist almost exclusively of perennial herbs.” The acceptance of the formation as the unit of vegetation took place slowly, but as a result of the work of Kerner (1863), Grisebach (1872), Engler (1879), Hult (1881, 1885), Goeze (1882), Beck (1884), Drude (1889), and Warming (1889), this point of view came to be more and more prevalent. It was not, however, until the appearance of three works of great importance, Warming (1895), Drude (1896), and Schimper (1898), that the concept of the formation became generally predominant. With the growing recognition of the formation during the last decade has appeared the inevitable tendency to stereotype the subject of ecology in this stage. The present need, in consequence, is to show very clearly that the idea of the formation is a fundamental, and not an ultimate one, and that the proper superstructure of ecology is yet to be reared upon this as the foundation.

=5. Plant succession.= The fact that formations arise and disappear was perceived by Biberg as early as 1749, but it received slight attention until Steenstrup’s study of the succession in the forests of Zealand (1844 prox.). In the development of formations, as well as in their recognition, nearly all workers have confined themselves to the investigation of particular changes. Berg (1844), Vaupell (1851), Hoffmann (1856), Middendorff (1864), Hult (1881), Senft (1888), Warming (1890), and others have added much to our detailed knowledge of formational development. Notwithstanding the lapse of more than a half century, the study of plant successions is by no means a general practice among ecologists. This is a ready explanation of the fact that the vast field has so far yielded but few generalizations. Warming (1895) was the first to compile the few general principles of development clearly indicated up to this time. The first critical attempt to systematize the investigation of succession was made by Clements (1904), though this can be considered as little more than a beginning on account of the small number of successions so far studied. Future progress in this field will be conditioned not only by the more frequent study of developmental problems by working ecologists, but also, and most especially, by the application of known principles of succession, and by the working out of new ones.

=6. Ecological phytogeography.= Until recent years, the almost universal tendency was to give attention to formations from the standpoint of vegetation alone. While the habitat was touched here and there by isolated workers, and plant functions were being studied intensively by physiologists, both were practically ignored by ecologists as a class. The appearance of Warming’s Lehrbuch der oecologischen Pflanzengeographie (1896) and of Schimper’s Pflanzengeographie auf physiologischer Grundlage (1898) remedied this condition in a measure by a general discussion of the habitat, and by emphasizing the importance of the ecological or physiological point of view. Despite their frank recognition of the unique value of the habitat, the major part of both books was necessarily given to what may be termed the general description of formations. For this reason, and for others arising out of an almost complete dearth of methods of investigation, ecology is still almost entirely a floristic study in practice, although there is a universal recognition of the much greater value of the viewpoint which rests upon the relation between the formation and its habitat.

=7. Experimental ecology.= Properly speaking, the experimental study of ecology dates from Bonnier (1890, 1895), though it is well understood that experimental adjustment of plants to certain physical factors had been the subject of investigation before this time. The chief merit of Bonnier’s work, however, lies in the fact that it was done out of doors, under natural conditions, and for these reasons it should be regarded as the real beginning of this subject. Bonnier’s experiments were made for the purpose of determining the effect of altitude. Culture plots of certain species were located in the Alps and the Pyrenees, and the results were compared with control cultures made in the lowlands about Paris. In 1894 he also made a comparative study of certain polydemic species common to the arctic islands, Jan Meyen and Spitzenberg, and to the Alps. Both of these methods are fundamental to field experiment, but the results are inconclusive, inasmuch as altitude is a complex of factors. As no careful study was made of the latter, it was manifestly impossible to refer changes and differences of structure to the definite cause. In a paper that has just appeared, E. S. Clements (1905) has applied the method of polydemic comparison to nearly a hundred species of the Rocky mountains. In this work, the all-important advance has been made of determining accurately the decisive differences between the two or more habitats of the same species in terms of direct factors, water-content, humidity, and light. In his own investigations of Colorado mountain vegetation, the author has applied the method of field cultures by planting seeds of somewhat plastic species in habitats of measured value, and has thought to initiate a new line of research by applying experimental methods to the study of vegetation as an organism. In connection with this, there has also been developed a method of control experiment in the plant house under definitely measured differences of water and light.

=8. Ecology of the habitat.= Since the time of Humboldt, there have been desultory attempts to determine the physical factors of habitats with some degree of accuracy. The first real achievement in this line was in the measurement of light values by Wiesner in 1896. In 1898 the writer first began to study the structure of habitats by the determination of water-content, light, humidity, temperature, wind, etc., by means of instruments. These methods were used by one of his pupils, Thornber (1901), in the study of a particular formation, and by another, Hedgcock (1902), in a critical investigation of water-content. Two years later, similar methods of measuring physical factors were put into operation in connection with experimental evolution under control in the plant house. E. S. Clements (1905), as already indicated, has made the use of factor instruments the foundation of a detailed study of polydemic species, i. e., those which grow in two or more habitats, and which are, indeed, the most perfect of all experiments in the production of new forms. In a volume in preparation upon the mountain vegetation of Colorado, the writer has brought the use of physical factor instruments to a logical conclusion, and has made the study of the habitat the basis of the whole work. Out of this investigation has come a new concept of vegetation (Clements 1904), namely, that it is to be regarded as a complex organism with structures and with functions susceptible of exact methods of study.

=9. The evidence from historical development.= This extremely brief resume of what has been accomplished in the several lines of ecological research makes evident the almost complete absence of correlation and of system. The whole field not merely lacks system, but it also demands a much keener perception of the relative value of the different tendencies already developed. It is inevitable from the great number of tyros, and of dilettante students of ecology in comparison with the few specialists, that the surface of the field should have received all of the attention. It is, however, both unfortunate and unscientific that great lines of development should be entirely unknown to all but a few. There is no other department of botany in which the superficial study of more than half a century ago still prevails to the exclusion of better methods, many of which have been known for a decade or more. It is clear, then, that the imperative need of ecology is the proper coordination of its various points of view, and the working out of a definite system which will make possible a ready recognition of that which is fundamental and of that which is merely collateral. The historical development, as is well understood, can furnish but a slight clue to this. It is a fact of common knowledge that the first development of any subject is general, and usually superficial also. True values come out clearly only after the whole field has been surveyed. For these reasons, as will be pointed out in detail later, the newer viewpoints are regarded as either the most important or the most fundamental. Experimental ecology will throw a flood of light upon plant structure and function, while exact methods of studying the habitat are practically certain of universal application in the future.

PRESENT STATUS OF ECOLOGY

=10. The lack of special training.= The bane of the recent development popularly known as ecology has been a widespread feeling that anyone can do ecological work, regardless of preparation. There is nothing in modern botany more erroneous than this feeling. The whole task of ecology is to find out what the living plant and the living formation are doing and have done in response to definite complexes of factors, i. e., habitats. In this sense, ecology is practically coextensive with botany, and the student of a local flora who knows a few hundred species is no more competent to do ecological work than he is to reconstruct the phylogeny of the vegetable kingdom, or to explain the transmission of ancestral qualities. The comprehensive and fundamental character of the subject makes a broad special training even more requisite than in more restricted lines of botanical inquiry. The ecologist must first of all be a botanist, not a mere cataloguer of plants, and he must also possess a particular training in the special methods of ecological research. He must be familiar with the various points of attack in this field, and he must know the history of his subject thoroughly. Ecology affords the most striking example of the prevalent evil of American botanical study, i. e., an indifference to, or an ignorance of the literature of the subject. The trouble is much aggravated here, however, by the breadth of the field, and the common assumption that a special training is unnecessary, if not, indeed, superfluous. Ignorance of the important ecological literature has been a most fertile source of crude and superficial studies, a condition that will become more apparent as these fields are worked again by carefully trained investigators.

=11. Descriptive ecology.= The stage of development of the subject at the present time may be designated as descriptive ecology, for purposes of discussion merely. This is concerned with the superficial description of vegetation in general terms, and results from the fact that the development has begun on the surface, and has scarcely penetrated beneath it. The organic connection between ecology and floristic has produced an erroneous impression as to the relative value of the two. Floristic has required little knowledge, and less preparation: it lends itself with insidious ease to chance journeys or to vacation trips, the fruits of which are found in vague descriptive articles, and in the multiplication of fictitious formations. While there is good reason that a record should be left of any serious reconnaissance, even though it be of a few weeks’ duration, the resulting lists and descriptive articles can have only the most rudimentary value, and it is absurd to regard them as ecological contributions at all. No statement admits of stronger emphasis, and there is none that should be taken more closely to heart by botanists who have supposed that they were doing ecological work. An almost equally fertile source of valueless work is the independent treatment of a restricted local area. The great readiness with which floristic lists and descriptions can be made has led many a botanist, working in a small area, or passing hurriedly through an extended region, to try his hand at formation making. From this practice have resulted scores of so-called formations, which are mere patches, consocies, or stages in development, or which have, indeed, no existence other than in the minds of their discoverers. The misleading definiteness which a photograph seems to give a bit of vegetation has been responsible for a surplus of photographic formations, which have no counterparts in nature. Indispensable as the photograph is to any systematic record of vegetation, its use up to the present time has but too often served to bring it into disrepute. There has been a marked tendency to apply the current methods of descriptive botany to vegetation, and to regard every slightly different piece of the floral covering as a formation. No method can yield results further from the truth. It is evident that the recognition and limitation of formations should be left absolutely to the broadly trained specialist, who has a thorough preparation by virtue of having acquainted himself carefully with the development and structure of typical formations over large areas.

=12. The value of floristic.= In what has been said above, there is no intent to decry the value of floristic. The skilled workman can spare the material which he is fashioning as readily as the ecologist can work without an accurate knowledge of the genera and species which make up a particular vegetation. Some botanists whose knowledge of ecology is that of the study or the laboratory have maintained that it is possible to investigate vegetation without knowing the plants which compose it. Ecology is to be wrought out in the field, however, and the field ecologist—none other, indeed, should bear the name—understands that floristic alone can furnish the crude material which takes form under his hands. It is the absolute need of a thorough acquaintance with the flora of a region which makes it impossible for a traveler to obtain anything of real ecological value in his first journey through a country. As the very first step, he must gain at least a fair knowledge of the floristic, which will alone take the major part of one or more growing seasons. This information the student of a local flora already has at the tip of his tongue; in itself it can not constitute a contribution to ecology, but merely the basis for one. In this connection, moreover, it can not be used independently, but becomes of value only after an acquaintance with a wide field. Floristic study and floristic lists, then, are indispensable, but to be of real value their proper function must be clearly recognized. They do not constitute ecology.

=13. Reconnaissance and investigation.= In striving to indicate the true value and worth of ecological study, it becomes necessary to draw a definite line between what we may term reconnaissance and investigation. By the former is understood the preliminary survey of a region, extending over one or two years. The objects of such a survey are to obtain a comprehensive general knowledge of the topography and vegetation of the region, and of its relation to the other regions about it. The chief purpose, however, is to gain a good working acquaintance with the flora: a reconnaissance to be of value must do this at all events. Certain general facts will inevitably appear during this process, but they will invariably need the confirmation of future study. It would be an advantage to real ecology if reconnaissance were to confine itself entirely to the matter of making a careful floristic survey. Investigation begins when the inquiry is directed to the habitat, or to the development and structure of the formation which it bears, i. e., when it takes up the manifold problems of the οἶκος. Such a study must be based upon floristic, but the latter becomes a part of investigation only in so far as it leads to it. Standing by itself, it is not ecological research: it is the preparation for it. This distinction deserves careful thought. The numerous recruits to ecology have turned their attention to what lay nearest to hand, with little question as to its value, or to where it might lead. The result has been to make reconnaissance far outweigh investigation in amount, and to give it a value which properly belongs to the latter. Furthermore, this mistaken conception has in many cases, without doubt, prevented its leading to valuable research work.

=14. Resident investigation.= Obviously, if reconnaissance is a superficial survey, and investigation thorough extensive study, an important distinction between them is in the time required. While one may well be the result of a journey of some duration, the other is essentially dependent upon residence. In the past the great disparity between the size of the field and the number of workers has made resident study too often an ideal, but in the future it will be increasingly the case that a particular region will be worked by a trained ecologist resident in it. This may never be altogether true of inaccessible and sterile portions of the globe. It may be pointed out, however, that, between the tropics and the poles, residence during the summer or growing period is in essence continuous residence. In the ultimate analysis, winter conditions have of course some influence upon the development of vegetation during the summer, but the important problems which a vegetation presents must be worked out during the period of development. For temperate, arctic, and alpine regions, then, repeated study during the growing period for a term of years has practically all the advantages of continuous residence. For all practical purposes, it is resident study.

=15. The dangers of a restricted field.= In the resident study of a particular region, the temptation to make an intensive investigation of a circumscribed area is very strong. The limits imposed by distance are alone a sufficient explanation of this, but it is greatly increased by the inclination toward detailed study for which a small field offers opportunity. This temptation can be overcome only by a general preliminary study of the larger region in which the particular field is located. The broader outlook gained in this way will throw needed light upon many obscure facts of the latter, and at the same time it will act as a necessary corrective of the tendency to consider the problems of the local field in a detached manner, and to magnify the value of the distinctions made and the results obtained. Such a general survey has the purpose and value of a reconnaissance, and is always the first step in the accurate and detailed investigation of the local area or formation. Each corrects the extreme tendency of the other, and thorough comprehensive work can be done only by combining the two methods. When the field of inquiry is a large area or covers a whole region, the procedure should be essentially the same. A third stage must be added, however, in which a more careful survey is made of the entire field in the light of the thorough study of the local area. The writer’s methods in the investigation of the Colorado vegetation illustrate this procedure. The summers of 1896, 1897, 1898 were devoted to reconnaissance; those of 1899–1904 were given to detailed and comprehensive study by instrument and quadrat of a highly diversified, representative area less than 20 miles square, while the work of the final summer will be the application of the results obtained in this localized area to the region traversed from 1896–98. This is practically the application of methods of precision to an area of more than 100,000 square miles. It also serves to call attention to another point not properly appreciated as yet by those who would do ecological work. This is the need of taking up field problems as a result of serious forethought, and not as a matter of accident or mere propinquity. A carefully matured plan of attack which contemplates an expenditure of time and energy for a number of years will yield results of value, no matter how much attention an area may have received. On the other hand, an aimless or hurried excursion into the least known or richest of regions will lead to nothing but a waste of time, especially upon the part of the ecologist, who must read the articles which result, if only for the purpose of making sure that there is nothing in them.

APPLICATIONS OF ECOLOGY

=16. The subjects touched by ecology.= The applications of ecological methods and results to other departments of botany, and to other fields of research are numerous. Many of these are both intimate and fundamental, and give promise of affording new and extremely fruitful points of view. It has already been indicated that ecology bears the closest of relations to morphology and histology on the one side, and to physiology on the other—that it is, indeed, nothing but a rational field physiology, which regards form and function as inseparable phenomena. The closeness with which it touches plant pathology follows directly from this, as pathology is nothing more than abnormal form and functioning. Experimental work in ecology is purely a study of evolution, and the facts of the latter are the materials with which taxonomy deals. Forestry has already been termed “applied ecology” and in its scientific aspects, which are its foundation, it is precisely the ecology of woody plants, and of the vegetation which they constitute. Apart from botany, the physical side of ecology is largely a question of soil physics, and of physiography. On the other hand, vegetation is coming more and more to be regarded as a fundamental factor in zoogeography and in sociology. Furthermore, with respect to the latter, it will be pointed out below that the principles of association which have been determined for plants, viz., invasion, succession, zonation, and alternation, apply with almost equal force to man.

=17. Physiology and pathology.= The effect of ecology in emphasizing the intrinsically close connection between physiology and morphology has already been mentioned. Its influence in normalizing the former by forcing it into the field as the place for experiment, and by directing the chief attention to the plant as an organism rather than a complex of organs, is also rapidly coming to be felt. Ecology will doubtless exert a corrective influence upon pathology in the near future. This is inevitable as the latter ceases to be the merely formal study of specific pathogenic organisms, and turns its attention to the cause of all abnormality, which is to be found in the habitat, whether this be physical, as when the water-content is low, or biotic, when a parasitic fungus is present. The relative ease with which specific diseases can be studied has helped to obscure the essential fact that the approach to pathology must be through physiology. Much indeed of the observational physiology of the laboratories has been pathology, and it will be impossible to draw a clear line between them until precise experiment in the habitat has come into vogue.

=18. Experimental evolution.= As a result of the extremely fragmentary character of the geological record, nothing is more absolute than that there can be no positive knowledge of the exact origin of a form or species, except in those rare cases of the present day, where the whole process has taken place under the eye of a trained observer. The origin of the plant forms known at present must forever lie without the domain of direct knowledge. If it were possible, by a marvel of ingenuity and patience to develop experimentally Myosurus from Selaginella, this would not be absolutely conclusive proof that Myosurus was first derived in this way. When all is said, however, this would be the very best of presumptive evidence. It must also be recognized that this is the nearest to complete proof that we shall ever attain, and with this in mind it becomes apparent at once that evidence from experiment is of paramount importance in the study of evolution (the origin of species).

The phase of experimental ecology which has to do with the plant has well been called experimental evolution. While this is a field almost wholly without development at present, there can be little question that it is to be one of the most fertile and important in the future. Attention will be directed first to those forms which are undergoing modification at the present time. The cause and direction of change will be ascertained, and its amount and rapidity measured by biometrical methods. The next step will be to actually change the habitat of representative types, and to determine for each the general trend of adaptation, as well as the exact details. By means of the methods used and the results obtained in these investigations, it will be possible to attack the much more difficult problem of retracing the development of species already definitely constituted. This will be accomplished by the study of the derived and the supposed ancestral form, but owing to the great preponderance of evolution over reversion, the study of the ancestral form will yield much more valuable results.

The general application of the methods of experimental ecology will mark a new era in the study of evolution. There has been a surplus of literary investigation, but altogether too little actual experiment. The great value of De Vries’ work lies not in the importance of the results obtained, but in calling attention to the unique importance of experimental methods in contributing to a knowledge of evolution. The development of the latter has been greatly hindered by the dearth of actual facts, and by a marked tendency to compensate for this by verbiage and dogmatism. This is well illustrated by the present position of the “mutation theory,” which, so far as the evidence available is concerned, is merely a working hypothesis. An incredible amount of bias and looseness of thought have characterized the discussion of evolution. It is earnestly to be hoped that the future will bring more work and less argument, and that the literary evolutionists will become less and less reluctant to leave the relative merits of variation and mutation to experiment.

=19. Taxonomy.= Taxonomy is classified evolution. It is distinct from descriptive botany, which is merely a cataloguing of all known forms, with little regard to development and relationship. The consideration of the latter is peculiarly the problem of taxonomy, but the solution must be sought through experimental evolution. The first task of the latter is to determine the course of modification in related forms, and the relationships existing between them. With this information, taxonomy can group forms according to their rank, i. e., their descent. The same method is applicable to the species of a genus, and, in a less degree, perhaps, to the genera which constitute a family. The use to which it may be put in indicating family relationships will depend largely upon the gap existing between the families concerned. While interpretation will always play a part in taxonomy, the general use of experiment will leave much less opportunity for the personal equation than is at present the case. Taxonomy, like descriptive botany, is based upon the species, but, while there may exist a passable kind of descriptive botany, there can be no real taxonomy as long as the sole criterion of a species is the difference which any observer thinks he sees between one plant and another. The so-called species of to-day range in value from mere variations to true species which are groups of great constancy and definiteness. The reasons for this are obvious when one recalls that “species” are still the product of the herbarium, not of the field, and that the more intensive the study, the greater the output in “species.” It would seem that careful field study of a form for several seasons would be the first requisite for the making of a species, but it is a precaution which is entirely ignored in the vast majority of cases. The thought of subjecting forms presumed to be species to conclusive test by experiment has apparently not even occurred to descriptive botanists as yet. Notwithstanding, there can be no serious doubt that the existing practice of re-splitting hairs must come to an end sooner or later. The remedy will come from without through the application of experimental methods in the hands of the ecologist, and the cataloguing of slight and unrelated differences will yield to an ordered taxonomy.

Experimental evolution will solve a taxonomic problem as yet untouched, namely, the effect of recent environment upon the production of species. It is well understood that some species grow in nature in various habitats without suffering material change, while others are modified to constitute a new form in each habitat. It is at once clear that these forms (or ecads) are of more recent descent than the species, i. e., of lower rank. It must also be recognized that a constant group and a highly plastic one are essentially different. If constancy is made a necessary quality of a species, one is a species, the other is not. If both are species, then two different kinds must be distinguished. Among the species of our manuals are found many ecads, alongside of constant and inconstant species. These can be distinguished only by field experiment, and their proper coordination is possible only after this has been done. Indeed, the whole question of the ability or the inability of environmental variation to produce constant species is one that must be referred to repeated and long-continued experiment in the field.

A minor service of considerable value can be rendered taxonomy by working over the diagnosis from the ecological standpoint. Many ecological facts are of real diagnostic value, while others are at least of much interest, and serve to direct attention to the plant as a living thing. The loose use of terms denoting abundance, which prevails in lists and manuals, should be replaced by the exact usage which the quadrat method has made possible for vegetation. The designation of habitats could be made much more exact, and the formation, as well as the habitat form or ecad, and the vegetation form or phyad, should be indicated in addition. The general terms drawn from pollination, seed-production, and dissemination might also be included to advantage.

=20. Forestry=, if the purely commercial aspects be disregarded, is the ecology of a particular kind of vegetation, the forest. Therefore, in pointing out the connection between them, it is only necessary to say that whatever contributes to the ecology of the forest is a contribution to forestry. There are, however, certain lines of inquiry which are of fundamental importance. First among these, and of primary interest from the practical point of view, are the questions pertaining to the distribution of forests and their structure. Of even greater significance are the problems of forest development, movement, and of reforestation, which are comprised in succession. The gradual invasion of the plains and prairies by the forest belt of the east and north is in full conformity with the laws of invasion, and the ecological methods to be employed here serve not merely to determine the actual conditions at present, but also to forecast them with a great deal of accuracy. The slow but certain development of forests on new soils, and their more rapid reestablishment where the woody vegetation has been destroyed by burning or lumbering, are ordinary phenomena of succession, for which the ecologist has already worked out the laws, and determined the methods of investigation. Having once ascertained the original and adjacent vegetation and the character of the habitat, the ecologist can indicate with accuracy not only the character of the new forest that will appear, but also the nature of the antecedent formations. A full knowledge of the character and laws of succession will prove of the greatest value to the forester in all studies of forestation and reforestation. Forests which now seem entirely unrelated will be seen to possess the most intimate developmental connection, and the fuller insight into the life history gained in this way will have a direct bearing upon methods of conservation, etc. It will further show that the forester must know other vegetations as well, since grassland and thicket formations have an intimate influence upon the course of the succession, as well as upon the advance of a forest frontier.

One of the greatest aids which modern ecology can furnish forestry, however, is the method of determining the physical nature of the habitat. So far, foresters have been obliged to content themselves with a more or less superficial study of the structure of forest formations, without being able to do more than guess at the physical causes which control both structure and development. This handicap is especially noticeable in the case of forest plantings in non-forested regions, where it has been impossible to estimate the chances of success, or to determine the most favorable areas except by actual plantations. Equipped with the proper instruments for measuring water-content, humidity, light and temperature, the ecologist is able to determine the precise conditions under which reproduction is occurring, and to ascertain what non-forested areas offer the most nearly similar conditions. A knowledge of habitats and the means of measuring them enables the forester to discover the causes which control the vegetation with which he is already familiar, and to forecast results otherwise hidden. Furthermore, it makes it possible for him to enter a new region and to determine its nature and capabilities at a minimum of time and energy.

=21. Physiography.= Physiographic features play an important part in determining the quantity of certain direct factors of the habitat. Perhaps a more important connection between physiography and ecology is to be found in succession. The beginning of all primary, and of many secondary successions is to be sought in the physiographic processes which produce new habitats, or modify old ones. On the other hand, most of the reactions which continue successions exert a direct influence upon the form of the land. The most pronounced influence of terrestrial successions is found in the stabilization which their ultimate stages exert upon land forms, even where these are highly immature. The chief effect of aquatic successions is to be found in the “silting up” and the formation of new land, which result from the action of vegetation upon silt-bearing waters. The closeness of the relation between succession and the forms of the land has led to the application of the term “physiographic ecology” to that part of the subject which deals with the development of vegetation, i. e., succession.

=22. Soil physics.= This subject is as much a part of ecology as is forestry. It is intrinsically that subdivision of ecology which deals with the edaphic factors of the habitat, and their relation to the plant. Since the basis is physics, there has been a general tendency to overvalue the determination of soil properties, and to ignore the fact that these are decisive only when considered with reference to the living plant. As the soil contains the water which is the factor of greatest importance to plants, soil physics is an especially important part of ecology. Its methods are discussed under the habitat.

=23. Zoogeography.= Since animals are free for the most part, and hence not confined so strictly to one spot as plants, their dependence upon the habitat is not so evident. The relation is further obscured by the fact that no physical factor has the direct effect upon them which water or light exerts upon the plant. Vegetation, indeed, as the source of food and protection, plays a more obvious, if not a more important part. This is especially true of anthophilous insects, but it also holds for all herbivorous animals, and, through them, for carnivorous ones. The animal ecology of a particular region can only be properly investigated after the habitats and plant formations have been carefully studied. Here, as in floristics, a great deal can be done in the way of listing the fauna, or studying the life habits of its species, without any knowledge of plant ecology; but an adequate study must be based upon a knowledge of the vegetation. Although animal formations are often poorly defined, there can be no doubt of their existence. Frequently they coincide with plant formations, and then have very definite limits. They exhibit both development and structure, and are subject to the laws of invasion, succession, zonation, and alternation, though these are not altogether similar to those known for plants, a fact readily explained by the motility of animals. Considered from the above point of view, zoogeography is a virgin field, and it promises great things to the student who approaches it with the proper training.

=24. Sociology.= In its fundamental aspects, sociology is the ecology of a particular species of animal, and has in consequence, a similar close connection with plant ecology. The widespread migration of man and his social nature have resulted in the production of groups or communities which have much more in common with plant formations than do formations of other animals. The laws of association apply with especial force to the family, tribe, community, etc., while the laws of succession are essentially the same for both plants and man. At first thought it might seem that man’s ability to change his dwelling-place and to modify his environment exempts him in large measure from the influence of the habitat. The exemption, however, is only apparent, as the control exerted by climate, soil, and physiography is all but absolute, particularly when man’s dependence upon vegetation, both natural and cultural, is called to mind.

THE ESSENTIALS OF A SYSTEM

=25. Cause and effect: habitat and plant.= In seeking to lay the foundation for a broad and thorough system of ecological research, it is necessary to scan the whole field, and to discriminate carefully between what is fundamental and what is merely collateral. The chief task is to discover, if possible, such a guiding principle as will furnish a basis for a permanent and logical superstructure. In ecology, the one relation which is precedent to all others is the one that exists between the habitat and the plant. This relation has long been known, but its full value has yet to be appreciated. It is precisely the relation that exists between cause and effect, and its fundamental importance lies in the fact that all questions concerning the plant lead back to it ultimately. Other relations are important, but no other is paramount, or able to serve as the basis of ecology. Ecology sums up this relation of cause and effect in a single word, and it may be that this advantage will finally cause its general acceptance as the proper name for this great field.

In the further analysis of the connection between the habitat and the plant, it is evident that the causes or factors of the habitat act directly upon the plant as an individual, and at the same time upon plants as groups of individuals. The latter in no wise decreases the importance of the plant as the primary effect of the habitat, but it gives form to research by making it possible to consider two great natural groups of phenomena, each characterized by very different categories of effects. Ecology thus falls naturally into three great fundamental fields of inquiry: habitat, plant, and formation (or vegetation). To be sure, the last can be approached only through the plant, but as the latter is not an individual, but the unit of a complex from the formational standpoint, the formation itself may be regarded as a sort of multiple organism, which is in many ways at least a direct effect of the habitat. In emphasizing this fundamental relation of habitat and vegetation, it is imperative not to ignore the fact that neither plant nor formation is altogether the effect of its present habitat. A third element must always be considered, namely, the historical fact, by which is meant the ancestral structure. Upon analysis, however, this is in its turn found to be the product of antecedent habitats, and in consequence the essential connection between the habitat and the plant is seen to be absolute.

=26. The place of function.= In the foregoing it is understood that the immediate effect of the physical factors of the habitat is to be found in the functions of the plant, and that these determine the plant structure. Function has so long been the especial theme of plant physiology that methods of investigation are numerous and well known, and it is unnecessary here to consider it further than to indicate its general bearing. The essential sequence in ecological research, then, is the one already indicated, viz., habitat, plant, and formation, and this will constitute the order of treatment in the following pages. That portion of floristic which is not mere descriptive botany belongs to the consideration of the formation, and in consequence there will be no special treatment of floristic as a subdivision of ecology.