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[[1]] (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.