Imágenes de páginas
PDF
EPUB

evinced. Thus the postulate of Darwin, that wide-ranging species vary most, must be modified after a consideration of the facts given here. But to pass over to certain apparent exceptions to the rule. Falco columbarius, breeding chiefly north of the United States, and migrating in winter as far as South America, has a variety (suckleyi) on the Pacific coast from Sitka to California ; Helminthophila ruficapilla, breeding as far north as Hudson's Bay, and migrating in winter as far as Guatemala, has a variety (gutturalis) from the Rocky Mts. to the Pacific coast, in winter to Mexico; and a number of similar cases could be mentioned, where the species, although it has a wide range of migration, and a breeding area which is not extraordinarily extensive, has, nevertheless, the tendency to geographical variation. But such apparent exceptions to the rule are, in fact, not valid objections, since in these cases the geographical variety is much more restricted in the range of its migration than the type species, or vice versa. And in any of these cases, the species, including the variety, is to be regarded as a number of individuals, some of which undertake extensive migrations, while others migrate not at all or through much shorter distances. Therefore, these are not true exceptions to the law, that the extent of the migration stands in inverse ratio to the amount of the tendency to produce geographical varieties; since a number of the individuals do not undertake extensive migrations. Real exceptions may, however, be found in such cases where the individuals of the type species as well as its varieties make prolonged periodic migrations; and after a careful examination of all the North American species and their varieties, I have found only four species which represent such exceptions to the rule: Dendroica æstiva, with its variety morcomii, Seiurus noveboracensis, with the subspecies notabilis, Sylvania pusilla, with the variety pileolata, and Turdus ustulatus, with its eastern variety swainsonii. These four species represent cases where, with not very extensive breeding area, both races of the species possess extensive migration ranges. But I think that the importance of these cases as exceptions to the rule is diminished, when we consider that in each case the migration route of the variety is different from

that of the species, one being west of, while the other is east of the Rocky Mts. And hence, since not only in its breeding area but also in its migration range, the variety is subjected to conditions of environment different from those influencing the type species, we would naturally expect that the species (as a whole) would become differentiated into two geographical

races.

The reason for the law, that extensive migration acts as a check upon the production of geographical varieties, is not far to seek. The barn swallow, for instance, remains in its breeding area from four to five months each year, spending the remainder of its time, except that consumed by its actual migration to and fro, in its tropical winter quarters. Roughly speaking, we may say that it spends about half a year in its breeding area, and the remainder in its winter home. In other words, the swallow is subjected to one environment for half the time of its existence, and to a more or less different environment during the remainder of its life. The result of this on the organism is obvious: the action of the two environments during approximately the same length of time, would prevent it from becoming more particularly adapted to the one than to the other, and would lead to the production of more generalized characters, fitted to respond more or less equally to both environments. In this way individuals of the species could not become especially adapted to a certain portion of the breeding area, if such adaptations should be unfavorable for its existence in the winter quarters, and vice versa ; in other words, the influence of the winter environment acts as a check upon the acquisition of adaptations suited alone to the summer environment. This is, to my mind, the only adequate explanation for the law that extensive migration exerts a check upon the production of geographical varieties. Species with wide-ranging breeding areas, on the other hand, but with none or only restricted migrations, may give rise to geographical varieties, suited respectively to the diverse conditions found in different portions of its habitat, since such species are influenced by the conditions of but one environ. ment, owing to the absence or restriction of migration.

11 Sept., 1895. West Chester, Penna.

[graphic]

THE PLANT-GEOGRAPHY OF GERMANY.

By RoscoE POUND.

In a recently published address Dr. Coulter speaks of a new movement in botany which is sending botanists "to the great laboratory of nature," and replacing collecting trips by biological surveys. "The old-fashioned collection of plants,” he says,

“ will hold no more relation to the new field work than the old geology, with its scattered collection of fossils, holds to the topographic geology of to-day.” Geographical botany as it is now understood is comparatively a recent development. Collectors and cataloguers for a long time have been gathering a portion of the bare facts upon which geographical botany must proceed, and the facts of plant-distribution have been more or less ascertained. But the systematic collating and grouping of these facts and the application of biological and physiological facts to them is a matter of the last few years and is still going on. At first localities were catalogued, and collectors were eager to add new and rare stations to those recorded for species; then came statistical comparison of families and genera, especially in relation to altitude and the media of plant-migration. The limits of distribution of species were ascertained, particularly of those which are characteristic and controlling in vegetation. Such work laid the foundations of geographical botany.

But the statistics as to the distribution of families, which have been worked out ir. one method and another, gave no promise of leading to important results. It was not until biological groups began to be made for the purpose of comparison, and statistics began to be applied to those groups, that such work acquired importance. It is apparent that a mere statement of the number of species of the various natural plantgroups occurring in a certain region tells us very little of the vegetation of that region except in the most general way. A group represented by comparatively few species may yet as far as the occupation of the soil is concerned be dominant and controlling. To understand the vegetation of a region one must ascertain not only what are its physical, meteorological and geological features, but much more what sorts of plants control its water, meadow, plain, or forest vegetation. Directed towards the latter ends, statistics have a very different meaning. Such work is the aim of the new geographical botany. “When we hear of a district," say Schroeter and Stebler, " that it is covered with extended fields of turf-rush or of bromegrass, that tells us more of the nature of the region than long lists of meteorological data. It also tells us more than the mere occurrence of the species in question of itself”

A notable contribution to this department of the science is Dr. Drude's new work, “ Deutschlands Pflanzengeographie,” of which the first part appeared in January last. The sub title of the work gives a clue to its purpose. It is stated to be "ein geographisches Charakterbild der Flora von Deutschland." Much has been done in recent years towards such characterization of restricted districts, or for large areas as regards certain kinds of vegetation. But Dr. Drude in giving a complete picture of the vegetation of as large a country as Germany has, in one sense, made an epoch in geographical botany. Such a work demonstrates that the era of preparation is passed. A mere cursory examination of the work serves to convince the reader that the theory and system of plantgeography have been thoroughly worked out, and that henceforth workers will be busied chiefly with their application to other regions rather than with devising new methods.

As has been remarked, in order to be of value, statistics must be based not upon the systematic groups of plants but upon groups founded on biological considerations, so far as they indicate a positive role in the vegetation of the region in question. Such groups are called vegetation-groups. Dr. Drude points out also that the proportions of the number of representatives of the several orders, genera, or other systematic groups are not to be reckoned with the whole flora of a region as represented by a certain number of species, but with the biological plant-community of the region. Accordingly he constructs some thirty-five vegetation-groups for the flora of Germany. The thoroughness of this may be judged from the fact that he begins with trees and ends with plankton-algæ.

Germany belongs to the Middle-European region which, bounded by the Pyrenees, the Alps, and the Balkan system, stretches along the north west border of the Russian steppes to the arctic flora which extends over the north of Europe. The region includes also the wooded portions of the Scandinavian countries. Throughout this large region, as regards the distribution of families and genera, the same fundamental character prevails. Carrying the principles of division further, and observing on the one hand lesser influences of climate and physiognomy and on the other the division of the floral-elements into “Genossenschaften,” subdivisions, or “Vegetationsregionen” are made. Germany and the neighboring regions of the Alps and Carpathians fall into five such divisions; the region of the north-Atlantic lowlands, the region of the southBaltic lowlands and uplands, the region of the middle and south German highlands and lower mountain districts, the region of the higher mountain districts and subalpine formations, and the region of the higher mountain formations of the Alps and Carpathians. The region of central France and the west-Pontic region, to which belong the southwestern and southeastern neighbors of Germany respectively, include also isolated spots in Germany itself. Dr. Drude's maps show that the first two regions are continuous in extent. The first includes Holland and North Germany west of the Elbe and the western portion of the Danish peninsula, the second EastPrussia and Pomerania, being bounded roughly by the Oder on the west. Between the Elbe and the Oder is a neutral zone, transitional between the two regions. The whole of middle and south Germany to the Alps constitutes the third region. But along the northern borders of the Alps and here and there throughout south Germany, as for instance the Harz forest, the Thuringian forest, the Black forest, in isolated spots, we find the fourth region, the region of subalpine forests. Along the upper Rhine here and there are localities belonging to the region of central France, and in the southeastern portion are many localities belonging to the west-Pontic region.

« AnteriorContinuar »