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It is at once apparent that almost all the species of North. American birds which are divisible into geographical varieties are classed in this third list, that is, that those species evincing the greatest tendency to geographical variation, are also those which undertake migrations of the least extent. Thus, for instance, Melospiza fasciata is usually resident in most localities throughout the whole year, and has become differentiated into a number of geographical races, while Melospiza georgiana is migratory, and though it breeds in an area nearly equal in extent to that of fasciata, has not produced local varieties; the non-migratory Megascops asio shows great geographical variation, while the migratory Asio acciptrinus, though almost cosmopolitan in its breeding area, shows no tendency toward such variation. And, in fact, an examination and comparison of List III with Lists I and II, will lead to the conclusion, that given any two species of equally extensive breeding areas, the one with the smaller range of periodic migration will, as a rule, evince a greater tendency to produce geographical varieties than will the species with the greater range of migration. This conclusion may be concisely formulated as follows; it is the rule that the amount of geographical variation in species with more or less extensive breeding areas, stands in inverse ratio to the extent of its periodic migrations. Naturally, this law is only applicable to species with extended breeding areas, since diverse conditions in different sections of this area are necessary, according to the theory of Natural Selection, for the production of geographical subspecies or varieties; and in a limited breeding area, throughout which the conditions of the environment are similar, there could be no cause to produce geographical varieties, irrespective of the migratory or nonmigratory habits of the species.

I have not meant to imply, in the preceding pages, that species with migration ranges of 30° lat., or more, are all sharply definable, i. e., that such species are never divisible into geographical varieties; but, on the contrary, that this tendency to produce geographical races is less in the species with extensive migrations, than in those with shorter ranges of migration. For it is usual, even in species with extensive migrations, whose

breeding areas are extraordinarily great, so as to include the whole of the arctic region, or northern America together with northern Eurasia, for them to subdivide into two geographical varieties, occupying respectively the eastern and western hemispheres. Thus, the eurasiatic Colymbus nigricollis is represented by a variety (californicus) in western North America; and to give other examples where an eurasiatic form, which undertakes long periodic migrations, is represented by a geographical variety in North America, may be mentioned one species of Fratercula, 1 Uria, 1 Larus, 1 Hydrochelidon, 2 Aythya, 1 Glaucionetta, 1 Somateria, 1 Anser, 1 Tringa, 1 Limosa, 1 Charadrius, 2 Falco, 1 Pandion, and others. But no species with extensive migration ranges shows any tendency to geographical variation, unless its breeding areas are also very large in extent. And the species with the least demonstrable tendency to produce local races, are those in which the wing power is greater, and the range of migration more extensive, than in any other species of birds, namely, those enumerated in List I. Further, we find it to be the rule, that in those avian families most of the species of which undertake long migrations, if species are present which are divisible into geographical varieties, that these latter are more restricted in their migrations than the former; examples are Uria troile, Rissa tridactyla, Fulmarus glacialis (only North American species of the family presenting geographical varieties), Rallus longirostrus, Porzana jamaicensis, Aegialitis wilsonia and Ae. meloda, and others. After the consideration of these facts it is certainly permissible to conclude that, as a rule, species which undertake annual migrations of comparatively great extent, through distances of 30° lat., or more, evince no tendency to give rise to geographical varieties, unless their breeding areas are very extensive; and, conversely, that species which do not undertake extensive migrations, owing to insufficient wing power or to some other cause, and which occupy broad breeding areas, have the tendency to produce geographical varieties. Consequently, also, extended migration acts as a check upon the production of varieties; and the extent of the range of migration will, therefore, stand in inverse ratio to the amount of geographical variation

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 environment, owing to the absence or restriction of migration. 11 Sept., 1895.

West Chester, Penna.

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 in 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

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