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I remember seeing a recently made track over the Bogong Pass, in Victoria, which had been rendered impracticable for horse traffic though being filled up in some places with masses of rock, logs of wood, and other débris. These had rolled down the steep sides of the mountain on account of their supports of humus and soil having been scratched away by these birds. Again, it is often impossible to exactly locate a survey trench or peg in lyre bird ("pheasant,” as it is called by selectors) country, sometimes even shortly after they have been placed there. This causes some difficulty when the blaze (ax-cut mark) on the tree has been destroyed by bush fires. This habit of scratching among decayed vegetation and soil may account for the abundance of lice which Mr. W. Bragwanath, jr., informs me, are to be found on most of these birds in the Baw Baw district of Gippsland.

I have spoken of the flight of lyre birds as a floating. As far as I have noticed they do not rise upward in the air like a soaring bird, and can not make a proper upward flight. But in going to a lower place they simply jump off a log or rock into the air with their wings outspread, and float or glide through it down a slope into a gully, sometimes taking advantage, every here and there, of a log or rock from which to get an additional spring. In a few seconds they can descend several hundreds of feet with very little apparent motion of the wings. It is an interesting sight to see the dark brown form of a departing lyre bird as it hops on a fallen tree, floats under a tree fern, or jumps off into space-silent, save for its first shrill whistle of alarm. While lyre birds are fond of tree-fern gullies and dogwood (Cassinia aculeata and C. longifolia), "native hop" (Daviesia latifolia), and "wild hop" (Goodenia ovata) slopes and ridges, they are very partial to the patches of "blanket wood" (Senecio bedfordi). This plant grows into small trees with lateral branches, and large, thick leaves, arranged more or less horizontally, thus forming a canopy. The ground beneath is usually not covered with ferns and small plants, but with decayed leaves and twigs, while the branches form convenient perches for the lyre birds. There are thus open spaces between the foliage and the ground, and the birds are fond of moving about in them, hence the scrub is called locally "pheasant scrub."

My thanks are tendered to Messrs. A. J. Campbell, D. Le Souéf, and F. P. Godfrey for six of the photographs which illustrate this article.

THE INFLUENCE OF PHYSICAL CONDITIONS IN THE

GENESIS OF SPECIES.

By JOEL A. ALLEN.

Among biologists who accept the modern theory of evolution as the only reasonable hypothesis available for the explanation of the diversity of structure among organized beings, there is a wide difference of opinion as to what are the leading causes of differentiation. The doctrine of natural selection, or the survival of the fittest, has recently been brought prominently forward as the key to this complex problem, and is upheld by a large class of enthusiastic adherents, who accept it as the full solution of the whole question. By others the conditions of environment are believed to be far more influential in effecting a certain class of modifications, at least, than the necessarily precarious influence of natural selection, which must take its origin in isolated instances of variation in favorable directions, and depend for its continuance upon these fortuitous advantages being inherited by the descendants of the favored individuals in which they originate. The modifying influence of conditions resulting from geographic or climatic causes was long since noticed, and for nearly a century has been considered by many writers as explanatory of much of the diversity existing not only in the human race but among animals. It has, however, remained until recently vaguely grounded, being based more in conjecture than on observed facts. Scarcely, indeed, have two decades passed since the real nature and extent of geographical variation among animals, and even as yet among only a few species, began to receive careful attention, while only within the last fifteen years has any attempt been made to correlate the observed differences with the climatic or geographical conditions of habitat. Only within recent years have the differences in general size, and in the relative size of different parts, been ascertained by careful measurement, and the differences in the character of the tegumentary covering (as the pelage in mammals) and in color, in indi

a Reprinted, with note and bracketed additions by the author, from the Radical Review, I, 1877, pp. 108-140,

viduals of the same species inhabiting distant portions of a common habitat, been duly recorded. In the work of registering these instructive data, it has fallen to Americans to take a leading part, large credit in the matter being due not only to the activity of our professional biologists, but to the liberality of the General Government in attaching competent natural history observers and collectors to the numerous surveying parties it has sent out during the last twenty years to explore the till then practically unknown geography and productions of our western Territories.

The combined fruits of their labors, together with those of the agents and correspondents of the Smithsonian Institution, have resulted in the accumulation of an amount of material far exceeding that elsewhere accessible to single investigators, representing, as it does, at least two of the vertebrate classes of animals from the whole North American continent so fully that generalizations may be made from their study which could not otherwise have been reached for many years and for which no similar facilities for any other equal area as yet exist. The recent investigations of American mammalogists and ornithologists have been in consequence largely directed to the subject of geographical variation, and their publications teem with tabulated measurements and records of variations in form and color that accompany differences in the climatic or geographical conditions of habitat. Among the results that have followed are the discovery of numerous interesting geographical varieties or subspecies, and the demonstration of the complete intergradation of many forms, often quite widely diverse in color, size, and proportion of parts, formerly regarded (and properly so as then known) as unquestionably distinct species, which discoveries have of course necessitated a large reduction in the number of recognized "specific" or nonintergrading forms. But most important of all has been the correlation of local variations with the conditions of environment, and the deduction there from of certain laws of geographical variation. Upon these have been based hypotheses that go far toward explaining many of the phenomena of intergradation and differentiation observed among existing animals. In the present paper will be given not only a summary of the results thus far attained, but enough of the details of the subject to show the nature of the evidence on which rest the conclusions already reached. These results, it is claimed, show that other influences than natural selection operate powerfully in the differentiation of specific forms, and that geographical causes share more largely in the work than naturalists have heretofore been prepared to admit-at least to consider as proven.

As is well known, animals vary greatly in respect to the extent of the areas they inhabit. While a few species are nearly or quite cosmopolitan, many others are restricted to single small islands or to

limited portions of a continent. Not a few range over the greater part of whole hemispheres, while by far the larger number are confined within comparatively narrow limits. Of the numerous species of mammals and birds inhabiting North America, none are equally common throughout the whole extent of the continent. The habitats of a few only extend from the Barren Grounds of the Arctic regions to Mexico, and from the Atlantic coast westward to the Pacific; one or two only among the mammals range over the whole continent from Alaska to Central America, while some occupy merely the extreme boreal parts of the continent. The latter, in many cases, range also over the Arctic and sub-Arctic regions of the Old World. Others extend from Arctic America southward to the United States. Still others occupy only the middle or more temperate latitudes, being unrepresented in the extreme north or the extreme south. Others, again, first appear in the middle or more southerly parts, and range thence southward far into the Tropics. A large number are restricted to the region east of the Rocky Mountains; others are confined to a narrow belt along the Pacific coast; and others still to limited areas of the great Rocky Mountain Plateau. In general, their distribution accords with climatal regions or zones, their respective ranges being limited in part by latitude and in part by geographical barriers, as treeless, arid plains, or high mountain ranges. The northern and southern boundaries of the habitat of a species are found to agree, not generally with the arbitrary parallels of the geographer, but with isothermal lines, these being more or less different for each species. The geographical distribution of a species is thus mainly determined by climatic or other physical causes, though in part, doubtless, by its organic constitution. In most cases species that are wide ranging are the most variable, as would naturally follow from their being subjected, in the different portions of their habitats, to widely different environing circumstances.

Hence such species are often found to run into numerous local races, some of them greatly differing from others, but still inseparably connected by individuals inhabiting the intervening regions. Over districts slightly diversified, even if of large extent, species generally preserve comparative constancy of character, while, conversely, local races are of frequent occurrence in regions of alternating valleys, mountain ranges, and table lands, and more especially is this true if the highly diversified region be situated in the warmer latitudes. Small islands, remotely situated from other lands, have usually many species peculiar to themselves, their differentiation being proportionate to the geologic antiquity of, the islands and their remoteness from larger In islands of recent origin and not widely separated from continental lands, the ancestral stock of the species is still often

SM 1905-28

clearly apparent, the forms thus differentiated through insular influences not having passed beyond the varietal stage; in other cases they are specifically different from their nearest continental allies, or may even have advanced far toward generic distinctness, while their origin may still remain tolerably apparent.

Plasticity, or susceptibility to the influences of physical surroundings, often differs even among quite closely allied species, as those of the same family or even genus, and different species are evidently affected differently by the same circumstances. Variability in color may or may not accompany variability in size or in the character of particular organs. Generally, however, a species which varies greatly in one feature varies to a similar degree in many others. Species having a wide geographical range not only commonly run into a greater or less number of local races, but they generally present more than the average amount of strictly individual variation, as though species ranging widely in space were originally more plastic than those having more circumscribed habitats, and were thus able more casily to adapt themselves to their surroundings; they are also more persistent, their fossil remains being far more frequently met with in the quaternary deposits than are those of the more local and generally more specialized forms.

Geographical variation, as exhibited by the mammals and birds of North America, may be summarized under the following heads, namely, (1) variation in general size, (2) in the size of peripheral parts, and (3) in color, the latter being subdivisible into (a) variation. in color with latitude and (b) with longitude. As a rule, the mammals and birds of North America increase in size from the south northward. This is true not only of the individual representatives of each species, but generally the largest species of each genus and family are northern. There are, however, some strongly marked exceptions, in which the increase in size is in the opposite direction, or southward. There is for this an obvious explanation, as will be presently shown, the increase being found to be almost invariably toward the region where the type or group to which the species belongs receives its greatest numerical development and where the species attain the largest size, and are also most specialized. Hence the representatives of a given species increase in size toward its hypothetical center of distribution, which is in most cases doubtless also its original center of dispersal. Consequently there is frequently a double decadence in size within specific groups, and both in size and numerically in the case of species when the center of development of the group to which they belong is in the warm temperate or tropical regions. This may be illustrated by reference to the distribution of the higher classes of vertebrates in North America. Among the species occurring north of Mexico there are very few that may not be supposed to have had

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