FOOTNOTES:
[37] Rheopresbe.
[38] Bryttosus.
[39] Leuciscus hakuensis Günther.
[40] Leuciscus jouyi.
[41] Formerly, but no longer, called Yeso in Japan.
[42] Called Nippon on foreign maps, but not so in Japan, where Nippon means the whole empire.
[43] Pleuronichthys cornutus, Hexogrammos otakii, etc.
[44] As Halichœres, Tetrapturus, Callionymus, Ariscopus, etc.
[45] Of these, the principal ones are Oxystomus, Myrus, Pagrus, Sparus, Macrorhamphosus, Cepola, Callionymus, Zeus, Uranoscopus, Lepidotrigla, Chelidonichthys.
[46] Among these are Beryx, Helicolenus, Lotella, Nettastoma, Centrolophus, Hoplostethus, Aulopus, Chlorophthalmus, Lophotes.
[47] Beryx, Hoplostethus.
[48] Antigonia, Etelis, Emmelichthys.
[49] Lepadogaster, Myrus; Lophotes, thus far recorded from Japan, the Mediterranean, and the Cape of Good Hope, is bassalian and of unknown range. Beryx, Trachichthys, Hoplostethus, etc., are virtually cosmopolitan as well as semi-bassalian.
[50] In this group we must place Cepola, Callionymus, Pagrus, Sparus, Beryx, Zeus, all of which have a very wide range in Indian waters.
[51] Cryptocentrus, Asterropteryx. The range of neither of these genera of small shore fishes is yet well known.
[52] As Crenilabrus, Labrus, Symphodus, Pagellus, Spondyliosoma, Sparisoma.
[53] As Chætodon, Lethrinus, Monotaxis, Glyphisodon, etc.
[54] Hoplopagrus, Xenichthys, Xenistius, Xenocys, Microdesmus, Cerdale, Cratinus, Azevia, Microlepidotus, Orthostœchus, Isaciella, etc.
[55] Hæmulon, Anisotremus, Gerres, Centropomus, Galeichthys, Hypoplectrus, Mycteroperca, Ulæma, Stellifer, Micropogon, Bodianus, Microspathodon.
[56] Among these are perhaps Teuthis (Acanthurus), Ilisha, Salarias, Myripristis, Thalassoma. Some such which have not crossed the Isthmus are Cirrhitus, Sectator, Sebastopsis, and Lophiomus.
[57] "Fishes of Central America," 1869, 397.
[58] Proc. U. S. Nat. Mus., 1885, 393.
[59] Introduction to the "Study of Fishes," 1880, p. 280.
[60] Proc. U. S. Nat. Mus., 1891, pp. 124-126.
[61] Citharichthys spilopterus and C. gilberti.
[62] Hæmulon steindachneri and Gymnothorax castaneus of the west coast probably being identical with H. schranki and Gymnothorax funebris of the east coast.
[CHAPTER XVI]
DISPERSION OF FRESH-WATER FISHES[63]
Dispersion of Fishes.—The methods of dispersion of fishes may be considered apart from the broader topic of distribution or the final results of such dispersion. In this discussion we are mainly concerned with the fresh-water fishes, as the methods of distribution of marine fishes through marine currents and by continuity of shore and water ways are all relatively simple.
The Problem of Oatka Creek.—When I was a boy and went fishing in the brooks of western New York, I noticed that the different streams did not always have the same kinds of fishes in them. Two streams in particular in Wyoming County, not far from my father's farm, engaged in this respect my special attention. Their sources are not far apart, and they flow in opposite directions, on opposite sides of a low ridge—an old glacial moraine, something more than a mile across. The Oatka Creek flows northward from this ridge, while the East Coy runs toward the southeast on the other side of it, both flowing ultimately into the same river, the Genesee.
It does not require a very careful observer to see that in these two streams the fishes are not quite the same. The streams themselves are similar enough. In each the waters are clear and fed by springs. Each flows over gravel and clay, through alluvial meadows, in many windings, and with elms and alders "in all its elbows." In both streams we were sure of finding trout,[64] and in one of them the trout are still abundant. In both we used to catch the brook chub,[65] or, as we called it, the "horned dace"; and in both were large schools of shiners[66] and of suckers.[67] But in every deep hole, and especially in the millponds along the East Coy Creek, the horned pout[68] swarmed on the mucky bottoms. In every eddy, or in the deep hole worn out at the root of the elm-trees, could be seen the sunfish,[69] strutting in green and scarlet, with spread fins keeping intruders away from its nest. But in the Oatka Creek were found neither horned pout nor sunfish, nor have I ever heard that either has been taken there. Then besides these nobler fishes, worthy of a place on every schoolboy's string, we knew by sight, if not by name, numerous smaller fishes, darters[70] and minnows,[71] which crept about in the gravel on the bottom of the East Coy, but which we never recognized in the Oatka.
There must be a reason for differences like these, in the streams themselves or in the nature of the fishes. The sunfish and the horned pout are home-loving fishes to a greater extent than the others which I have mentioned; still, where no obstacles prevent, they are sure to move about. There must be, then, in the Oatka some sort of barrier, or strainer, which keeping these species back permits others more adventurous to pass; and a wider knowledge of the geography of the region showed that such is the case. Farther down in its course, the Oatka falls over a ledge of rock, forming a considerable waterfall at Rock Glen. Still lower down its waters disappear in the ground, sinking into some limestone cavern or gravel-bed, from which they reappear, after some six miles, in the large springs at Caledonia. Either of these barriers might well discourage a quiet-loving fish; while the trout and its active associates have some time passed them, else we should not find them in the upper waters in which they alone form the fish fauna. This problem is a simple one; a boy could work it out, and the obvious solution seems to be satisfactory.
Generalizations as to Dispersion.—Since those days I have been a fisherman in many waters,—not an angler exactly, but one who fishes for fish, and to whose net nothing large or small ever comes amiss; and wherever I go I find cases like this.
We do not know all the fishes of America yet, nor all those well that we know by sight; still this knowledge will come with time and patience, and to procure it is a comparatively easy task. It is also easy to ascertain the more common inhabitants of any given stream. It is difficult, however, to obtain negative results which are really results. You cannot often say that a species does not live in a certain stream. You can only affirm that you have not yet found it there, and you can rarely fish in any stream so long that you can find nothing that you have not taken before. Still more difficult is it to gather the results of scattered observations into general statements regarding the distribution of fishes. The facts may be so few as to be misleading, or so numerous as to be confusing, and the few writers who have taken up this subject in detail have found both these difficulties to be serious. Whatever general propositions we may maintain must be stated with the modifying clause of "other things being equal"; and other things are never quite equal. The saying that "Nature abhors a generalization" is especially applicable to all discussions of the relations of species to environment.
Still less satisfactory is our attempt to investigate the causes on which our partial generalizations depend,—to attempt to break to pieces the "other things being equal" which baffle us in our search for general laws. The same problems, of course, come up on each of the other continents and in all groups of animals or plants; but most that I shall say will be confined to the question of the dispersion of fishes in the fresh waters of North America. The broader questions of the boundaries of faunæ and of faunal areas I shall bring up only incidentally.
Questions Raised by Agassiz.—Some of the problems to be solved were first noticed by Prof. Agassiz in 1850, in his work on Lake Superior. Later (1854), in a paper on the fishes of the Tennessee River,[72] he makes the following statement:
"The study of these features [of distribution] is of the greatest importance, inasmuch as it may eventually lead to a better understanding of the intentions implied in this seemingly arbitrary disposition of animal life....
"There is still another very interesting problem respecting the geographical distribution of our fresh-water animals which may be solved by the further investigation of the fishes of the Tennessee River. The water-course, taking the Powell, Clinch, and Holston Rivers as its head waters, arises from the mountains of Virginia in latitude 37°; it then flows S.W. to latitude 34° 25', when it turns W. and N.W., and finally empties into the Ohio, under the same latitude as its source in 37°.
Fig. 185.—Horned Dace, Semotilus atromaculatus (Mitchill). Aux Plaines River, Ills. Family Cyprinidæ.
"The question now is this: Are the fishes of this water system the same throughout its extent? In which case we should infer that water communication is the chief condition of geographical distribution of our fresh-water fishes. Or do they differ in different stations along its course? And if so, are the differences mainly controlled by the elevation of the river above the level of the sea, or determined by climatic differences corresponding to differences of latitude? We should assume that the first alternative was true if the fishes of the upper course of the river differed from those of the middle and lower courses in the same manner as in the Danube, from its source to Pesth, where this stream flows nearly for its whole length under the same parallel. We would, on the contrary, suppose the second alternative to be well founded if marked differences were observed between the fish of such tracts of the river as do not materially differ in their evolution above the sea, but flow under different latitudes. Now, a few collections from different stations along this river, like that sent me by Dr. Newman from the vicinity of Huntsville, would settle at once this question, not for the Tennessee River alone, but for most rivers flowing under similar circumstances upon the surface of the globe. Nothing, however, short of such collections, compared closely with one another, will furnish a reliable answer.... Whoever will accomplish this survey will have made a highly valuable contribution to our knowledge."
Conclusions of Cope.—Certain conclusions were also suggested by Prof. Cope in his excellent memoir on the fishes of the Alleghany region[73] in 1868. From this paper I make the following quotations:
"The distribution of fresh-water fishes is of special importance to the questions of the origin and existence of species in connection with the physical conditions of the waters and of the land. This is, of course, owing to the restricted nature of their habitat and the impossibility of their making extended migrations. With the submergence of land beneath the sea, fresh-water fish are destroyed in proportion to the extent of the invasion of salt water, while terrestrial vertebrates can retreat before it. Hence every inland fish fauna dates from the last total submergence of the country.
"Prior to the elevation of a given mountain chain, the courses of the rivers may generally have been entirely different from their later ones. Subsequent to this period, they can only have undergone partial modifications. As subsequent submergences can rarely have extended to the highlands where such streams originate, the fishes of such rivers can only have been destroyed so far as they were unable to reach those elevated regions, and preserve themselves from destruction from salt water by sheltering themselves in mountain streams. On the other hand, a period of greater elevation of the land, and of consequent greater cold, would congeal the waters and cover their courses with glaciers. The fishes would be driven to the neighborhood of the coast, though no doubt in more southern latitudes a sufficient extent of uncongealed fresh waters would flow by a short course into the ocean, to preserve from destruction many forms of fresh-water fishes. Thus, through many vicissitudes, the fauna of a given system of rivers has had opportunity of uninterrupted descent, from the time of the elevation of the mountain range, in which it has its sources....
"As regards the distinction of species in the disconnected basins of different rivers, which have been separated from an early geologic period, if species occur which are common to any two or more of them, the supporter of the theory of distinct creations must suppose that such species have been twice created, once for each hydrographic basin, or that waters flowing into the one basin have been transferred to another. The developmentalist, on the other hand, will accept the last proposition, or else suppose that time has seen an identical process and similar result of modification in these distinct regions.
Fig. 186.—Chub of the Great Basin, Leuciscus lineatus (Girard). Heart Lake, Yellowstone Park. Family Cyprinidæ.
"Facts of distribution in the eastern district of North America are these. Several species of fresh-water fishes occur at the same time in many Atlantic basins from the Merrimac or from the Hudson to the James, and throughout the Mississippi Valley, and in the tributaries of the Great Lakes. On the other hand, the species of each river may be regarded as pertaining to four classes, whose distribution has direct reference to the character of the water and the food it offers: first, those of the tide-waters, of the river channels, bayous, and sluggish waters near them, or in the flat lands near the coast; second, those of the river channels of its upper course, where the currents are more distinct; third, those of the creeks of the hill country; fourth, those of the elevated mountain streams which are subject to falls and rapids."
In the same paper Prof. Cope reaches two important general conclusions, thus stated by him:
"I. That species not generally distributed exist in waters on different sides of the great watershed.
"II. That the distribution of the species is not governed by the outlet of the rivers, streams having similar discharges (Holston and Kanawha, Roanoke and Susquehanna) having less in common than others having different outlets (Kanawha, or Susquehanna and James).
Fig. 187.—Butterfly-sculpin, Melletes papilio Bean, a fish of the rock-pools. St. Paul, Pribilof Islands.
"In view of the first proposition, and the question of the origin of species, the possibility of an original or subsequent mingling of the fresh waters suggests itself as more probable than that of distinct origin in the different basins."
Questions Raised by Cope.—Two questions in this connection are raised by Prof. Cope. The first question is this: "Has any destruction of the river faunæ taken place since the first elevation of the Alleghanies, when the same species were thrown into waters flowing in opposite directions?" Of such destruction by submergence or otherwise, Prof. Cope finds no evidence. The second question is, "Has any means of communication existed, at any time, but especially since the last submergence, by which the transfer of species might occur?" Some evidence of such transfer exists in the wide distribution of certain species, especially those which seek the highest streamlets in the mountains; but except to call attention to the cavernous character of the Subcarboniferous and Devonian limestones, Prof. Cope has made little attempt to account for it.
Prof. Cope finally concludes with this important generalization:
"It would appear, from the previous considerations, that the distribution of fresh-water fishes is governed by laws similar to those controlling terrestrial vertebrates and other animals, in spite of the seemingly confined nature of their habitat."
Views of Günther.—Dr. Günther[74] has well summarized some of the known facts in regard to the manner of dispersion of fishes:
"The ways in which the dispersal of fresh-water fishes has been affected were various. They are probably all still in operation, but most work so slowly and imperceptibly as to escape direct observation; perhaps they will be more conspicuous after science and scientific inquiry shall have reached a somewhat greater age. From the great number of fresh-water forms which we see at this present day acclimatized in, gradually acclimatizing themselves in, or periodically or sporadically migrating into, the sea, we must conclude that under certain circumstances salt water may cease to be a barrier at some period of the existence of fresh-water species, and that many of them have passed from one river through salt water into another. Secondly, the headwaters of some of the grandest rivers, the mouths of which are at opposite ends of the continents which they drain, are sometimes distant from each other a few miles only. The intervening space may have been easily bridged over for the passage of fishes by a slight geological change affecting the level of the watershed or even by temporary floods; and a communication of this kind, if existing for a limited period only, would afford the ready means of an exchange of a number of species previously peculiar to one or the other of these river or lake systems. Some fishes provided with gill-openings so narrow that the water moistening the gills cannot readily evaporate, and endowed, besides, with an extraordinary degree of vitality, like many Siluroids (Chlarias, Callichthys), eels, etc., are enabled to wander for some distance over land, and may thus reach a water-course leading them thousands of miles from their original home. Finally, fishes or their ova may be accidentally carried by water-spouts, by aquatic birds or insects, to considerable distances."
Fresh-water Fishes of North America.—We now recognize about six hundred species[75] of fishes as found in the fresh waters of North America, north of the Tropic of Cancer, these representing thirty-four of the natural families. As to their habits, we can divide these species rather roughly into the four categories proposed by Prof. Cope, or, as we may call them,
(1) Lowland fishes; as the bowfin,[76] pirate-perch,[77] large-mouthed black bass,[78] sunfishes, and some catfishes.
(2) Channel-fishes; as the channel catfish,[79] the mooneye,[80] garpike,[81] buffalo-fishes,[82] and drum.[83]
(3) Upland fishes; as many of the darters, shiners, and suckers, and the small-mouthed black bass.[84]
(4) Mountain-fishes; as the brook trout and many of the darters and minnows.
To these we may add the more or less distinct classes of (5) lake fishes, inhabiting only waters which are deep, clear, and cold, as the various species of whitefish[85] and the Great Lake trout;[86] (6) anadromous fishes, or those which run up from the sea to spawn in fresh waters, as the salmon,[87] sturgeon,[88] shad,[89] and striped bass;[90] (7) catadromous fishes, like the eel,[91] which pass down to spawn in the sea; and (8) brackish-water fishes, which thrive best in the debatable waters of the river-mouths, as most of the sticklebacks and the killifishes.
As regards the range of species, we have every possible gradation from those which seem to be confined to a single river, and are rare even in their restricted habitat, to those which are in a measure cosmopolitan,[92] ranging everywhere in suitable waters.
Characters of Species.—Still, again, we have all degrees of constancy and inconstancy in what we regard as the characters of a species. Those found only in a single river-basin are usually uniform enough; but the species having a wide range usually vary much in different localities. Such variations have at different times been taken to be the indications of as many different species. Continued explorations bring to light, from year to year, new species; but the number of new forms now discovered each year is usually less than the number of recognized species which are yearly proved to be untenable. Four complete lists of the fresh-water fishes of the United States (north of the Mexican boundary) have been published by the present writer. That of Jordan and Copeland,[93] published in 1876, enumerates 670 species. That of Jordan[94] in 1878 contains 665 species, and that of Jordan and Gilbert[95] in 1883, 587 species. That of Jordan and Evermann[96] in 1898 contains 585 species, although upwards of 130 new species were detected in the twenty-two years which elapsed between the first and the last list. Additional specimens from intervening localities are often found to form connecting links among the nominal species, and thus several supposed species become in time merged in one. Thus the common channel catfish[97] of our rivers has been described as a new species not less than twenty-five times, on account of differences real or imaginary, but comparatively trifling in value.
Where species can readily migrate, their uniformity is preserved; but whenever a form becomes localized its representatives assume some characters not shared by the species as a whole. When we can trace, as we often can, the disappearance by degrees of these characters, such forms no longer represent to us distinct species. In cases where the connecting forms are extinct, or at least not represented in collections, each form which is apparently different must be regarded as a distinct species.
The variations in any type become, in general, more marked as we approach the tropics. The genera are represented, on the whole, by more species there, and it would appear that the processes of specific change go on more rapidly under the easier conditions of life in the Torrid Zone.
We recognize now in North America twenty-five distinct species of fresh-water catfishes,[98] although nearly a hundred (93) nominal species of these fishes have been from time to time described. But these twenty-five species are among themselves very closely related, and all of them are subject to a variety of minor changes. It requires no strong effort of the imagination to see in them all the modified descendants of some one species of catfish, not unlike our common "bullhead,"[99] an immigrant probably from Asia, and which has now adjusted itself to its surroundings in each of our myriad of catfish-breeding streams.
Meaning of Species.—The word "species," then, is simply a term of convenience, including such members of a group similar to each other as are tangibly different from others, and are not known to be connected with these by intermediate forms. Such connecting links we may suppose to have existed in all cases. We are only sure that they do not now exist in our collections, so far as these have been carefully studied.
When two or more species of any genus now inhabit the same waters, they are usually species whose differentiation is of long standing,—species, therefore, which can be readily distinguished from one another. When, on the other hand, we have "representative species,"—closely related forms, neither of which is found within the geographical range of the other,—we can with some confidence look for intermediate forms where the territory occupied by the one bounds that inhabited by the other. In very many such cases the intermediate forms have been found; and such forms are considered as subspecies of one species, the one being regarded as the parent stock, the other as an offshoot due to the influences of different environment. Then, besides these "species" and "subspecies," groups more or less readily recognizable, there are varieties and variations of every grade, often too ill-defined to receive any sort of name, but still not without significance to the student of the origin of species. Comparing a dozen fresh specimens of almost any kind of fish from any body of water with an equal number from somewhere else, one will rarely fail to find some sort of differences,—in size, in form, in color. These differences are obviously the reflex of differences in the environment, and the collector of fishes seldom fails to recognize them as such; often it is not difficult to refer the effect to the conditions. Thus fishes from grassy bottoms are darker than those taken from over sand, and those from a bottom of muck are darker still, the shade of color being, in some way not well understood, dependent on the color of the surroundings. Fishes in large bodies of water reach a larger size than the same species in smaller streams or ponds. Fishes from foul or sediment-laden waters are paler in color and slenderer in form than those from waters which are clear and pure. Again, it is often true that specimens from northern waters are less slender in body than those from farther south; and so on. Other things being equal, the more remote the localities from each other, the greater are these differences.
Fig. 188.—Scartichthys enosimæ Jordan and Snyder, a fish of the rock-pools of the sacred island of Enoshima, Japan. Family Blenniidæ.
In our fresh-water fishes each species on an average has been described as new from three to four times, on account of minor variations, real or supposed. In Europe, where the fishes have been studied longer and by more different men, upwards of six or eight nominal species have been described for each one that is now considered distinct.
Special Creation Impossible.—It is evident, from these and other facts, that the idea of a separate creation for each species of fishes in each river-basin, as entertained by Agassiz, is wholly incompatible with our present knowledge of the specific distinctions or of the geographical distribution of fishes. This is an unbroken gradation in the variations from the least to the greatest,—from the peculiarities of the individual, through local varieties, geographical subspecies, species, sub-genera, genera, families, super-families, and so on, until all fish-like vertebrates are included in a single bond of union.
Origin of American Species of Fishes.—It is, however, evident that not all American types of fishes had their origin in America, or even first assumed in America their present forms. Some of these are perhaps immigrants from northern Asia, where they still have their nearest relatives. Still others are evidently modified importations from the sea; and of these some are very recent immigrants, land-locked species which have changed very little from the parent stock.
The problems of analogous variation or parallelism without homology are very often met with among fishes. In shallow, swift brooks in all lands there are found small fishes which hug the bottom—large-finned, swift of movement, with speckled coloration, and with the air-bladder reduced in size. In the eastern United States these fishes are darters, dwarf perches; in northern India they are catfishes; in Japan, gobies or loaches; in Canada, sculpins; in South America, characins. Members of various groups may be modified to meet the same conditions of life. Being modified to look alike, the thought of mutual affinity is naturally suggested, but in such cases the likeness is chiefly external. The internal organs show little trace of such modifications. The inside of an animal tells what it really is, the outside where it has been. In other words, it is the external characters which are most readily affected by the environment. Throughout all groups of animals and plants, there are large branches similarly affected by peculiarities of conditions.
This is the basis of the law of "Adaptive Radiation." Prof. H. F. Osborn thus states this law:
"It is a well-known principle of zoological evolution that an isolated region, if large and sufficiently varied in its topography, soil, climate, and vegetation, will give rise to a diversified fauna according to the law of adaptive radiation from primitive and central types. Branches will spring off in all directions to take advantage of every possible opportunity of securing food. The modifications which animals undergo in this adaptive radiation are largely of mechanical nature; they are limited in number and kind by hereditary stirp or germinal influences, and thus result in the independent evolution of similar types in widely separated regions under the law of parallelism or homoplasy."