Popular Science Monthly/Volume 45/July 1894/Latitude and Vertebrae
A STUDY IN THE EVOLUTION OF FISHES.
By DAVID STARR JORDAN.
IN this paper is given an account of a curious biological problem and of the progress which has been made toward its solution. The discussion may have a certain popular interest from the fact that it is a type of many problems in the structure and distribution of animals and plants which seem to be associated with the laws of evolution. In the light of these laws they may be more or less perfectly solved. On any other hypothesis than that of organic evolution the solution of the present problem, for example, would be impossible. On the hypothesis of special creation a solution would be not only impossible but inconceivable.
It has been known for some years that in several groups of fishes (wrasse fishes, flounders, and "rock cod," for example) those species which inhabit northern waters have more vertebræ than those living in the tropics. Certain arctic flounders, for example, have sixty vertebræ; tropical flounders have, on the average, thirty. The significance of this fact is the problem at issue. In science it is assumed that all facts have significance, else they would not exist. It becomes necessary, then, to find out first just what the facts are in this regard.
Going through the various groups of nonmigratory marine fishes we find that such relations are common. In almost every group the number of vertebræ grows smaller as we approach the equator, and grows larger again as we pass into southern latitudes.
It would be tedious to try to prove this here by statistical tables, but the value of generalization in science depends on such evidence. This proof I have elsewhere given in detail. Suffice it to say that, taking an average netful of fishes of different kinds at different places along the coast, the variation would be evident. At Point Barrow or Cape Farewell or North Cape a seineful of fishes would perhaps average eighty vertebræ apiece, the body lengthened to make room for them; at Sitka or St. Johns or Bergen, perhaps, sixty vertebræ; at San Francisco or New York or St. Malo, thirty-five; at Mazatlan or Pensacola or Naples, twenty-eight; and at Panama or Havana or Sierra Leone, twenty-five. Under the equator the usual number of vertebræ in shore fishes is twenty-four. Outside the tropics this number is the exception. North of Cape Cod it is virtually unknown.
The next question which arises is whether we can find other conditions that may affect these numbers. These readily appear. Fresh-water fishes have in general more vertebræ than salt-water fishes of the same group. Deep-sea fishes have more vertebræ than fishes of shallow waters. Pelagic fishes and free-swimming fishes have more than those which live along the shores, and more than localized or nonmigratory forms. The extinct fishes of earlier geological periods had more vertebræ than the corresponding modern forms which are regarded as their descendants. To each of these generalizations there are occasional partial exceptions, but not such as to invalidate the rule.
All these effects should be referable to the same group of causes. They may, in fact, be combined in one statement. All other fishes have a larger number of vertebræ than the marine shore fishes of the tropics. The cause of the reduction in numbers of vertebræ must therefore be sought in conditions peculiar to the tropical seas. If the retention of the primitive large number is in any case a phase of degeneration, the cause of such degeneration must be sought in the colder seas, in the rivers, and in oceanic abysses. What have these waters in common that the coral reefs, rocky islands, and tide pools of the tropics have not?
In this connection we are to remember that the fewer vertebræ indicates generally the higher rank. When vertebræ are few in number, as a rule each one is larger. Its structure is more complicated, its appendages are larger and more useful, and the fins with which it is connected are better developed. In other words, the tropical fish is more intensely and compactly a fish, with a better fish equipment, and in all ways better fitted for the business of a fish, especially for that of a fish that stays at home.
In my view the reduction in number and increase of importance of the individual vertebræ are simply part of this work of making a better fish. Not a better fish for man's purposes—for Nature does not care a straw for man's purposes—but a better fish for the purposes of a fish. The competition in the struggle for existence is the essential cause of the change. In the center of competition no species can afford to be handicapped by a weak backbone and redundant vertebræ. Those who are thus weighted can not hold their own. They must change or perish.
The influence of cold, darkness, monotony, and isolation is to limit the struggle for existence, and therefore to prevent its changes, preserving through the conservation of heredity the more remote ancestral conditions, even though they carry with them disadvantages and deficiencies. The conditions most favorable to fish life are among the rocks and reefs of the tropical seas. About the coral reefs is the center of fish competition. A coral archipelago is the Paris of fishes. In such regions is the greatest variety of surroundings, and therefore the greatest number of possible adjustments. The struggle is between fish and fish, not between fishes and hard conditions of life. No form is excluded from the competition. Cold, darkness, and foul water do not shut out competitors, nor does any evil influence sap the strength. The heat of the tropics does not make the water hot. It is never sultry nor laden with malaria. The influence of tropical heat on land animals is often to destroy vitality and check self-activity. It is not so in the sea.
From conditions otherwise favorable in arctic regions the majority of competitors are excluded by their inability to bear the cold. River life is life in isolation. To aquatic animals river life has the same limitations that island life has to the animals of the land. The oceanic islands are behind the continents in the process of evolution. In like manner the rivers are ages behind the seas.
Therefore the influences which serve as a whole to intensify fish life, and tend to rid the fish of every character or structure it can not "use in its business," are most effective along the shores of the tropics. One phase of this is the reduction in numbers of vertebræ, or, more accurately, the increase of stress on each individual bone.
Another phase is the process of cephalization, the process by which the head becomes emphasized and the shoulder bones and other structures become connected with it or subordinated to it. Still another is the reduction and change of the swim-bladder and its utter loss of the function of lung or breathing organ which it occupied in the ganoid ancestors of modern fishes.
Conversely, as these changes are still in operation, we should find that in cold waters, deep waters, dark waters, fresh waters. inclosed waters, and in the waters of past geological epochs, the process would be less completed, the numbers of vertebræ would be larger, while the individual vertebra? remain smaller, less complete, and less perfectly ossified.
This, in a general way, is precisely what we do find in examining the skeletons of a large variety of fishes.
The life of the tropics, so far as fishes are concerned, offers many analogies to the life of cities, viewed from the standpoint of human development. In the cities in general, the conditions of individual existence for the man are most easy, but there also competition of life is most severe. The struggle for existence is not a struggle with the forces and conditions of Nature. It is not a struggle with wild beasts, unbroken forests, or stubborn soil, but a competition between man and man for the opportunity of living.
It is in the city where the influences which tend to modernization and concentration of the characters of the species go on most rapidly. It is adaptation or death to each individual in the city: every quality not directly useful tends to become lost or atrophied.
Conversely, it is in the "backwoods," the region farthest from human conflicts, where primitive customs, antiquated peculiarities, and useless traits are longest and most persistently retained. The life of the "backwoods" may be not less active or vigorous, but it will lack specialization. It is from the unused possibilities of the "backwoods" that the progress of the future comes. The high specialization of favored regions unfits its subjects for life under changed conditions. The loss of muscular power is often one of the results of skeletal specialization.
The coral reef is the metropolis of the fish. The deep sea, the arctic sea, and the isolated rivers—these are the ichthyological backwoods.
An exception to the general rule in regard to the numbers of vertebræ is found in the case of the eel. Eels inhabit nearly all seas, and everywhere they have many vertebræ. The eels of the tropics are at once more specialized and more degraded. They are better eels than those of northern regions, but, as the eel is a degraded type, they have gone further in the loss of structures in which this degeneration consists.
It is not well to push this analogy too far, but perhaps we can find in the comparison of the tropics and the cities some suggestion as to the development of the eel.
In the city there is always a class which follows in no degree the general line of development. Its members are specialized in a wholly different way. By this means they take to themselves a field which others have neglected, making up in low cunning what they lack in humanity or intelligence.
Thus, among the fishes, we have in the regions of closest competition this degenerate and non-fishlike type, lurking in holes among the rocks, or creeping in the sand, thieves and scavengers among fishes. The eels thus fill a place otherwise left unfilled. In their way they are perfectly adapted to the lives they lead. A multiplicity of vertebral joints is useless to the typical fish, but to the eel, strength and suppleness are everything. No armature of fin or scale or bone is so desirable as its power of escaping through the smallest opening.