VIII.

LATITUDE AND VERTEBRÆ.

A STUDY IN THE EVOLUTION OF FISHES.

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 the derivation of species 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.

Northern fishes have most vertebræ.

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 non-migratory 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 show 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 each, 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 saltwater fishes of the same group. Deepsea fishes have more vertebræ than fishes of shallow waters. Pelagic fishes and free-swimming fishes have more than those which live along the shores,

Fewest vertebræ in shore fishes of the tropics.

* In a more technical paper on this subject entitled Relations of Temperature to Vertebræ among Fishes, published in the Proceedings of the United States National Museum for 1891, pp. 107– 120. Still fuller details are given in a paper contained in the Wilder Quarter-Century Book, 1893.

and more than localized or non-migratory 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

* This is especially true among those fishes which swim for long distances, as, for example, many of the mackerel family. Among such there is often found a high grade of muscular power, or even of activity, associated with a large number of vertebræ, these vertebræ being individually small and little differentiated. For long-continued muscular action of a uniform kind there would be perhaps an advantage in the low development of the vertebral column. For muscular alertness, moving short distances with great speed, the action of a fish constantly on its guard against enemies or watching for its prey, the advantage would be on the side of few vertebræ. There is often a correlation between the freeswimming habit and slenderness and suppleness of body, which again is often dependent on an increase in numbers of the vertebral segments. These correlations appear as a disturbing element in the problem rather than as furnishing a clew to its solution. In some groups of fresh-water fishes there is a reduction in numbers of vertebræ, not associated with any degree of specialization of the individual bone, but correlated with simple reduction in size of body. This is apparently a phenomenon of degeneration, a survival of dwarfs where conditions are unfavorable in full growth.

in common that the coral reefs, rocky islands, and tide pools of the tropics have not?

FIG. 16.-Skeleton of the spotted greenling (Hexagrammos decagrammus). From nature, by W. S. Atkinson. A northern fish, with vertebræ numerous and small.

In this connection we are to remember that the fewer vertebræ indicates generally the higher rank. When

Fewer vertebræ indicates greater specialization.

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,

FIG. 17. Skeleton of the scarlet rock-fish (Sebastodes miniatus). From nature, by W. S. Atkinson. A species of temperate waters; the vertebræ in moderate number.

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 a process we may call ichthyization, the work of making a better fish. Not a better fish for man's purposes for Nature does not care for man's purposesbut 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 centre of competition no species can afford to be handicapped by a weak back

FIG. 18.-Skeleton of angel fish (Angelichthys ciliaris). From nature, by W. S. Atkinson. A tropical species; the vertebræ few and large.

bone and redundant vertebræ. Those who are thus weighted can not hold their own. They must change or perish.

Coral reefs the centre of fish competition.

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 favourable to fish life are among the rocks and reefs of the tropical About the coral reefs is the centre of fish competition. A coral archipelago is the Paris of fishes. In such regions is the greatest variety of surroundings, and

seas.