Relation of Number of Vertebræ to Temperature and the Struggle for Existence.—One of the most remarkable modifications of the skeleton of fishes is the progressive increase of the number of vertebræ as the forms become less specialized, and that this particular form of specialization is greatest at the equator.[21]
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.
Fig. 154.—Skeleton of Pike, Esox lucius Linnæus, a river fish with many vertebræ.
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. 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 tropical and semi-tropical waters this number is the exception. North of Cape Cod it is virtually unknown.
Number of Vertebræ.—The numbers of vertebræ in different groups may be summarized as follows:
Lancelets.—Among the lancelets the numbers of segments range from 50 to 80, there being no vertebræ.
Lampreys.—In this group the number of segments ranges from 100 to 150.
Elasmobranchs.—Among sharks and skates the usual number of segments is from 100 to 150 and upwards. In the extinct species as far as known the numbers are not materially different. The Carboniferous genus, Pleuracanthus, has about 115 vertebræ. The Chimæras have similar numbers; Chimæra monstrosa has about 100 in the body and more than as many more in the filamentous tail.