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. [Illustration: 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. _Cycliæ._--_Palæospondylus_ has about 85 vertebræ. _Arthrodires._--There are about 100 vertebræ in _Coccosteus_. _Dipnoans._--In Protopterus there are upwards of 100 vertebræ, the last much reduced in size. Figures of _Neoceratodus_ show about 80. _Crossopterygians._--_Polypterus_ has 67 vertebræ; _Erpetichthys_, 110; _Undina_, about 85. _Ganoids._--In this group the numbers are also large--95 in _Amia_, about 55 in the short-bodied _Microdon_. The Sturgeons all have more than 100 vertebræ.