2 1 2 Adaptations of Fishes 



Relation of Numbers to Conditions of Life. — Fresh-water fishes 

 have in general more vertebrae than marine fishes of shallow 

 waters. Pelagic fishes and deep-sea fishes have more than 

 those which live along the shores, and more than localized or 

 non-migratory forms. To each of these generalizations there 

 are occasional partial exceptions, but not such as to invalidate 

 the rule. 



The presence of large numbers of vertebras is noteworthy 

 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 vertebrae, these vertebrae being individ- 

 ually small and little differentiated. For long-continued mus- 

 cular action of a uniform kind there would be perhaps an ad- 

 vantage 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 a 

 few vertebras. There is often a correlation between the free- 

 swimming habit and slenderness and suppleness of the body, 

 which again is often dependent on an increase in numbers of 

 the vertebrfil segments. These correlations appear as a dis- 

 turbing element in the problem rather tham as furnishing a 

 clew to its solution. In some groups of fresh-Avater fishes there 

 is a reduction in number of vertebrae, not associated with any 

 degree of speciaHzation of the indi\'idual bone, but correlated 

 with simple reduction in size of body. This is apparently a 

 phenomenon of degeneration, a survival of dwarfs, where con- 

 ditions are unfavorable in full growth. 



All these effects should be referable to the same group of 

 causes. They may, m fact, be combined in one statement. All 

 other fishes now extant, as well as all fishes existing prior to 

 Cretaceous times, have a larger number of vertebne than the 

 marine shore fishes of the tropics of the present period. There 

 is good reason to believe that in most groups of spiny-rayed 

 fishes, those with the smaller number of segments are at once 

 the most highly organized and the most primitive. This is true 

 among the blennies, the sculpins, the flounders, the perches, and 

 probably the labroid fishes as well. The present writer once 



