LATITUDE AND VERTEBRA. 347 



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 vertebrae than salt-water 

 fishes of the same group. Deep-sea fishes have more vertebrae 

 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 vertebrae than the correspond- 

 ing modern forms which are regarded as their descendants. To 

 each of these generalizations there are occasional partial excep- 

 tions, 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 vertebrae than the marine 

 shore fishes of the tropics. The cause of the reduction in num- 

 bers of vertebrae must therefore be sought in conditions peculiar 

 to the tropical seas. If the retention of the primitive large num- 

 ber is in any case a phase of degeneration, the cause of such de- 

 generation 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 verte- 

 brae indicates generally the higher rank. When vertebrae 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 impor- 



* This is especially true among those fishes which swim for long distances, as, for ex- 

 ample, 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 ver- 

 tebra? being individually small and little differentiated. For long-continued muscular action 

 of a uniform kiad there would be perhaps an advantage in the low development of the 

 vertebi'al 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 advan- 

 tage would be on the side of few vertebrae. There is often a correlation between the free- 

 swimming 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 dis- 

 turbing 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 vertebrae, 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 to full growth. 



