LATITUDE AND VERTEBRA. 



223 



and more than localized or non-migratory forms.* The 

 extinct fishes of earlier geological periods had more ver- 

 tebrae than the corresponding modern forms which are 

 regarded as their descendants. To each of these gener- 

 alizations 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 vertebrae than the marine shore fishes of the tropics. 

 The cause of the reduction in numbers of vertebrae 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 vertebrae, 

 these vertebrae 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 vertebrae. 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 reduc- 

 tion 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 unfavor- 

 able in full growth. 



16 * 



