LATITUDE AND VERTEBRA. 223 
and more than localized or non-migratory forms.* The 
extinct fishes of earlier geological periods had more ver- 
tebree 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 vertebre than the marine shore fishes of the tropics. 
The cause of the reduction in numbers of vertebre 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 vertebra, 
these vertebre 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 vertebre. 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 vertebre, 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. 
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