Page:Popular Science Monthly Volume 45.djvu/363

Rh 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 vertebræ than salt-water fishes of the same group. Deep-sea fishes have more vertebræ 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 vertebræ than the corresponding modern forms which are regarded as their descendants. To each of these generalizations 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 vertebræ than the marine shore fishes of the tropics. The cause of the reduction in numbers of vertebræ 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 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 vertebræ indicates generally the higher rank. When vertebræ 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