Temperature and VertebrtE 29 



CONCLUSION. 



From the foregoing examples we ma}' conclude that, other 

 things lieing equal, the numbers of vertebrae are lowest in 

 the shore-fishes of the tropics, and especially in those of local 

 habits, living about rocks and coral reefs. 



The cause of this is to be found in the fact that in these lo- 

 calities the influences of natural selection are most active. 

 The reduction of vertebrse may be regarded as a phase in the 

 process of specialization which has brought about the typical 

 spiny-rayed fish. 



These influences are most active in the warm, clear waters 

 of tropical shores, because these regions offer conditions most 

 favorable to fish life, and to the life of the greatest variety of 

 fishes. No fish is excluded from competition. There is the 

 greatest variety of competitors, the greatest variety of fish- 

 food, and the greatest variety of conditions to which adapta- 

 tion is po.ssible. The number of species visiting any single 

 area is vastly greater in the tropics than in cold regions. 



A single drawing of the net on the shores of Cuba''- will 

 obtain more different kinds of fish than can be found on the 

 coasts of Maine in a j^ear. Cold, monotony, darkness, isola- 

 tion, foul water ; all these are characters opposed to the 

 formation of variety in fish life. The absence of these is a 

 chief feature of life in the tropical waters. 



The life of the tropics, so far as the fishes are concerned, 

 offers analogies to the life of cities, viewed from the stand- 

 point of human development. In the same way the other re- 

 gions under consideration are, if we may .so speak, a sort of 

 ichthyological backwoods. In the cities, in general, the con- 

 ditions of individual existence are most easy, but the compe- 

 tition is most severe. The struggle for existence is not a 

 struggle with the forces and conditions of nature. ' It is not a 

 struggle with wild beasts, unbroken forests, or a stubborn 

 soil, but a competition between man and man for the oppor- 

 tunity of living. 



■* In 1884 a single haul of a net in a shallow bay on Key West brought 

 in seventy-five species of shore-fishes. A week's work about Martha's 

 Vineyard yielded but forty-eight kinds. 



