Temperature and Vertebra 15 



segments has come about through degeneration, the cause of 

 such degeneration must be sought for in the colder seas, in the 

 rivers and in the oceanic ab3'sses. What have these in com- 

 mon that the sandy shores, rocky islands and coral reefs of the 

 tropics have not? 



STATEMENT OF THEORY. 



For the purpose of this discussion we may assume the deri- 

 vation of species by means of the various influences and pro- 

 cesses, for which, without special analysis, we may use the 

 term "natural selection." 



By the influence of natural selection, the .spiny-rayed fish, 

 so characteristic of the present geological era, has diverged 

 from its soft-rayed ancestry. 



The influences which have produced the spiny-raj'ed fish 

 have been most active in the tropical seas. It is there that 

 "natural selection " is most potent, so far as fishes are con- 

 cerned. The influence of cold, darkness, monotony, and re- 

 striction is to limit the direct struggle for existence, and there- 

 fore to limit the resultant changes. In general the external 

 conditions most favorable to fish life are to be found in the 

 tropical seas, among rocks and along the coral reefs near the 

 shore. Here is the center of competition. From conditions 

 otherwise favorable to be found in arctic regions, the majority 

 of competitors are excluded by their inability to bear the cold. 

 In the tropics is found the greatest variety in surroundings, 

 and therefore the greatest variety in the possible adjustments 

 of series of individuals to correspond with these surroundings. 



The struggle for existence in the tropics is a struggle be- 

 tween fish and fish, and among the individuals of a very great 

 number of species each one acquiring its own peculiar points 

 of advantage. No form is excluded from competition. No 

 competitor is handicapped by loss of strength on account of 

 cold, darkness, foul water, or any condition adverse to fish 

 life. Very few fishes are excluded from the tropical seas by 

 the heat of the water. The land heat of the tropics is often 

 unfavorable to life and especially to activity. But in the sea 

 the temperature is never unfavorable to self activity. The 

 water is never sultry, nor laden with malaria. 



