108 TEMPERATURE AND FISH VERTEBRA JORDAN. 



In the present paper, I wish to consider the extent to wliich these 

 statements are true and to suggest a line of exi)lanation which cover 

 all these generalizations alike. 



STATEMENT OP THEORY. 



i 



For the purpose of this discussion we may assume the derivation of 

 species by means of the various influences and processes, for which, 

 without special analysis, we may use the term "natural selection." 



By the influence of natural selection, the spiny-rayed fish, so char- 

 acteristic of the present geological era, has diverged from its soft- 

 rayed ancestry. 



The influences which have produced the spiny-rayed fish have been 

 most active in the tropical seas. It is there that "natural selection" 

 is most potent, so far as fishes are concerned. The influence of cold, 

 darkness, monotony, and restriction is to limit the direct struggle for 

 existence, and therefore 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 favor- 

 able to be found iu Arctic regions, the majority of competitors are ex- 

 cluded 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 between fisli 

 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. 



The influences which serve as a whole to make a fish more intensely 

 and compactly a fish, and which tend to rid it of every character and 

 every organ not needed in fish life, should be most effective along the 

 rocks and shores of the tropics. 



For this iirocess of intensification offish-like characters, which finds 

 its culmination in certain specialized spiny rayed* fishes of the coral 

 reefs, we may conveniently use the term " Ichthyizatiou." 



"low." It is certain, however, that the spiny-rayed fishes deviate farthest from the 

 primitive stock, and that the qualities that distinguish fislies as a group are most 

 intensified. In other words, it is in the spiny-rayed fishes that the jjrocess of " ichthy-^ 

 ization" or fish-forming has gone farthest. A third category would comprise the 

 Anacanthines (cods, flounders, etc.), fishes auatou.ically similar to the spiny-rayed 

 forms, hut without spines to their fins, with weaker skeletons and smaller and more 

 numerous vertebne. They are " degenerate" or more " generalized " ofi'shoots from 

 the spiny-rayed types, as the eels are from some soft-rayed type. 



"The Parrot-fishes (Scaridw), Trigger-fishes {Balislidw), Angel-fishes (CJuvtodon- 

 tidw), etc. 



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