1 6 David Starr Jordan 



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 process of intensification of fish-like characters, 

 which finds its culmination in certain specialized spin)' rayed* 

 fishes of the coral reefs, we may conveniently use the term 

 ' ' Ichthyization ' ' 



If " ichthyization " is in some degree a result of conditions 

 found in the tropics, we may expect to find a less degree of 

 specialization in the restricted and often unfavorable condi- 

 tions which prevail in the fresh waters, in the cold and exclu- 

 sion of the polar seas, and especiall}' in the monotony, dark- 

 ness, and cold of the oceanic abysses where light cannot pen- 

 etrate and where the temperature scarcely rises above the 

 freezing point. 



An important factor in "ichthyization " is the reduction of 

 the number of segments or vertebrae, and a proportionate in- 

 crease in the size and complexit}^ of the individual segment 

 and its appendages. 



If the causes producing this change are still in operation, we 

 should naturally expect that in cold water, deep water, dark 

 water, the fresh waters, and in the waters of a past geologi- 

 cal epoch the process would be less complete and the numbers 

 of vertebrae would be larger, while the individual vertebrae re- 

 main smaller, less .specialized and often imperfectly ossified. 

 And this, in a general way, is precisel)^ what we find in the 

 examination of skeletons of a large series of fishes. 



If this view is correct, we have a possible theory of the re- 

 duction in numbers of vertebrae as we approach the equator. 

 It should, moreover, not surprise us to encounter various 

 modifications and exceptions, for we know little of the hab- 

 its and scarcely anything of the past history of great numbers 

 of species. The present characters of species maj^ depend on 

 occurrences in the past concerning which even guesses are 

 impossible. 



*The Parrot-fishes {Scaridcz), Trigger-fishes {Balistidce), Angel-fishes 

 {Ckcetodontidce), etc. 



