TEMPERATURE AND VERTEBRA— A STUDY IN 

 EVOI.UTION. 



Being a Discossion of thk Relations of the Numbers of Verte- 

 bra AMONG Fishes, to the Temperature of the Water and to 

 THE Character of the Struggle for Existence. 



By DAVID STARR JORDAN. 



The present paper is an attempt to find a relation of cause 

 and effect in connection with the fact that in many groups of 

 fishes the species which live in the warmest water have the 

 fewest vertebrae. As here given, it is a modified reprint, with 

 some additional matter, of a paper entitled "Relations of 

 Temperature to Vertebrae among Fishes," published by the 

 author in Volume XIV of the Proceedings of the U. S. 

 National Museum for 1891, pages 107 to 120. 



STATEMENT OF THE PROBLEM. 



It has been known for many years that in certain groups of 

 fishes the northern or cold-water representatives have a larger 

 number of vertebrae than those members which are found in 

 tropical regions. To this generalization, first formulated by 

 Dr. Gill in 1863 and applied by him to the Labridce, we may 

 add certain others which have been more or less fully appre- 

 ciated by ichthyologists, but which for the most part received 

 their first formal statement from the writer in 1891. In groups 

 containing fresh-water and marine members, the fresh-water 

 forms have in general more vertebrae than those found in the 

 sea. The fishes inhabiting the depths of the sea have more 

 vertebrae than their relatives living near the shore. In free- 

 swimming pelagic fishes the number of vertebrae is also great- 

 er than in the related shore fishes of the same regions. The 

 fishes of the earlier geological periods have for the most part 

 numerous vertebrae, and those fishes with the low numbers (24 

 to 26) found in the specialized spiny-rayed fishes appear only 

 in comparatively recent times. In the same connection we 



