I04 



SCIENCE. 



[Vol. XVIII. No 446 



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RELATIONS OF TEMPERATURE TO VERTEBRAE 

 AMONG FISHES.' 



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 for- 

 mulated by Dr. Gill in 1863, we may add certain others 

 which have been more or less fully appreciated by ichthyol- 

 ogists, but which for the most part have never received for- 

 mal statement. In groups containing fresh-water and marine 

 members, the fresh-water forms have in general more verte- 

 bra than those found in the sea. The fishes inhabiting the 

 depths of the sea have more vertebrse than their relatives 

 living near the shore. In free-swimming pelagic fishes the 

 number of vertebrae is also greater than in the related shore 

 fishes of the same regions. Tlie fishes of the earlier geologi- 

 cal periods have for the most part numerous vertebrae, and 

 those fishes with the low numbers (21 to 26) found in the 

 specialized spiny-rayed fishes appear only in comparatively 

 recent times. In the same connection we may also bear in 

 mind the fact that those types of fishes (soft-rayed and ana- 

 canthine) which are properly characterized by increased 

 numbers of vertebrae predominate in the fresh waters, the 

 deep seas, and in Arctic and Antarctic regions. On the other 

 hand, the spiny-rayed fishes ai*e in the tropics largely in the 

 majority. 



In the present paper, I wish to consider the extent to which 

 these statements are true and to suggest a line of explanation 

 which covers all these generalizations alike. 



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

 rivation 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 characteristic of the pres- 

 ent geological era, has diverged from its soft-rayed ancestry. 



^ Abstract of a paper by David Starr Jordan, president of Leland Stanford, 

 Jr., University (Proceedings U.S. National Museum, XIV., 107). 



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 con- 

 cerned. 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 ex- 

 ternal 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 centre 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 

 IDoints of advantage. No form is excluded from competi- 

 tion. No competitor is handicapped by loss of strength on 

 account of cold, darkness, foul water, or any condition ad- 

 verse to fish life. 



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

 intensely and compactly a flsh, and which tend to rid it of 

 every character and every organ not needed in fish life, 

 should be most eff^ective along the rocks and shores of the 

 tropics. 



For this process of intensification of fi^sh-like characters, 

 which finds its culmination in certain specialized spiny-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 ex- 

 clusion of the polar seas, and especially in the monotony, 

 darkness, and cold of the oceanic abysses where light can not 

 penetrate 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 increase 

 in the size and complexity 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, fresh waters, and in the waters of a 

 past geological epoch the process would be less complete and 

 the numbers of vertebra3 would be larger. And this, in a 

 general way, is precisely what we find in the examination of 

 a large series of fishes. 



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

 reduction 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 habits 

 and scarcely anything of the past history of great numbers 

 of species. The present characters of species may depend on 

 occurrences in the past concerning which even guesses are 

 impossible. 



It may be taken for granted that the ancestry of the various 

 modern types of bony fishes is to be sought among the 

 Ganoids. All the fossil forms in this group have a notably 

 large number of vertebrae. The few now living are nearly 

 all fresh-water fishes, and among these, so far as known, the 

 numbers range from 65 to 110. 



Among the Teleostei or bony fishes, those which first ap- 

 pear in geological history are the Isospondyli, the allies of 



