Vol. VIII. January, 1905. Ko. 2 



BIOLOGICAL BULLETIN 



DIVERGENCE AND CONVERGENCE IN FISHES. 1 



CARL H. EIGENMANN. 

 ILLUSTRATIONS KY CLARENCE KENNEDY. 



The struggle for existence with the biological environment as 

 the result of the geometric rate of increase tends to divergence in 

 habit and form. It tends to divergence in habit and form by 

 preserving variants whenever such possess a character diverging 

 sufficiently in amount to give the variant a personal advantage 

 over his fellows --always provided the divergent character is 

 transmissible. 



Whether we call the diverging individuals variants in the old 

 sense, or mutants in the new, it is to the selection of those among 

 them best adapted to utilize the foods of various sorts, to escape 

 the enemies of various sorts and to leave others similar to them 

 in their place when they die that we owe the specific divergence 

 in structure, shape, color, food-habits and breeding-habits of a 

 given family - - say the American characins. The entire process 

 tends to the divergence and multiplicity of species. 



The characins are a family of fresh -water fishes that, in Amer- 

 ica, range from the border of the United States to some distance 

 south of Buenos Ayres. They form about one third of the entire 

 South American fresh-water fauna, and have diverged in adapta- 

 tion to diverse food, diverse habitat and diverse enemies to fill 

 nearly every niche open to fishes. The ends of the three lines 

 of adaptation to different food give us mud-eating forms, with 

 long intestinal tract and no teeth ; flesh-eaters with shear-like 

 teeth, that make bathing dangerous to life and that cut their way 

 out of nets ; and conical-toothed forms, with sharp, needle-like 

 teeth and comparatively huge fangs. Greater diversity could 



1 Contributions from the Zoological Laboratory of the Indiana University, No. 64. 



59 



