18 BLIND VERTEBRATES AND THEIR EYES. 



DIVERGENCE IN EPIGEAN AND CONVERGENCE IN SUBTERRANEAN FISHES. 



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 does this 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 occupy localities of various kinds, 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 America, range from 

 the border of the United States to some distance south of Buenos Aires. They 

 form about one-third of the entire South American fresh-water fauna, and have 

 diverged in adaptation to diverse food, diverse habitat, and diverse enemies to fill 

 nearly every niche open to fishes. The ends of three of the 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 scarcely be imagined, 

 and one is led to suspect that some of the forms are over-adapted. In their 

 divergence in form they have reached almost every conceivable shape as we shall 

 see in a moment. 



The struggle for existence with any unit of physical environment, whether 

 there be geometric rate of increase or not, tends to convergence in habit and form. 

 There is no more striking instance of this than the acceptance of the annual or 

 deciduous habit of most of the plants inhabiting the temperate zones with their 

 seasonal changes, nor is there a more striking illustration of the struggle with other 

 individuals than the diversity of form and habit of various forest plants for ground 

 and light space. Records of the simultaneous and similar changes in the form in 

 the mass of species of any area during changing physical conditions are not want- 

 ing. For instance, Scott says: 



The steps of modernization which may be observed in following out the history of many dif- 

 ferent groups of mammals are seen to keep curiously parallel, as may be noticed, for example, in 

 the series of skulls figured by Kowalevsky, where we find similar changes occurring in such families 

 as the pigs, deer, antelopes, horses, elephants, etc. Indeed, one may speak with propriety of a 

 Puerco, or Wasatch, or White River type of skull, which will be found exemplified in widely separate 

 orders. 



On some riffles of the San Juan River of Cuba I found a small fish that is very 

 strikingly like other fishes inhabiting similar localities in the eastern United States. 

 The former is a goby, a marine form, Philypnus dormitator, which has become 

 adjusted to conditions found about the riffles of streams; the others are darters, 

 Hadropterus, belonging to an entirely different family of fresh-water fishes. The 

 similarity of various "darters" which live on the bottom of our streams to various 



