Dispersion of Fresh- water Fishes 293 



Where species can readily migrate, their uniformity is pre- 

 served; but whenever a form becomes localized its representa- 

 tives assume some characters not shared by the species as a 

 whole. When we can trace, as we often can, the disappearance 

 by degrees of these characters, such forms no longer represent 

 to us distinct species. In cases where the connecting forms are 

 extinct, or at least not represented in collections, each form 

 which is apparently different must be regarded as a distinct 

 species. 



The variations in any type become, in general, more marked 

 as we approach the tropics. The genera are represented, on 

 the whole, by more species there, and it would appear that the 

 processes of specific change go on more rapidly under the easier 

 conditions of life in the Torrid Zone. 



We recognize now in North America twenty-five distinct 

 species of fresh- water catfishes,* although nearly a hundred (93) 

 nominal species of these fishes have been from time to time 

 described. But these twenty-five species are among themselves 

 very closely related, and all of them are subject to a variety 

 of minor changes. It requires no strong effort of the imagina- 

 tion to see in them all the modified descendants of some one 

 species of catfish, not unlike our common "bull-head," f an im- 

 migrant probably from Asia, and which has now adjusted itself 

 to its surroundings in each of our myriad of catfish-breeding 

 streams. 



Meaning of Species. The word "species," then, is simply a 

 term of convenience, including such members of a group similar 

 to each other as are tangibly different from others, and are not 

 known to be connected with these by intermediate forms. 

 Such connecting links we may suppose to have existed in all 

 cases. We are only sure that they do not now exist in our 

 collections, so far as these have been carefully studied. 



When two or more species of any genus now inhabit the 

 same waters, they are usually species whose differentiation is of 

 long standing, species, therefore, which can be readily dis- 

 tinguished from one another. When, on the other hand, we 

 have "representative species," closely related forms, neither 

 of which is found within the geographical range of the other, 



* Siluridas. f Ameiurus nebulosus. 



