DISPERSION OF FRESH-WATER FISHES. 97 



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 pro- 

 cesses 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 Cat-fishes,^ although 

 nearly a hundred (93) nominal species of these 

 fishes have been from time to time described. 

 But these twenty-five species are among them- 

 selves very closely related, and all of them are 

 subject to a variety of minor changes. It requires 

 no strong effort of the imagination to see in them 

 all the modified descendants of some one species 

 of Cat-fish, not unlike our Common " Bull-head,^ 

 — an immigrant probably from Asia, and which 

 has now adjusted itself to its surroundings in each 

 of our myriad of Cat-fish breeding streams. 



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 connect- 

 ing 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 



1 Siluridcz. 2 Ameiurus nsbtdostis. 



7 



