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 studiéd. 
When two or more species of any genus now 
inhabit the same waters, they are usually species 
1 Siluride. 2 Ameiurus nebulosus. 
