96 SCIENCE SKETCHES. 
have been published by the present writer. That 
of Jordan and Copeland,! published in 1876, enu- 
merates 670 species. That of Jordan? in 1878 
contains 665 species, and that of Jordan® in 1885, 
587 species, although upwards of 75 new species 
were detected in the nine years which elapsed be- 
tween the first and the last list. Additional spe- 
cimens from intervening localities are often found 
to form connecting links among the nominal spe- 
cies, and thus several supposed species become 
in time merged in one. Thus the Common Chan- 
nel Cat-fish* of our rivers has been described as 
a new species not less than twenty-five times, on 
account of differences real or imaginary, but com- 
paratively trifling in value. 
Where species can readily migrate, their uniform- 
ity is preserved; but whenever a form becomes 
localized its representatives assume some charac- 
ters 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degrees of these characters, such forms no longer 
represent to us distinct species.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 
1 Check List of the Fishes of the Fresh Waters of North Amer- 
ica, by David S. Jordan and Herbert E. Copeland. Bulletin of the 
Buffalo Society of Natural History, 1876, pp. 133-164. 
2 A Catalogue of the Fishes of the Fresh Waters of North 
America. Bulletin of the United States Geological Survey, 1878, 
PP. 407-442. 
8 A Catalogue of the Fishes known to inhabit the Waters of 
North America North of the Tropic of Cancer. Annual Report 
of the Commissioners of Fish and Fisheries for 1884 and 1885. 
4 Tctalurus punctatus Rafinesque. 
