GILBERT: FISHES OF THE SUBGENUS LUXILUS


appears to be the best of the several possibilities. In addition to other
factors cited previously, spread to such distant areas as the Atlantic
coast and the Rocky Mountains could have been accomplished most
readily from a geographically intermediate area.
 Following its initial invasion of the Atlantic coast the species
spread southward, possibly to the Roanoke River. The population
may then have become isolated from the remaining cornutus stock
farther north, resulting in the eventual evolution of a new form,
Notropis albeolus. The presence of other closely related species and
subspecies pairs in the James and Roanoke systems, such as Etheo-
stoma longimanum-Etheostoma podostemone, Moxostoma rhothoecum-
Moxostoma hamiltoni, and Notropis procne procne-Notropis procne
longiceps, tends to confirm this idea. It is equally plausible, as in-
dicated earlier, that albeolus reached the Atlantic coast via the Teays
River. This is substantiated by the similarity in distribution pattern
of this form with other species, such as Notropis ardens and N. ma-
tutinus, which are restricted to the Roanoke and neighboring systems
on the east coast, but whose affinities are with the Ohio Valley fauna.
The differentiation of albeolus into two rather distinct forms, one
occurring in the Roanoke and New drainages, the other in the Neuse,
Tar, and Cape Fear systems, suggests that albeolus may even have
evolved in some way from a union of stocks coming both from the
Atlantic coast and the Ohio Valley.
 The first glacial advance completely inundated the Laurentian
 River and thus split the range of cornutus into two major segments,
 one along the Atlantic coast, the other west of the glacier. Whether
 cornutus reinvaded the glaciated area during all the interglacial pe-
 riods, which is a possibility, cannot be determined. Some remixing
 of segments of the Atlantic coastal and western populations of cor-
 nutus probably did occur during the Pleistocene, for no taxonomic
 separation can be made between the eastern and western popula-
 tions of cornutus at the subspecific level. If these populations did
 not come into contact throughout the entire Pleistocene, the long pe-
 riod of isolation should have produced more pronounced morpho-
 logical differences. Furthermore cornutus is so widespread and adapt-
 able that it would be among the most likely species to expand into
 newly available territory.
 Dispersal of N. chrysocephalus to various parts of the Mississippi
 Valley evidently occurred throughout the Pleistocene. The Missis-
 sippi River was then slightly cooler, less turbid, and less sluggish,
 and probably did not pose the barrier to small-stream fishes that it