40 
Carter R. Gilbert 
served to increase the degree of isolation of the upper Oklawaha popu- 
lation. Johnson (1974) has shown that the present inland distribution of 
C. variegatus variegatus is sharply limited by the Pamlico terrace, which 
is the lowest (8 to 10 m) and most recent of the Pleistocene marine 
terraces (Fig. 6). Why this fish failed to become established farther 
inland during earlier marine high stands is unclear. An interesting point 
is that the factor (lowered sea levels) leading to the evolution of C. 
variegatus hubbsi in peninsular Florida is essentially the opposite of 
that resulting in evolution of the other fish taxa discussed earlier. 
The lower (i.e., northern) part of the St. Johns drainage had an 
independent origin from the upper area. Several tributaries of the lower 
St. Johns and Oklawaha rivers (the Oklawaha is the principal system 
within the St. Johns drainage), including Orange, Deep, Rice, Etonia 
and Black creeks (Fig. 3), were not inundated during Pleistocene inter- 
glacial or pre-Pleistocene rises in sea level (Fig. 4). This area, which has 
been called the ‘’Wicomico Peninsula” (Pirkle et al. 1974), is basically an 
eastern extension of the Cody Scarp. Several major topographic fea- 
tures were formed on the margins of this peninsula, including Trail 
Ridge, which probably originated as a beach ridge during the early 
Pleistocene, and the Duval Upland and Baywood Promontory, which 
developed as regressional plains as sea levels dropped (Fig. 3). The 
streams in this area served as refugia for various fishes and inverte- 
brates, and their isolation was of sufficient duration to permit the evolu- 
tion of several endemic species of invertebrates (Burgess and Franz 
1978: Table 1). Although no endemic fishes are present here, four 
( Notropis welaka , Notropis cummingsae, Ictalurus brunneus and 
Etheostoma olmstedi ) have isolated relict populations centered in the 
area. None of these species is found in the adjacent Suwannee or St. 
Marys drainages (Gilbert 1980e; Gilbert and Burgess 1980a,c; Lee and 
McAllister 1980), and only N. cummingsae occurs in any immediately 
adjacent drainages. These four fishes are believed to have entered the 
lower St. Johns drainage via the upper Santa Fe River (a major tribu- 
tary of the Suwannee), which also was located on the Wicomico Penin- 
sula (Burgess and Franz 1978) (Fig. 3). Burgess and Franz (1978) specu- 
lated that current absence of these species from the Santa Fe system, 
and elsewhere in the Suwannee drainage, could have been caused by 
two related factors: reduction, during glacial maxima, in the overall 
amount of surface water as a result of solution draining; and increased 
interspecific competition. The ecological conditions existing today in the 
area appear unsuitable for N. welaka , but seem favorable for the other 
three species. Several other fishes ( Elassoma zonatum, Acantharchus 
pomotis and Umbra pygmaea ) have similarly restricted distributions in 
