1903] DISTRIBUTION OF ISOETES SACCHARATA 199 
station to the mouth of the river in which it occurs, thence up or 
down the coast to a neighboring river, and up that river to the 
fresh-water portion near the head of tide water. Such a trans- 
portation, if at all possible, must depend upon the most excep- 
tional of circumstances. 
Probably a truer explanation is found in relation to the geo- 
logical history of the bay. It is believed by some geologists '* 
‘that the region about Chesapeake Bay is now sinking, and it is 
certain that it has recently sunk after a period of elevation. In 
fact, it seems to have been elevated and depressed several times 
in its Pleistocene history." 
The position of old shore lines with their sea cliffs and ter- 
races gives evidence of the amount of subsidence of the land at 
each period of sinking, but no evidences remain as to the height 
to which the land rose during each period of elevation. The 
present elevation of the land is such that the water of the bay 
is fresh to Spesutie Island, about ten kilometers below the mouth 
of the Susquehanna River. During periods of greater elevation 
the water was fresh further to the south. When the land was so 
elevated that the water was fresh at the mouth of the Potomac 
River, favorable habitats along the shore of the bay must have 
been occupied by the progenitors of the /soetes saccharata colonies 
which now occur in the upper estuarine portion of the tributary 
rivers. As the land sank and the rivers were ponded farther 
and farther from their mouths, new areas became adapted to the 
Srowth of Isoetes, and new colonies were formed. Simulta- 
neously the colonies furthest down stream were destroyed by the 
advance of salt water. In this way there came to be, instead of 
a single colony or group of colonies at the head of Chesapeake 
Bay, as many distinct colonies as there were ponded tributaries. 
So long as the land continued to sink, the successful reproduc- 
tion was on the up-stream side, and destruction followed pari 
Passu on the down-stream side until the present condition of 
widely separated colonies was brought about. In periods of 
“Cook, GEORGE H., Geology of N. J. 1868: 343 ef seg. 
‘Ss MCGEE, W J, Am. Jour. Sci. 35 : 463-466. 1888. Darron, N. H., Bull. Geol. 
Soc. Am. 2: 450. Ap 1891. SHATTUCK, GEORGE B., Am. Geol. 28: 100-105. Ag 1901. 
