116 ON THE SPECIFIC RANK OP POTAMOGBTON ZIZII. 



and "Flora" gravels, or rarely with boulder clay) which resisted 

 the successive denudations that formed the "fenland basin." As 

 these islands are of small extent, with the exception of the Island 

 of Ely, they possessed no rivers, and probably no water-courses, 

 which did not become dried up in the summer. Hence such 

 Potamogetons as grew on them previously to the separation from 

 the mainland must soon become extinct ; P. crispus, which is 

 equally at home in the smallest cattle-pond or the largest river, 

 alone excepted. This is at present the only species found on these 

 elevations — locally termed "high-lands" ; the surrounding peat 

 being called black, or fen land. 



As the fens became drier a series of moors and bogs were 

 formed, alike around the base of the islands and at the foot of the 

 mainlands which then formed the shores of the fenland lake. The 

 vegetation of these moors is everywhere different from that of the 

 " deep fens," which remained more or less submerged until com- 

 paratively recent times ; and in making researches into the topo- 

 graphical botany of the district this should always be borne in mind. 

 Cultivation has almost destroyed this moorland flora, but it may 

 yet be seen at Wicken and Burwell on the mainland of Cambridge, 

 and at Holme in Huntingdonshire. Traces of it exist around the 

 islands also, and in nearly all cases it is marked by the presence of 

 two Potamogetons, heterophylhis and plant agineus, which are not met 

 with in the deep fens. These latter are tenanted by all the species 

 which now inhabit the rivers which flow from the counties which 

 surround the fens, but the two above-named plants are at present 

 absent from rivers and deep fens alike, just as the moorland 

 vegetation is absent. 



P. Zizii is not found in the upland rivers of the two counties, 

 nor in any of the moorland fens of the old mainland, but only near 

 the islands, and then only where P. heterophylhis now grows, or in 

 slightly outlying stations to which it was evidently carried by now 

 disused systems of local drainage. This peculiar distribution first 

 suggested the connection between the two species. 



Potamogeton lucens grows almost everywhere in the fens, on 

 moorland and deep fen alika; but, although our natural and 

 artificial "rivers" often differ in no physical respect from the 

 larger "drains" of the fens, no forms of P. Zizii are found in 

 them. The river Potamogetons are natans, lucens, pralongiis, 

 perfoliatm, and crispus, with others which do not demand con- 

 sideration in the present note ; and it is not a little remarkable 

 that although in some instances they seem perfectly adapted for the 

 growth of both heterophylhis and Zizii, yet neither of these species 

 is found in them. 



When P. heterophylhis grows apart from lucens, no Zizii is to be met 

 with, but whenever the two grow together, Zizii is always present! 

 From whence did it come ? I think the question is answered in 

 what follows on the different local races of both species. 



Recently, on comparing a large ] imber of fenland and British 

 P. Zizii with one another, I was sm-prised to find that, with one or 



two exceptions, each of the local races of this species differed to a 



