ON THE SPECIFIC RANK OP POTAMOGETON ZIZII. 115 



P. Incens and P. graminem. The earlier writers seem, with a few 

 exceptions, to have regarded it as a variety or subspecies of 

 graminem; while Fries, and those who followed him, described it as 

 a variety of hicens ; and a few have, from its first discovery, accorded 

 it full specific rank. These diverse views have led to much 



uncertainty in naming the more doubtful forms, and especially to 

 a confusion of small states of lueens with true Zizii. This un- 

 certainty is perhaps of little consequence in itself, but it has led 

 to what I venture to think is a serious error — the arrangement of 

 P. gramineus L. in the same natural group as P. lueens, or even to 

 regarding it as a subspecies of the latter. 



Until I began to suspect the hybrid origin of many of our 

 accepted "species" of Potamogeton, I adopted this arrangement, 

 although I found it a constant source of difficulty in attempting a 

 natural arrangement of the broad-leaved forms of the genus ; and 

 that if the principles on which it was founded were consistently 

 carried out, we must also place P. natans and P. perfoliatus as sub- 

 species of one superspecies. 



In studying a genus which is supposed to produce many hybrid 

 forms, it is of the first importance to ascertain those which are 

 termed "true species," that is, those produced by "evolution" or 

 u direct creation." Unless we can do this with some degree of 

 probability, our attempts towards a natural arrangement of the 

 species can have no certain basis ; and the wider question of 

 hybridisation being one of the methods by which species, as we now 

 know them, were formed can have no possible solution. 



In this place I will not dwell on the possibility of plants of 

 distinct species producing perfectly fertile hybrid offspring, but 

 will refer the student to the ninth chapter of Darwin's Origin of 

 Species, where the subject is most impartially discussed. The 

 immediate object of this note is to detail the observations made in 

 the field, rather than in the herbarium, and which have induced 

 me to regard the typical forms of P. Zizii as more or less fertile 

 hybrids between P. lueens and P. gramineus L. Other forms which 

 are sometimes assigned to Zizii seem to be merely extreme states 

 of Incens, or even of heteropkyllus auct. ; but these can only be worked 

 out by botanists who have opportunities of studying them in a 

 living state. 



In the first place it is necessary to describe the fenland locality 

 in which P. Zizii grows, and the probable means by which its 

 supposed parents became established there ; and then to relate 

 such facts as I have been able to collect in support of the theory of 

 the hybrid origin of this plant — a theory solely suggested by those 

 facts, not one formed antecedently. 



The fens of Cambridge and Huntingdon are not entirely covered 

 with vegetable soil, but are here and there studded with slight 

 elevations which rise a few feet above the surrounding level ; in 

 ancient times these were true islands rising from the water which 

 then covered the vast extent now defined by the peat, which grew 

 up as the water gradually diminished. These islands are masses of 

 Oxford or Kimmeridge Clay (frequently capped with " Palaeolithic" 



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