part 1] ARCTIC FLORA OF THE CAM VALLEY. 13 



any two such fossil floras is to be expected, even if they were 

 actually contemporary ; for it could rarely happen, in the case of 

 two rivers, that the areas drained supported exactly the same plant- 

 formations in precisely the same proportions. 



Difference, then, in detail, must almost always be expected, 

 though, naturally, two contemporary floras so differing may still 

 bear the stamp of the more general conditions under which they 

 flourished — such, for instance, as the climatic conditions. Com- 

 parison, with the view of establishing the relative age of two floras, 

 seen in such close perspective as in the case of those from the 

 Pleistocene, is therefore difficult ; and the difficulty is increased 

 by the fact that in any two cases conditions of preservation may 

 have varied, so that whereas, in the one instance, certain delicate 

 forms were preserved, in the other, on the contrary, they were 

 destroyed before fossilization could take place. Further, an in- 

 adequate study of one or of both of two deposits may mean that 

 characteristic forms escaped notice, and this risk should always 

 be borne in mind in any attempted comparison, however much 

 material was examined ; for the fossil content frequently differs in 

 richness from seam to seam, and it must sometimes happen that 

 the investigation of one more sample of peat would reveal the 

 presence in a flora of plants previously unrecorded. 



All that can be said safeh^ therefore, in comparing two Pleisto- 

 cene floras is that both, say, are Temperate, or both Arctic, so 

 that they in?ij have been contemporary. This is essentially true 

 of the Lea and Cam-Yalley Floras, both of which yielded Arctic 

 plants, and both of which, judging by stratigraphical evidence, 

 appear to have been of Upper Palaeolithic date. 



The late Clement Keid, m a note mentioned previously,^ sug- 

 gested the contemporaneity of these two floras on the grounds 

 that : 



(a) there was a correspondence in the plant-assemblages, and that 



(b) not only did the same species occur, but the same Arctic species were 



missing. 



Recent evidence does not accord with these statements, for, beyond 

 the fact that both floras were Arctic, yielding certain of the same 

 widely-distributed Arctic and Temperate forms, there appears to 

 be no close correspondence between the two plant-assemblages, as 

 will be shown subsequently. Moreover, some of the Arctic species 

 which were supposed to be absent from the two areas were found 

 ]ately at Barnwell, illustrating once more the fact that inferences 

 based on negative evidence are always liable to modification as the 

 result of subsequent discovery. 



This being borne in mind, some conclusions in regard to the 

 differences between the floras have been deduced from the available 

 ■evidence. But the fact that these conclusions are purely tentative 

 cannot be too strongly emphasized, for at any moment furtlier 

 research m.ay render them untenable. First, then, while the flora 



1 See J. E. Marr, Q. J. G. S. vol. Ixxv (1919-20) p. 227. 



