152 MRS. E. M. REID : A COMPARATIVE [vol. lxxvi, 



with palasozoology and stratigraphy, for the determination of the age of 

 strata the geological position of which is unknown. 



As a corollary, it follows that the positions indicated for the two floras of 

 unknown age — the Castle Eden and the Reuverian — are very approximately 

 correct. The Castle-Eden flora may be regarded as belonging to the middle 

 of the Middle Pliocene ; the Reuverian as coming low down in the Lower 

 Pliocene. 



(3) By carrying the curve beyond the Pliocene into the Miocene, to the 

 point marked by the co-ordinates of 100 — which point, it may be noted, lies 

 very approximately on the curve — it will be seen, that at a period some way 

 back in the Miocene, probably about the middle, or a little earlier, the whole 

 of the flora of Western Europe, north of the east-and-west mountain-ranges, 

 was of the Chinese-North American type. If, as I have suggested, the division 

 between Pliocene and Miocene should lie farther back on the curve, it would 

 indicate that this period lies rather later — not much later — in the Miocene. 



I would here remark that the curve depends upon the recognition of the 

 Chinese-North American element in the West European Pliocene flora, but is 

 quite independent of any theory as to why this flora is found here, or what 

 led to its extermination. 



Recognizing this, it is noteworthy that the locus of 100 on the two 

 co-ordinates indicates that the extermination of the Chinese-North American 

 element began, or perhaps it maybe more correct to say, the period at which 

 it was both exterminated, and supplanted by other elements, began at exactly 

 the period which stratigraphy and palseozoology indicate as that at which the 

 elevation of the mountain-ranges, extending from Morocco to Indo-China, 1 

 attained its maximum. But it was precisely to the presence of these moun- 

 tain-ranges that we chiefly attributed the extermination of the Chinese- 

 North American flora in Western Europe. 2 Consequently, the curve bears the 

 ' strongest confirmatory and independent testimony to the truth of our theory 

 of extermination, of which I have given a digest at the beginning of this 

 paper. 



(4) In the form of the curve two facts are stated by implication only -., 

 these are, the rate of change of the exotic element in the Pliocene flora of 

 Western Europe, other than the Chinese-North American element ; and the rate 

 of increase of that part of the flora which survived to form part of the living 

 flora of Western Europe. It is to be noted that, whereas the Chinese-North 

 American element is outgoing, both these elements are incoming, and that 

 they only began to affect the flora of Western Europe during the Miocene 

 Epoch. The questions at once arise — What are they ? Whence did they 

 come ? I am not prepared to answer these questions, although there are 

 points to which it is well to draw attention, as offering a possible, and 

 perhaps a probable, solution. 



Our analysis of the Pliocene Floras (op. jam cit. pp. 27-50), further corrobo- 

 rated by the study of the Castle-Eden and Pont-de-Gail floras, shows that, of 

 the Pliocene species which at present form part of the West European flora, 

 those with a circumpolar distribution are, in by far the majority of cases, 

 marsh- or water-plants ; land-plants have, save exceptionally, a more limited 

 distribution ; most have a range extending in varying degrees from the 

 Himalaya throughout the Near East, the Caucasus, Central Europe, and the 

 Mediterranean region ; a few also range into China and Japan. 3 A very 

 similar distribution of aquatic and land-plants is seen among the incoming 

 exotic species ; which points to the indigenous and exotic elements having a 

 common origin. 



1 A. de Lapparent, ' Traite de Geologie ' 4th ed. vol. iii (1900) pp. 1513, 

 1777, 1807. 



2 ' The Pliocene Floras of the Dutch-Prussian Border ' 1915, pp. 18, 19. 



3 It is possible that some of these may be derived from late waves of Polar 

 migrants. 



