part 2] beview or pliocene floras. 147" 



To explain this second fact, it is necessary to consider the- 

 climatic conditions which succeeded the cooling of the Northern) 

 Hemisphere throughout Pliocene time. These were the somewhat 

 rapid transitions of climate from cold to warmth — how many it 

 does not here concern us — which followed in the succeeding; 

 Pleistocene Epoch. 



During a cold period, the warmest Hora to survive in any given- 

 district must have heen that inhabiting the plains or valley-bottoms.. 

 As the climate ameliorated, the plains would become too hot for- 

 tius flora, and, in order to escape destruction by heat, as it had. 

 formerly escaped destruction by cold, it must migrate. In a> 

 country of great plains, if the changes of climate were rapid,, 

 movement to other latitudes might be too slow to counteract the 

 change of climate ; but in a mountain country, comparatively small 

 vertical movements would afford the necessary change. Hence, 

 in a warm period following upon a cold — our present condition — 

 we should expect to meet the migrants, when inhabiting more 

 southern lands than those in which their fossil relations occur,, 

 not on the plains (where mostly they had been exterminated),, 

 but upon the mountains. 



The Lower Pliocene flora of Western Europe is now found 

 most commonly at a height of from 1500 to 2000 metres in the- 

 Himalaya and in the mountains of Western China. 



Such briefly are the theories put forward in ' The Pliocene 

 Floras of the Dutch- Prussian Border,' in order to account for- 

 the facts that I have mentioned. The knowledge of these facts 

 was obtained chiefly from the study of three floras, the Cromerian, 

 Teglian, and Reuverian. 



In these three floras we recognized three stages in the chain of" 

 events suggested above. The age of the two earlier stages was 

 known from the evidence afforded by mollusca or mammalia; the- 

 Cromerian lay at the top of the Pliocene ; the Teglian near the 

 base of the Upper Pliocene, being the equivalent of the Norwich 

 Crag. The age of the large Reuverian flora was not known, but 

 it contained many more exotic and extinct species, and many more- 

 belonging to the Chinese-North American association, than the- 

 Teglian, and must therefore be older than it. We hazarded a 

 guess as to its probable age, and suggested that it might represent 

 the top of the Middle Pliocene, a suggestion I now know to be- 

 wrong. It should be placed in the Lower Pliocene. 



About two years after the research had reached this stage, I 



Unfortunately he does not give the reference. Dr. Daydon Jackson and 

 Mr. S. A. Skan have very kindly looked up the matter for me, and have 

 found two papers written in Russian, but with Latin as well as Russian 

 titles, which appear to be those to which Dr. Kryshtofovich refers. The refer- 

 ences to these are as follows : — Vladimir Leontjivic Komarov, ' Prolegomena, 

 ad Floras Chinas necnon Mongolian ' Fasc. 1 (1908) pp. 1-176, pis. i-iv & 

 2 maps; Fasc. 2 (1909) pp. 179-388, pis. v-xx & text-figs., Acta Horti Petr.. 

 xxix. (I have since heard that these are the papers.] 



