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



■certain theories, by which to account for the facts discovered during 

 the course of our investigations. The facts were these : that 

 the Pliocene Epoch had witnessed the existence and 

 •extinction in Western Europe, of a flora closely allied 

 to the living floras of the Far East of Asia, and of 

 North America; and, that in whatever part of the 

 Northern Hemisphere the nearest living relations of 

 these exotics, or of any Pliocene exotics, were now 

 found, provided that they were found in lower lati- 

 tudes, they were nearly always mountain-plants. 



The explanation that we gave of these facts shows that the two 

 ■are related. 



To account for the first we extended the theory put forward by 

 Asa Gray l to explain the relationship between the living floras of 

 Japan and North America — that they were two divergent streams 

 of migrants from some Polar source. 



In the Pleistocene Flora of Western Europe we recognized a 

 third stream. 2 All three were driven south by the ever-increasing 

 cold of the Pliocene. For the East Asian and North American 

 ■streams the way to the Tropics was open, and they escaped. For 

 the West Asian and European stream, from the Atlantic seaboard 

 till the coastal plain of China was reached, the way was everywhere 

 closed in temperate regions, by impassable East-and-West barriers 

 •of mountains, seas, and, perhaps, deserts. Against these barriers 

 successive waves of migrants were driven, and perished, so that by 

 the end of the Pliocene (Cromerian) scarcely a trace of their 

 former existence was to be found, save in their fossil remains. 



Had this been all, we might expect to find some part at least of 

 the living elements of the exotic flora established on the plains 

 •of North America and China wherever the climate was siiitable ; 

 but this is not the case. Nearly always the survivors are 

 mountain-plants. 3 



K. Akad. Wetensch. Amsterdam, Verslag. Afd. Natuurk. pt. xix (1910) 

 pp. 262-71, pi. ; 'Les Vallees Pliocenes avec Lignite de Bidart, Ccnitz & Cha- 

 biague (Basses-Pyrenees)' Bull. Soc. G-eol. France, ser. 4, vol. sv (1915) 

 pp. 403-27 & pi. vii ; and ' The Pliocene Floras of the Dutch-Prussian Border ' 

 Med. Eijksopsporing van Delfstoffen, No. 6, 1915. 



1 ' Diagnostic Characters of New Species of Phamogamous Plants, collected 



in Japan With Observations upon the Relations of the Japanese 



Flora to that of North America & of other parts of the Northern Temperate 

 Zone ' Mem. Amer. Acad. Arts & Sci. n. s. vol. vi (1859) pp. 377-449. 



2 ' The Pliocene Floras of the Diitch-Prussian Border ' pp. 15-26. 



3 If the theory is true, that successive waves of migrants have found 

 refuge throughout Later Tertiary time in the countries of the Chinese- 

 North American plant-association, it follows that these countries must now 

 ■exhibit a rich and varied flora representing many Tertiary survivals. Having 

 this view in mind. I was much interested to receive at the end of last year 

 •(1919) a letter from Dr. A. Kryshtofovich, informing me that M. V. Komarov 

 ■had also been led to regard the mountains of Western China as an ' asylum ' 

 for plant-life. This conclusion was reached, so Dr. Kryshtofovich tells 

 me, by the study of certain typical Chinese genera, and he adds that 

 M. Komarov's papers were published in the ' Acta Horti Petropolitani.' 



