758 ^ CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



of the general conditions of the time. Strictly, pollen analysis is on the borders 

 of plant geography, especially where that subject impinges on archaeology, but 

 within its limitations it has been and doubtless will continue to be a valuable 

 adjunct. 



The second development was the discussion which grew up around what is 

 usually called the "nunatak theory," the view that some elements of the pre- 

 glacial floras survived those ages on refuges actually within the ice-cap but not 

 themselves glaciated. This idea again was not in itself new, but in the nineteen- 

 twenties it received much fresh impetus from the explorations of Fernald in 

 the region of the St. Lawrence River, and from his energetic writings (e.g., 1925). 

 The general suggestion of which this was a particular expression is an attractive 

 one, namely, that tlie explanation of certain puzzling phytogeographical facts 

 is to be found in the occurrence of refuges where plants may have been able to 

 avoid the worst consequences of climatic change and survive. But there is as yet 

 no overwhelming evidence that it is necessary to invoke this explanation. 



The last few paragraphs deal with matters which, despite their apparent 

 diversity, nevertheless have a considerable common element, revealing clearly 

 the main trend in the study of plant geography between the two wars, namely, its 

 concentration on the historical-developmental aspects of the subject. Major works 

 in this tradition soon appeared, and price of place may be given to Irmscher's 

 Pflanzenverhreitung und Entivicklung der Kontinente, published in 1922 and 

 followed in 1926 by Hayek's AUgemeine Pflanzengeographie, which, although 

 much more of a textbook, had the same approach. Both of these are important, 

 but it is fair to regard a later book as the real primer of the new interest. This 

 was Wulff's An Introduction to Historical Plant Geography (1943), which was 

 composed much more in the new idiom than either of the earlier books. This 

 publication is important, too, as marking the entry into the field of the great 

 new Russian school of botanists which had grown up since the Revolution, a 

 school whose full influence is still impeded by barriers of alphabet and language. 

 Fortunately Wulff's first volume, which was published in 1932, was translated 

 into English during the war, but his second and much larger volume is still 

 available only in Russian. This is also true of the monumental Flora URSS 

 edited by Komarov, begun in 1934 and still in active progress. It contains a great 

 mass of information about the distribution of plants over the huge but hitherto 

 little studied tracts of much of central and eastern Asia. 



This is perhaps the best place at which, ignoring chronology for the moment, 

 to refer briefly to two later publications, because, with that of Wulff, just men- 

 tioned, they form a mutually complementary trilogy, covering with reasonable 

 adequacy most aspects of modern plant geography and giving as complete a 

 picture of the present situation as can, in all circumstances, be expected. These 

 are Cain's Foundations of Plant Geography, which appeared in 1944, and the 

 present writer's book, The Geography of the Flowering Plants, written before 

 the late war but unavoidably delayed in production until 1947. Both have his- 

 torical plant geography as their chief emphasis, but while the former is of spe- 

 cial interest for its treatment of many particular aspects of the relation between 

 evolution and plant geography, such as polyploidy, the latter is rather more 

 a review of the facts of angiosperm distribution and a reconsideration, from a 

 developmental point of view, of the factors which have caused them. All three 



