82 PROCEJEUINQS OF THE 



the vice-counties and divisions vrere classed into four groups 

 according to tliese percentages, and the groups differentiated 

 in the map hv three degrees ot" shading ana by blanks. Thus, 

 black is used for 40 per cent, and upwards, hatched shading 

 for 25-39 per cent., stippled shading for 12-24 per cent., blanks 

 for less than 12 per cent. 8imilarly the black of the continental 

 areas in maps 2 and 3 means that the area so marked contains 

 40 or more percent, of the species of its class, tiie hatched sliading 

 stands for 25-39 percent,, and so on. Only in map 1 (Southera 

 elements) was it found desirable to introduce a different value 

 in the t»se< map, the percentages being 70, 40, and 15 respec- 

 tively, it is, of course, understood that the term " tSouthern " is 

 used throughout exclusively with reference to species indigenous 

 in the British Isles, and therefore excludes species which might 

 be " Southern '' on the Continent, but do not occur in the British 

 Isles. Finally, out of the insets of the area maps of the species 

 making up the " Southern Element " a small number was chosen 

 to iilusti'ate the varying degrees of discontinuity of area which 

 are represented by these species. 



At the meeting of the Britisli Association for the Advancement 

 of Science at Portsmouth in 1911 a discussion took place on the 

 relation of the present plant population of the British Isles to the 

 Glacial pei-iod. It Mas opened by Mr. Clement Eeid in an 

 address in which he advocated the theory that no temperate flora 

 could have survived the conditions prevailing in the islands during 

 the Glacial period, that the existing flora, apart from a few arctic 

 and alpine species, came in towards the end of, and after, that 

 period, and that especially the " Atlantic or Lusitaniau " plants 

 (also referred to as " Pyrenean ") and the " American "and " lime- 

 stone " elements arrived and, may be, still arrive by chance intro- 

 ductions of seeds, now mainly due to birds driven by exceptional 

 gales. I then expressed my agreement with the speaker's view as 

 to the effect of the glaciation of the British Isles on the flora, and 

 the reimmigration of the bulk of the latter in post-glacial times, 

 but combated the supposition of the presence of the peculiar 

 American, Atlantic, and limestone elements being due to chance 

 introduction over great distances. Since then Dr. Scharff (3) has 

 thrown doubt on the theory of a wholesale destruction of the 

 pre-glaeial flora of Great Britain and Ireland and refuted the idea 

 of the introduction of the "Pyrenean" element by migrating or 

 gale-driven birds. In my opinion the question of the presence of 

 those peculiar elements and especially of the so-called " Atlantic,"" 

 " Pyrenean " or " Lusitaniau " plants has in a general way already 

 been solved bj' Engler (4) in his ' A^ersuch einer Entwicklungs- 

 geschichte der Pflanzenwelt ' more than thirty years ago. To 

 him their immigration or rather reimmigration took place in post- 

 glacial times- — for he too assumes the wiping out of the greater 

 part of the pre-glacial flora during the Glacial period — and it 



