November 9, 191 1] 



NATURE 



57 



tendency to specialise at particular meetings, e.g. in 

 physiology at Dublin. This year an international phyto- 

 geographical excursion had been arranged to visit the 

 British Isles during August, and to conclude with the 

 British Association week at Portsmouth. The advent of so 

 many eminent foreign phytogeographers predetermined that 

 the bias of this year's meeting should be ecological. Field 

 excursions, therefore, formed a prominent feature of the 

 programme. The localities visited included Kingley Vale 

 (yew woods), the New Forest (heath, valley-moors, and 

 woodland), Southampton Water (Spartina associations), 

 and Ditcham Park (beech-wood, &c.). In addition to the 

 excursions, there were two discussions and a number of 

 individual papers dealing with phytogeographical subjects. 

 These may be taken first. 



T/ie Relation of the Present Plant Population of the 

 British Isles to the Glacial Period. 



A joint meeting of botanists, geologists, and geographers 



as arranged for Monday morning, September 4, the sub- 

 ject under discussion being the relation of the present plant 

 population of the British Isles to the Glacial period. 



The discussion was opened by Mr. Clement Reid, 

 F.R.S., who first gave a brief historical summary, and 

 then proceeded to discuss some of the problems which par- 

 ticularly need solution. The first question is, " Are any 

 of our plants survivors that managed to live through the 

 cold of the Glacial period in some warm nook in Britain? " 

 This he answered in the negative, except in the case of 

 certain arctic and alpine species, which thus, he believes, 

 form the oldest element of the British flora. Discussing 

 (with the aid of specially prepared maps) the distribution 

 of ice during the period of maximum intensity of cold, he 

 concluded that the whole of the temperate flora must have 

 been swept away as completely as the celebrated volcanic 

 eruption of 1883 destroyed the vegetation of Krakatoa. 

 0ealing now with the question of reimmigration, he could 

 find no evidence for the existence of post-glacial land- 

 bridges connecting the mainland of Great Britain with 

 either the Scilly Isles, Ireland, or the Continent. Mr. 

 Reid then mentioned some of the well-known peculiarities 

 of distribution in our flora, especially the cases of the 

 Pyrenean, Atlantic and Germanic elements. Most of the 

 species composing these elements are not really maritime 

 plants, though they have a marked coastal distribution. 

 This he explained as due to their comparatively recent 

 introduction from the nearest continental shores, the lapse 

 of time being such that the slow process of spreading 

 inland has only as yet extended a few miles. Mr. Reid 

 strongly urged the view that chance introduction of seeds 

 (e.g. by birds driven by exceptionally strong gales) during 

 thousands of years explains the existing peculiarities of 

 geographical distribution in a way that no changes of sea 

 or land or climate will do. 



The president then read a letter which Mr. Reid had 

 received from Dr. A. R. Wallace, F.R.S. Dr. Wallace 

 said that he was firmly convinced that plants had great 

 powers of distribution over the sea, in rare cases even 

 for thousands of miles. Referring to the flora of the 

 Azores, he said " there is absolutely no doubt that the 

 whole of its plants have been gradually introduced during 

 the latter half of the Tertiary period, over a width of 

 ocean of about 1000 miles." But he could not accept Mr. 

 Reid's view that the whole of our flora had been ex- 

 terminated. " Temperature is only one of many, very 

 many, factors that determine the distribution of species ; 

 and it is also certain that at the southern limit of the ice- 

 sheet the winter temperature may have been quite mild 

 enough to support a large number of our species." Dr. 

 Wallace added that the covering of snow during the winter 

 mav have been a compensation for the low temperature. 



Dr. Scharff discussed the problem from the zoological 

 PO'/it of view, dealing principally with the larger mammals 

 living in Ireland in present and past times. From the 

 evidence of animal remains in the Irish turf, marl and 

 cave deposits, Dr. Scharff maintained that many of the 

 larger herbivores, e.g. Irish elk, reindeer, &c., survived in | 

 Ireland during the Glacial period. As these animals would 

 require an abundant supply of vegetable food, he differed 

 'pm Mr. Reid as regards the survival of both animals and 

 ■ ' ints in Ireland during the cold of the Glacial period. 



NO. 2193, VOL. 88] 



Dr. O. Stapf, F.R.S., expressed his agreement with the 

 opener's view as to the extinction and reimmigration of 

 the temperate flora of the British Isles, but opposed the 

 view that the curious distribution of the American, 

 Atlantic, and limestone elements is due to chance introduc- 

 tion of seeds from great distances. He described the 

 present distribution of the American and Atlantic plants 

 in question, and pointed out that in both cases there 

 existed (if the different species composing the two groups 

 are considered) such gradations of discontinuity as to con- 

 nect the extreme cases with cases of almost continuous 

 areas. These extreme instances of discontinuous areas, 

 then, would merely represent the last phase of disintegra- 

 tion. Dr. Stapf concluded by emphasising the importance 

 of preserving and coordinating all records (of rare finds, 

 &c.) which bear on the history of the flora of the British 

 Isles. 



Prof. C. Schroter dealt with the theories of Nathorst 

 and Brockman with regard to the post-glacial history of 

 the Swiss flora. He also pointed out that new evidence 

 has accumulated showing the great importance of wind 

 as a factor in the dispersal of plants in the Swiss Alps. 



Mr. Wright directed attention to the presence of deeply 

 submerged forests and peat-beds in the southern half of 

 the British Isles. This indicates, though it scarcely proves, 

 that a land connection with the Continent existed in post- 

 glacial times. This connection seems to be demanded for 

 the entry of the larger mammals which have found their 

 way into England and Ireland since the Ice age. The 

 evidence as to the total extinction of all life on Krakatoa 

 has been questioned on the ground that seeds may have 

 been preserved in the old surface deposits beneath the 

 mantle of ash, and subsequently exposed for growth by 

 the rapid formation of rain gullies known to have followed 

 the eruption. 



Prof. P. F. Kendall ^ entirely agreed with the con- 

 clusions of Mr. Reid, and instanced the case of the Isle of 

 Man as indicating on a small scale what has probably 

 happened in the British Isles as a whole. The Isle of 

 Man presents a great variety of topographical, hydro- 

 graphic, and other features, and thus offers conditions 

 favourable to the maintenance of an equally varied flora 

 and fauna. But the island is remarkably poor in the 

 number of species of both animals and plants. The ex- 

 planation seems clear that since the departure of the great 

 ice-sheet the island has been repopulated by plants and 

 animals introduced by chance agencies across the Irish 

 Sea. 



Dr. J. E. Marr, F.R.S.,' said it is generally admitted 

 that after the great Ice age a period occurred which was 

 marked by widespread steppe conditions. It is to be ex- 

 pected that survivals of this period would persist in areas 

 not now under steppe conditions. He suggested that a 

 case of such persistence may be found in a group of xero- 

 phytes growing on the Brecklands of North Suffolk and 

 South Norfolk. 



Prof. O. Drude pointed out that in attempting to solve 

 a problem of this nature it was necessary to consider, not 

 Great Britain alone, but also the whole of middle Europe. 

 During what is known as the Baltic Ice age, Picca excelsa, 

 and even Hymenophyllum, survived in Saxony. It is not 

 impossible, therefore, that temperate forms, mingled with 

 boreal, may have survived in the south of England. 



Dr. F. J. Lewis was of opinion that the evidence of 

 submerged peat deposits and buried forests pointed to 

 considerably greater changes of level than those allowed 

 by Mr. Reid. He also thought that the conditions ^ of 

 Krakatoa were so different from those of Britain during 

 post-glacial times that comparisons between the two were 

 unsafe. 



Dr. C. H. Ostenfeld thought that the importance of 

 " nunataks " has been overestimated. It is very diflRcult 

 to be certain of whether a mountain summit has been 

 glaciated or not. Dr. Ostenfeld maintained that the bulk 

 of the temperate British flora had returned bv means of 

 a land-bridge, though he admitted that a few of the 

 Atlantic and the two American species had probably 

 arrived by chance. 



Mr. E. A. N. Arber believed that land connections existed 



1 Prof. Kendall and Dr. Marr sent written communications, as they were 

 unable to be present. 



