330 PLANTS, SEEDS, AND CURRENTS 



second in the migration of plants to and from the common meeting 

 ground of the eastern and western floras in the north, and that of 

 the third in its isolating influence varying in degree on the South 

 American, South African, South Asian, and Australian floras 

 (pp. 323-4). 



2. The views of Thiselton-Dyer are here adopted. They embody 

 those of Bentham, Hooker, and Asa Gray in a general principle 

 applicable to plant- distribution in geological time. From this 

 standpoint distribution becomes a problem of the northern hemi- 

 sphere, and we cannot get over this fact by raising problems in the 

 south (pp. 324-5). 



3. After remarking that zoologists have often appealed to the 

 hypothesis of a north polar centre of dispersal, it is observed that 

 the method of reconstructing the land-surface from the facts of 

 distribution is the least promising of all modes of attacking the 

 problem. The geological record is on the side of the views adopted 

 in this chapter ; and if we hold that the great change in climate and 

 in flora which has come over the north since Tertiary times has been 

 repeated in the earlier ages of the earth's history, we stand on much 

 safer ground than if we assume that we are here face to face with a 

 change unprecedented in the story of our planet (pp. 325-6). 



4. It is observed that discontinuous distribution, which is then 

 briefly dealt with, is the essence of the problem ; and it is remarked 

 that the error lies in regarding extreme cases, like that of Ravenala, 

 as requiring special explanation (pp. 326-7). 



5. The important feature of centrifugal dispersion from the north 

 is again alluded to, in order to emphasise the point that the south- 

 ward migration generally recognised as resulting from the last glacial 

 period illustrates a principle that has been in operation through the 

 ages. The opposing view, which is supported by some Australian 

 naturalists and has been very ably advocated mainly on zoological 

 grounds by Hedley, is then discussed. It holds that the community 

 of austral forms of life is the result of radiation along former land- 

 routes from the south polar region (pp. 328-9). 



6. A restatement of the position adopted by Thiselton-Dyer is 

 then given, and it is urged that his views remove more difficulties in 

 the study of distribution than any previous hypothesis (p. 329). 



LIST OF SOME OF THE WORKS QUOTED IN THIS AND IN THE 

 PRECEDING CHAPTER 



Andrews, E. C, The Development of the Natural Order Myrtaceae, Proc. Linn. Soc. 

 N.S. Wales, December 1913. 

 The Development and Distribution of the Natural Order Leguminosse, Journ. Proc. 

 Boy. Soc. N.S. Wales, November 1914. 



Cambage, R. H., Presidential Address, Roy. Soc. N.S. Wales, 1913. 

 Acacia Seedlings, Proc. Boy. Soc. N.S. Wales, July 1915. 



Dyer, W. T. Thiselton-, Geographical Distribution of Plants, Seward's Darwin and 

 Modern Science, 1909. 

 On the supposed Tertiary Antarctic Continent, Journ. Acad. Nat. Sci. Philadelphia, 

 XV., ser. 2, 1912. 



