DISTRIBUTION 



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Antarctic continent without committing oneself to the view that 

 Antarctica was a Tertiary abode of the flowering plants. Before 

 quitting this subject it may be added that Mr. Hedley's treatment 

 of the matter is mainly zoological, and that his arguments can only 

 be fairly met in their complete form by one who is similarly skilled 

 as a zoological investigator. 



Re-statement of the Views of Sir W. T. Thiselton-Dyer. — 

 The position adopted by him in his contribution to Darwin and 

 Modern Science may be again stated before we bring this chapter to 

 a conclusion. He contends that by postulating the permanency of 

 the general configuration of the earth's surface, and by assuming 

 that fluctuating conditions of climate have supplied an effective 

 means of propulsion from the north, a continuous and progressive 

 dispersal of species from the land-centre in the north polar regions 

 is inevitable. One may extend these remarks and say that if it can 

 be shown, as undoubtedly the general trend of the facts of distribu- 

 tion does show, that the divergence of plant-types responds to the 

 divergence of the great land-masses from the north and that dis- 

 similarity is intensified with distance from that pole, any evidence 

 for a Tertiary Antarctic centre for the flowering plants would be 

 discounted in advance. On this view the plant-types most differ- 

 entiated would be those that had met most rarely in the common 

 gathering ground of the north; that is, those of the tropics and of 

 the southern hemisphere. 



It was the recognition by Bentham and Hooker of the continuous 

 southward migration of the Scandinavian flora over a great part of 

 the globe that supplied the key for this interpretation of distribu- 

 tion; and it was, as Thiselton-Dyer remarks, from the geological 

 researches of Heer that it received early powerful support as a general 

 explanation of the geographical distribution of plants. Long ago, 

 as is again shown, Asa Gray held that the preservation of fragments 

 of the Cretaceous flora in Asia and America had its explanation in 

 their having had a common source in the north. As shaped and 

 promulgated in Darwin and Modern Science this theory removes 

 more difficulties from the path of the student of distribution than 

 any previous hypothesis. But it does even more, since in clearing 

 the road it opens a field of investigation which will take many years 

 to explore. How distribution may appear when regarded from this 

 standpoint is exemplified in the comparative study of Carex and 

 Sphagnum in the next chapter. 



Summary 



1. The differentiation theory could of itself explain distribution 

 only where a continuous land-area not affected by unstable climatic 

 conditions is concerned. As our globe presents itself, three factors 

 control and direct the operation of the differentiating agencies : the 

 divergence of the land-masses from the north, the secular fluctua- 

 tions of climate, and the barriers lying athwart the line of march of 

 migrating plants. The effect of the first is seen in the increasing 

 differentiation of floras with distance from the pole, that of the 



