DISTRIBUTION 



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only one of the ten species occurs outside the New World, namely, 

 in Madagascar. The number of genera mainly American which have 

 one or two solitary representatives in the Old World is remarkable. 

 Several remarkable cases of discontinuity could be cited from the 

 tropics of the Old World. For instance, Canarina (Campanulacece) 

 holds three species, found respectively in the Canary Islands, tropical 

 Africa, and the Moluccas. 



Whether in the family, in the tribe, or in the genus, discontinuous 

 distribution is a familiar feature in the plant-world of the southern 

 hemisphere. A very ancient history is implied in the representation 

 of the Proteacece by different genera in Australia, South Africa, and 

 South America. Students of fossil botany are persistent in claiming 

 a home for the family in Europe and North America in Cretaceous 

 and Eocene times; but Thiselton-Dyer, with much to gain from 

 such a valuable witness on behalf of his views, does not accept the 

 evidence. As an example of a tribe we may quote his reference to 

 the MutisiacecB, a tribe of the Compositce, characteristically southern 

 in its distribution in South America, South Africa, and Australia. 

 For the genera we may cite Librocedrus and Podocarpus of the 

 Coniferce. Both of them are indicated amongst the fossil Tertiary 

 remains of Northern Europe and North America, the first-named even 

 in Spitzbergen; and both reach extreme southern lands — in the 

 case of Podocarpus, Southern Chile, South Africa, and New Zealand, 

 and in that of Librocedrus, New Zealand and Chile. 



In the foregoing remarks I have very inadequately illustrated the 

 important subject of discontinuous distribution. Many of these 

 genera are now restricted to tropical regions, and the geological record, 

 as so far interpreted, tells no story of a sojourn in the north. But 

 many of the genera of trees now confined to temperate latitudes of 

 North America and Eurasia grew in Miocene times within the Arctic 

 Circle, and the implication is that in the case of the dissevered 

 tropical genera they also long ages ago were denizens of the north. 

 The history of Sequoia, now restricted to California and its vicinity, 

 but growing in Tertiary times in Arctic latitudes around the pole, is 

 the story of a genus that has failed. So also there may have been a 

 similar failure with some of the tropical genera that are now found in 

 only one of the two hemispheres, either in the Old or in the New World. 



The Centrifugal Dispersion from the North during the 

 last Ice Age. — The associated processes of centrifugal dispersion 

 from the north and of differentiation with distance from the pole are 

 well described by Prof. Harshberger in his Phyto geographic Survey of 

 North America (p. 181) ; but he limits its operation to the last of the 

 great migrations that was connected with the Glacial Period. As 

 the herd of glacial plants moved south from the far north into each 

 one of the continental masses, America, Europe and Asia, they were 

 subjected (thus he writes) to a great variety of conditions, " the 

 outcome being great differentiation of form and the development of new 

 species." This dispersion from the north during the last great Ice 

 Age has been the theme of Darwin, Asa Gray, Bentham, Hooker, 

 and many other eminent men of science, a fact to which allusion will 

 again be made at the close of the chapter. 



