326 PLANTS, SEEDS, AND CURRENTS 



southern regions separated by broad oceans from each other. Let 

 us connect with these two facts a third, that this migration south 

 has been associated with a secular change from warm to cold climatic 

 conditions. The conclusion to which they inevitably lead is rejected 

 by Dr. Scharff in the case of animals (p. 428) mainly on account of 

 the absence of geological evidence. But we have the geological 

 records on our side for the plants ; and if we hold the belief that the 

 great change both in climate and in flora which has come over the 

 north since Tertiary times has been repeated in the earlier ages of 

 the world's history, we stand on much safer ground than if we were 

 to assume that we are here face to face with a change unprecedented 

 in the story of our planet. We are not concerned with the north 

 polar regions as an evolutionary centre, but as the great mixing 

 ground through the ages of the plants of the eastern and western 

 hemispheres. 



Discontinuous Distribution. — Let us glance at the facts of 

 discontinuous distribution. Whether we take a genus (A) which, 

 although represented in the Tertiary deposits of the common meeting 

 ground of the continents in the north, is now divided by the oceans 

 in the south, or a genus (B) that is now hopelessly sundered and 

 isolated in tropical regions and has, as far as is yet known, left no 

 trace of its original existence in the north, or a genus (C) that is now 

 restricted in the main to the continents of the temperate latitudes 

 of the south, the lesson is the same. For discontinuity forms the 

 essence of the problem, the error lying in treating extreme cases, like 

 that of Ravenala, as things apart that require a special explanation. 

 Such cases raise not side-issues, but the main question, the whole 

 history of distribution being concerned with the effects of discon- 

 tinuity increasing with distance from the north polar area. 



The simplest cases of discontinuous distribution are those illus- 

 trated by such genera as Quercus, Fagus, Acer, Juglans, Tilia, etc., 

 all represented in the Tertiary deposits of the extreme north, and 

 now found on both sides of the great oceans to the south, reaching 

 in some instances, as in that of Fagus, New Zealand and Fuegia. 

 Less simple cases are those so frequently illustrated among tropical 

 genera where the geological record, as at present known to us, is 

 silent as to their original occurrence in the north. But it is legitimate 

 to assume that the same principle has been at work here, and to 

 infer with Thiselton-Dyer in his Philadelphia address that during 

 the warm periods of the Miocene and earlier ages purely tropical 

 types would have extended north to latitudes where the interchange 

 between the Old and the New World would not be impracticable. 

 The behaviour of Ravenala, which is represented by only two known 

 species, one in Madagascar and the other in tropical South America, 

 is repeated, though in a less striking degree, by numbers of tropical 

 genera. We have, for example, Thespesia (Malvaceae), mainly 

 American and Malayan, and Mammea (Guttiferw), equally shared by 

 the New World and Madagascar. Then there are Chrysobalanus 

 (Rosacea;) and Crudya (Leguminosos), which are chiefly American, 

 two genera that are only known in the Old World from Africa and 

 Malaya. Then there is Desmanthus, a leguminous genus, of which 



