82 PLANT GEOGRAPHY 



CHAPTER VIII 



BRIDGES 



THERE are difficulties in explaining plant-distribution, 

 especially that of island floras, which are met in widely 

 different ways by students of these questions. On the 

 one hand, some writers, notably Darwin and Dr. Wallace, 

 believing in the permanence of continents and oceans, 

 at least in their main features, through most of geological 

 time, and certainly throughout recent geological time, 

 emphasise the possibilities of occasional means of trans- 

 port to long distances. Unwilling to believe that land 

 has existed where there is now deep sea, they point to 

 the prolonged resistance that some seeds can offer to 

 sea-water, the distance to which others may be carried 

 by wind, the long flights of birds, and the possibilities of 

 transport by floating tree-trunks or by ice-rafts. For 

 them many islands have never been connected with the 

 mainland. 



On the other hand, many zoologists, and not a few 

 geologists, argue in favour of very extensive changes of 

 level resulting in land becoming water and water 

 becoming land even in Tertiary times. This school 

 believes that many volcanic and now remote islands 

 have been connected with one or other of the continents, 

 and that they have, on the whole, been stocked by 

 ordinary migration, i.e. by short, not by long steps. 



The questions at issue are largely geological and by 

 no means simple. While it is urged that true oceanic 

 islands are either coralline or volcanic and destitute of 

 ordinary sedimentary rocks, it has been pointed out: 

 (i.) that such rocks may be entirely masked by volcanic 

 outpourings, and (ii.) that if such a continent as Africa 

 were submerged, only a few peaks, mainly, if not entirely, 

 volcanic, might be left exposed. While some writers 

 do not hesitate to postulate great continental extensions, 

 others content themselves with the suggestion of 

 " bridges," which may have been narrow and isthmus- 

 like, or even of chains of islands like the Antillean 

 "festoon" which now extends between Yucatan and 

 the mouth of the Orinoco. 



