172 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



has taken place there, and suggested that the lands of the Far South 

 had once been connected with one another by way of the Antarctic 

 Continent. This provided a basis for the presumption that there 

 had probably been a centre of species-formation there too, and that 

 life originated in the lands encircling both geographical poles. 

 However, neither theory can well be accepted, for it is now recognized 

 that climatic zones have existed as long as there has been life on 

 earth, and that ice-ages occurred previously to the Pleistocene and 

 in parts of the world other than the present polar areas. Moreover, 

 other centres of plant development have been strongly advocated 

 ■ — particularly in the tropics. 



An interesting example of the importance of historical and other 

 factors in the diversity and constitution of present-day island floras 

 is illustrated in Fig. 45, B, kindly contributed by Mr. W. T. Stearn, 

 of the British Museum (Natural History). This shows that whereas 

 the flora of Jamaica is large and diverse although its area is small, 

 the flora of Ceylon is smaller although its area is larger than that 

 of Jamaica, while the flora of the many times larger British Isles 

 is smaller still. Each island having a comparable degree of topo- 

 graphic variation, and the climates of Jamaica and Ceylon being 

 similarly favourable and that of the British Isles not unfavourable, 

 it seems reasonable to presume that the greater floristic diversity of 

 Jamaica is due to its proximity to the Central American region of 

 continuous evolution from early geological times. Ceylon, on the 

 other hand, although also tropical and humid and likely to be capable 

 of supporting a large flora, is near the geologically much younger 

 and floristically poorer southern portions of India, and consequently 

 has had a less diverse flora to draw upon. In the British Isles, the 

 flora and proportion and also degree of endemism are all smaller 

 than in Jamaica and Ceylon, owing in part to relatively recent 

 glaciation, in part to the prevailingly less favourable climate, and 

 in part to the proximity of (and only recent separation from) the 

 mainland of Europe. Greenland, most of whose still greater area 

 is covered by ice, has an even smaller flora, owing, it seems clear, 

 to the widely inimical climate and severe Pleistocene glaciation. Its 

 about 475 species of vascular plants (including well-established 

 introductions and 31 Pteridophytes) show an extremely low propor- 

 tion and degree of endemism, with none at all among the Pterido- 

 phytes. (These are the latest statistics which have become available 

 since the preparation of Fig. 45, B.) 



