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SCIENCE 



[N. S. Vol. XLIV. No. 1131 



from cool upland or mountain regions, 

 where herbs are commoner than in low- 

 lands, and the herbaceous percentage is ac- 

 cordingly higher. In the lowland tropical 

 forest, however, as is shown in the selected 

 figures for the Amazon Valley only, her- 

 baceous species are extremely few. 



The most important limiting factor to 

 the spread of tropical vegetation seems 

 to be the occurrence, even for a very 

 short time during the year, of temperatures 

 near the freezing point. As to just what 

 the climate was like under which the angio- 

 sperms first appeared we are not altogether 

 certain, but freezing temperatures seem for 

 the most part to have been quite absent. 

 "We may reasonably infer that conditions 

 then favored an overwhelming development 

 of trees and shrubs such as we see in 

 winterless regions of the earth to-day. 



An examination of those floras which are 

 believed to be very ancient and to be com- 

 posed of plant types which have elsewhere 

 disappeared, is of especial interest for our 

 problem. The organic life of certain iso- 

 lated oceanic islands, in particular, is gen- 

 erally recognized as giving us a rough idea 

 of the fauna and flora which existed over 

 wider areas in ancient times; and the 

 "endemic" animals and plants — those 

 which are peculiar to the region and are 

 found nowhere else, and which in such 

 oceanic islands constitute a large propor- 

 tion of the species — are regarded as still 

 more ancient than the non-endemic ele- 

 ment ; for they must either have had a long 

 evolutionary history in the region or must 

 be remnants of older types which have 

 elsewhere become extinct. Table II. shows 

 the percentage of herbs among the non- 

 endemic species (most recent element) ; the 

 endemic species of non-endemic genera (in- 

 termediate element) ; and the species of the 

 endemic genera (most ancient element) in 

 certain of these insular floras. 



Hawaii (582 species) 



Fiji (563 species) 



Juan Fernandez (89 species) 



St. Helena (41 species) 



Socotra (517 species) 



Mauritius and the Seychelles 

 (587 species) 



Ancient 



Element, 



Per Cent. 



Herbs 



It is evident that the youngest element is 

 predominantly herbaceous, the intermediate 

 one less so, and the oldest almost entirely 

 woody. In fact, the great majority of 

 herbs in these insular floras apparently 

 arrived such a short time ago that they 

 have not yet developed into endemic types, 

 but are still identical with species in other 

 regions. This is the more noteworthy since 

 herbs, because of the brevity of their life- 

 cycles and their consequent multiplication 

 of generations, tend to change more rapidly 

 than woody plants. The vegetation of 

 these ancient islands thus seems to have 

 been, in times not very remote, even more 

 devoid of herbs than it is at present. In 

 such islands as Bermuda and the Azores, on 

 the contrary, where from the almost com- 

 plete absence of endemic species we have 

 reason to believe that the flora is not 

 ancient, the percentage of herbs is fully 

 as high as in continental areas of similar 

 climate. 



The larger land masses of the south tem- 

 perate zone — Australia, New Zealand, 

 southern South America and South Africa 

 — which have also been isolated in a greater 

 or less extent from the continental areas of 

 the northern hemisphere, resemble ancient 

 oceanic islands to a certain degree in the 

 composition of their vegetation. In Table 

 III. are shown the percentages of herbs 

 among the species of the non-endemic 

 genera (recent) and among the endemic 

 genera (ancient) in the floras of these 

 regions. 



