240 Dr. J. D. Hooker on the Vegetation 



nigh levels, where the tropical proportion no longer obtains, it is probable 

 that Mr. Brown's results are the more accurate. If the Galapagos number of 

 Monocotyledones is small as compared with that of the continent, it is even 

 more so with regard to that of other tropical islands : thus in St. Helena they 

 equal nearly \ of the Dicotyledones, in the Society Islands \%, and in the 

 Sandwich Islands \. This paucity is not due to the sterility of the soil or 

 dryness of the climate, for the Cape Verd proportion is \, these islets being, as 

 I have mentioned above, equally barren with the Galapagos. 



Although I can offer no explanation of this apparent anomaly, it may not 

 be out of place to notice here, that the tropical islands in general possess 

 proportionally more Monocotyledones than do the continents. This is no 

 doubt due to the same causes which determine the increased proportion which 

 the temperate zone shows over the tropical, and is the more striking from this 

 circumstance, that the nearer the tropical islet is to a great continent, the 

 greater does the proportional number of X)ico/'3//ec?owe* become, as is shown by 

 the Galapagos and Cape Verds, both adjacent to great continents, possessing 

 more than the Sandwich Islands, Society group, St. Helena, or Ascension. 



The individual species of the very largest Monocotyledonous families being 

 more widely dispersed than any of equal extent amongst the Dicotyledonous, 

 is also a reason why the insular proportion of the latter should be different 

 from the continental*. 



If the insular tropical proportion be assumed to be \, it is larger than that for 

 the islands of the Atlantic immediately beyond the tropics. Thus for the Cana- 

 ries (lat. 28°) it is probably^, and for Madeira^ ; whence they again increase 



* And for the same reason great caution is required in deducing the continentsJ proportions from 

 small local collections, however complete in themselves. Tlie results obtained from a small area are 

 almost necessarily erroneous if applied to the greater one of which it forms a part, although the general 

 features of vegetation may be well displayed by the latter. To this I attribute the remarkable dis- 

 crepancy between the results obtained for West Tropical Africa by Mr. Brown, after an examination of 

 the Congo Expedition plants, and those which my examination of the Niger Expedition collections led 

 to, the Monocotyledones equalling \ in the former case, and ^ in the latter, of the Dicotyledones. This 

 appears to be owing to the same species of Cyperacea and Graminete (which form the majority of the 

 Monocotyledones in both instances) prevailing throughout a great extent of coast, but accompanied by 

 diflFerent species of Dicotyledones at different parts of the same tract. Hence it is probable that up 

 to a certain point the Monocotyledonous proportion will decrease as the area under examination is 

 extended, and the proportion for the tropics may even fall short of |. 



