696 GEOGEAPHICAL AND GEOLOGICAL BOTANY. 



St. Paul's, and Tristan d'Acunha. Very little is known of the vegetation 

 of these islets ; but a species of Phylica is supposed to form the forest of 

 the first-mentioned island, and to connect botanically the islands of St. 

 Helena, Tristan d'Acunha, and Amsterdam one with another and with 

 the African continent, species of Phylica being found in each, and most 

 abundantly in South Africa. Spartina arundinacea, a grass, is common to 

 the islands of Amsterdam, St. Paul's, and Tristan d'Acunha. Chenopo- 

 dium tomentosum, Ncrtera df presto, Dactylis ccespitosa, and Accena indi- 

 cate a connexion with the Fuegian and Antarctic floras, and through a 

 species of Pelargonium with the South-African. 



Sect. 4. STATISTICS OF VEGETATION. 



Number of Species. Various authors have made computations 

 from existing data, with a view to ascertain the total number of 

 existing species of Phanerogams; but as the opinions of authors as 

 to what limits a species are so extremely varied, it seems useless to 

 occupy space with such speculative matter. The computations 

 range from 100,000 to 300,000 species and upwards. It is some- 

 what more easy to lay down some general statistical facts regard- 

 ing the distribution, and particularly in reference to the relative 

 proportions of the more important Classes and Orders, in different 

 regions of the globe. 



Relative Proportion of the larger Groups. Materials are insufficient to 

 enable us to calculate the relative distribution of Cryptogams and Pha- 

 nerogams in different regions. The former appear to bear a higher pro- 

 portion to the latter as we recede from the equator to the poles ; but this 

 may depend upon our better acquaintance with the Cryptogamic Floras 

 of the northern temperate regions than with the Cryptogams of the 

 warmer climates. 



As regards the relative abundance of Monocotyledons and Dicotyledons 

 in different latitudes, it is generally agreed that the proportion of Mono- 

 cotyledons to Dicotyledons increases from the equator towards the poles 

 a retrogression of the proportional number taking place, however, in the 

 icy regions of the poles and on the alpine summits. Asa rule also, closely 

 connected with the above statements, Monocotyledons are more pre- 

 dominant in proportion to the greater moisture of a climate. 



Probably no Orders, except the Leguminosae and the Compositae, con- 

 tain a number of species amounting to 5 per cent, of the total number of 

 Phanerogamic species. Thus the existence of species of one Order in any 

 region exceeding in number 5 per cent, of all species found there, indicates 

 a predominance of that Order. If such predominance occur only in one 

 region, the Order becomes characteristic of that region ; if such predomi- 

 nance of the same Order occur in many regions, it indicates wide diffusion 

 of that Order. 



In a very long list of Floras, from all parts of the globe, compared by 

 Ajph. De Candolle, it was found that only 35 Orders of Phanerogamia 

 formed more than 5 per cent, in any one or several regions. 



