DISTRIBUTION OF SPECIES 19 



world, and under various geographical conditions, and thus 

 arrives at the causes of great extension of certain species from 

 Vilest to east in the north temperate zone, or along sea-shores 

 or river-banks in the tropics ; while the normal area is consid- 

 ered to be '^ massive " rather than elongated. 



Coming then to detailed facts, he shows that about 200 

 species (out of the total then known of about 120,000) have 

 areas equal to one-third or more of the entire land surface of 

 tlie globe. Further, in certain Families (usually called ^Nat- 

 ural Orders) there are plants which range from the Arctic 

 regions to the southern extremity of the great continents. 

 Among the former are our common Marsh Marigold (Caltha 

 palustris) and Common Sundew (Drosera rotund i folia), which 

 are found in all Northern Europe, Asia, and America ; while 

 our common Sowthistle (Sonchus oleraceus) is found scattered 

 over the whole globe, tropical as well as temperate, and is per- 

 haps the nearest of any known plant to being truly cosmopol- 

 itan. 



By a laborious comparison the author arrives at the con- 

 clusion that the average area occupied by the species of flower- 

 ing plants is rroth part of the whole land surface of the globe. 

 But the area varies enormously in different parts of the world. 

 Thus, in the wdiole Russian Empire, species have a mean area 

 of irVth the land surface, owing to the fact that so many range 

 east and west over a large part of Europe and Xorth Asia ; 

 while in South Africa the mean range is only W&oth of that 

 surface, which expresses the fact of the extreme richness of 

 the latter flora, many of the species composing w^hich have 

 extremely restricted ranges. He also reaches the eonclusion 

 that in passing from the pole to the equator the mean areas 

 of the species become smaller. A few examples of very lim- 

 ited areas are the following : — Several species of heaths are 

 found only on Table Mountain, Cape of Good Hope; Cam- 

 immda isopliylJa grows only on one promontory of the coast 

 of Genoa; the beautiful Alpine Gromwell (Lithospermum 

 Gastoni), on one cliff in the Pyrenees; Wulfenia Carintliiaca, 



