OH. xii] SIZE AND SPACE 117 



of the same age as a tree genus with 100, but both will follow 

 this principle as far as possible. It goes to show that on the whole, 

 as the area occupied increases, a genus tends to break up into 

 more and more species: only at times does the original species 

 of the genus cover the whole of its range when it has reached a 

 very large area, and then most often when the conditions are 

 very uniform, as in the case of Zannichellia for example. In the 

 case of the Podostemaceae, where the conditions are perhaps 

 even more uniform, and yet a great many species have arisen, 

 it is due, as I shall hope to show in a later publication, to the 

 fact that the plants are always under the influence of plagio- 

 tropism, to the greatest extent possible. 



If we take the 28 largest genera in the world (51), we find that 

 about 16 are cosmopolitan in their distribution, 5 are cosmo- 

 tropical, 4 tropical America, and Qiiercus Old World, leaving 

 only Erica and Mesembryanthemutn, whose large number of 

 species is correlated in both cases with the fact that they grow 

 in South Africa, where the extreme conditions seem to tend to 

 produce large numbers of species, though, as we shall hope to 

 show in later publications, there are other factors in the matter. 



Nearly half the species in the world (69,000 of 162,000) belong 

 to 1171 genera that occur in both worlds (average 59 species per 

 genus), while only 66,750 belong to 9671 genera that are con- 

 fined to a single continent (average 7), and the 2026 genera of 

 the northern palaeotemperate and the palaeotropical regions, 

 etc. {i.e. widely distributed in the Old World) have about 26,250 

 species (numbers from my Dictionary), and form, as one would 

 expect upon the hypothesis of Size and Space, an intermediate 

 between the other two groups (average 13). 



Of the 28 large genera named above, the British Isles contain 

 10, Ceylon 17, New Zealand 11, the Hawaiian Islands 14 and 

 the Galapagos 15. Solanum (1225 species). Euphorbia (750), and 

 Cyperus (400) occur on all five, and four others on four, of these 

 groups, the only ones that occur on none being Myrcia and Mam- 

 millaria. Of the 244 genera that contain o^•er 100 species, no 

 fewer than 166 occur in both Old and New Worlds, 28 in tropical 

 America, and 10 in the Old World tropics, leaving only 31 for 

 the remaining smaller divisions of tiie world, like tropical Asia, 

 which has only 7. 



In the same way, the smaller families usually occupy smaller 

 areas than the larger, and the question arises whether they 

 should be considered of equal rank to the latter. Guppy has 



