172 ENDEMISM AND DISTRIBUTION: GENERA [pt. ii 



There are two with nine species, one in the Andes from Mexico 

 to Chile, including Venezuela, and the other with eight species 

 in Pacific North America and one in Atlantic. So far the areas 

 occupied are closely correlated with the number of species, but 

 in the bigger genera there is more variation. Loeselia with twelve 

 species rmis from California and Texas to Colombia and Vene- 

 zuela. Gilia with 109 and Navarretia with 41 both occupy North 

 America, the Andes, and Argentina, while Phlox with 48 covers 

 North America and part of Siberia. Finally, Polemonium, which 

 has only 29 species\ covers North and South America, and north 

 temperate Europe and Asia. Except for this last genus, which 

 covers the whole family range, the area is roughly proportional 

 to the number of species (cf. Chapter xii) and the grouping is 

 just like that of the endemic species or genera. 



This type of distribution is very common indeed, showing in 

 perhaps the greater number of the families. One genus, usually 

 with many species, covers the whole or most of the family range, 

 the smaller genera, with more restricted ranges, being the more 

 numerous, and on the whole increasing in number the smaller 

 they are, and the more restricted their range. In the Polemoni- 

 aceae, there are eight genera below, and four above, the average 

 size for the family, one of the latter occupying the whole family 

 range. 



If one take the Cistaceae (37), one finds Halimium with 26 

 species covering the whole family range, while Helianthemum 

 with 70 covers the Old World from Macaronesia to Beluchistan 

 and Arctic Europe, and Lechea with 13 covers North and Central 

 America and the West Indies. The rest, with 20, 12, 9 and 3 

 species, cover smaller ranges within these. 



Or if one take the large "and widespread Menispermaceae (fig. 

 on p. 173), one finds (37) Cocculus and Cissampelos with a 

 distribution practically covering that of the family, Stephania 

 and Tinosjjora covering most of the Old World, and Hyperbaena 

 most of the New World, range. Within these are many genera 

 of smaller and smaller range till one comes down to the 12 

 in West Africa, 5 in Brazil. 5 in Madagascar, etc. There are 

 2 genera of maximum range, 4 of rather less (including Meni- 



1 This curious j)oint, that tlie most widely spread genus of all has fewer 

 species than some of the others, is by no means unique, but occurs in a 

 number of families, e.g. also in the Menispermaceae, Cistaceae, and Hydro- 

 phyllaceae. It requires careful investigation with the aid of palaeobotany, 

 for it seems to me not impossible that the deficiency in species may be 

 connected with the occurrence of the glacial period. 



