CH.xviJ ENDEMISM AND DISTRIBUTION: GENERA 171 



Inasmuch as a genus consists on the average of over twelve 

 species, which never all occupy the same area, it is obvious that 

 the average area occupied by a genus must be larger than that 

 occupied by a species, but that does not affect the argument. 



Some endemic genera occupy very small areas, e.g. Homalo- 

 petalum in three parishes in Jamaica, Itatiaia on one mountain 

 in southern Brazil, Sphagneticola in a suburb of Rio de Janeiro, 

 Leichhardtia on the Daintree River, Carpolyza in the immediate 

 suburbs of Cape Town, Traunia and Spondiopsis upon Kili- 

 mandjaro, Cladopus in one or two streams in Java, Alsiiiidendron 

 upon Oahu Island, Neohracea upon several of the Bahama islands, 

 Podadenia in the neighbourhood of Ratnapura in Ceylon, and 

 so on. Or if one take a single country, New Zealand, for" example, 

 and take a few of its endemic genera, one finds Siphonidium and 

 Toivnsonia upon very small areas, Pachycladus upon one slightly 

 larger. Colensoa reaches about 80 miles along New Zealand, 

 Tetrachondra about 100, Anagosperma about 140, Notospartiwn 

 about 240, Ixerba about 300, Hoheria 700, Tupeia 1000, and 

 Carpodetus the whole length of 1080 miles from North Cape to 

 the south of Stewart Island. Of the eighteen genera endemic to 

 New Zealand which have one species each (37), six are confined 

 to areas not over 140 miles in length, or 33 per cent, of the 

 genera upon areas not exceeding 13 per cent, of the whole, so 

 that the tendency even here is to give a hollow curve (cf. pre- 

 ceding chapter). 



In Ceylon, the Hawaiian Islands, and elsewhere one finds the 

 same type of distribution, and if one go on to larger and larger 

 areas one finds larger and larger areas for genera in the same 

 graduated way, until one comes to such a Avorld-ranging genus 

 as Senecio, or Astragalus. Though of course there are many 

 exceptions, on the whole the size of the genera (number of their 

 contained species) becomes steadily larger with the increasing 

 area, as we have already pointed out in Chapter xii ; of course 

 allied groups only being compared. 



If instead of taking individual genera, or the endemic genera 

 of a single country, one take all the genera of a small family, one 

 finds the same graduation of areas. Take, for example, the 

 Polemoniaceae (from the Pflanzenreich). Of its twelve genera, 

 three, with one species each, occupy (roughly) California and 

 Utah, Mexico and Guatemala, and the Pacific United States. 

 One with five species is found in California, Utah, Nevada, and 

 Arizona, one with six in the Andes from Colombia to Chile. 



