184 ENDEMISM AND DISTRIBUTION [pt. ii, ch. xvi 



genera, which are usually found only in large islands or other 

 large areas. If plotted as graphs the figures give the usual 

 hollow curves, and it is clear that the sizes of the genera depend 

 on factors similar to those that determine the sizes of areas 

 occupied by species. 



Still larger floras, e.g. those of single continents, or of the 

 whole world, show the same type of arrangement of the genera, 

 with many monotypes, fewer (but still many) ditypes, these two 

 making about half the total, while the larger genera taper away 

 steadily in number in a long tail. 



It is clear that neither the supposition that endemics and 

 small genera are relics, nor that they are special adaptations 

 will avail to explain the phenomena presented by the great mass. 



Endemic genera further prove to belong more to the large 

 families, just as endemic species belong to the larger genera. 



The case of islands, usually regarded as the typical home of 

 endemic genera, is then considered in more detail, and it is 

 shown that the proportions of endemic genera in (1) the islands 

 of the world, in (2) West Australia, South Africa, and Brazil, 

 and in (3) Australia, Africa, and South America, are much the 

 same for all three, for each group of ten families in order of size, 

 and this proportion is the same as occurs in the world for each of 

 these groups. Confirmatory evidence is also given, the result of 

 the whole being to show that in the mass endemic genera are 

 simply, like endemic species, young beginners, and probably the 

 descendants of other genera still existing. 



