CH. XVI] ENDEMISM AND DISTRIBUTION: GENERA 183 



like endemic species, and that both endemic species and genera 

 behave Hke non-endemic, it is clear that nothing but a mechanical 

 explanation will serve for the chief features of their distribution, 

 when one is dealing with the mass. Age supplies such an explana- 

 tion, but this is hardly possible to the supposition either that 

 they are chiefly relics, or that they are chiefly local adaptations. 

 It Avould thus seem to follow that endemics in the mass, whether 

 species or genera, are chiefly youpg beginners, descended in 

 general from the more widely distributed forms about them. 

 The smaller the area occupied, on the average, the younger the 

 species or genus. 



Only in comparatively rare cases can we look on forms of 

 small area as relics. The fact that in every family the monotypes 

 are from two to three times as numerous as the ditypes is fatal 

 to any idea of relic nature for the great bulk of them. Of course, 

 just as in the case of species, we must make varioiis provisos for 

 the use of Age and Area, such as that the genera be only com- 

 pared in groups of ten allies on either hand of the comparison, 

 that they be only taken in tens in any case (to lose the relics in 

 the crowd), and that conditions remain reasonably constant. 

 Species and genera are endemic simply because they have not 

 yet had time to spread abroad, or because they have been pre- 

 vented by barriers, sometimes physical, sometimes ecological. 



Summary 



Endemic genera occur in similar places to endemic species, 

 and instances are given of the numbers that occur in various 

 parts of the world, from which it appears that islands in general 

 have the smallest proportions. Avhile the proportion increases 

 with increasing area, up to 100 per cent, for the world. Examples 

 are quoted of very small areas occupied by many endemic genera, 

 usually monotypic, and more detail is given of the distribution 

 of genera in several families, showing that on the whole the area 

 varies roughly with the number of species, and that both types of 

 distribution seen in the preceding chapter— one species covering 

 the whole generic range, or several species dividing it among them 

 —can be matched in the families, and the genera pertaining to 

 them. 



It is then shown that endemic genera are distributed in different 

 countries in regular order, with many monotypes, fewer (but 

 still many) ditypes, and numbers tapering away to the larger 



