THE AGE AND AREA LAW 447 



arguments of both Ridley and Willis that incipient species are 

 easily killed out. Willis, however, " for the sake of argu- 

 ment," gives all Ridley's objections concerning the distribu- 

 tion of these seventy species their maximum effect, and shows 

 that the resulting corrections of the figures for the distribution 

 as a whole leaves the case for age and area as strong as before. 



In a still more recent contribution (19) Willis gives five 

 further examples of the action of age on area. Taking the 

 orchids of Jamaica and dividing the islands of Jamaica and 

 Cuba into a number of areas approximately 6| miles square, 

 he shows that the endemic species as a rule occupy about 

 3 squares, i.e. the average for all the endemics is 3 squares, 

 while the average for the species extending only to Cuba is 4*5 

 squares, and for the wider-spread species 5*7 squares. It is 

 clear, therefore, that the distribution of the orchids in Jamaica 

 follows the age and area law. As an example of an island 

 flora rich in endemics he takes the flora of the Hawaiian 

 Islands and points out that 74 out of the 149 non-endemic 

 species occur on all the chief islands, while only 41 out of the 

 581 endemic species cover that area. The average area 

 occupied by the non-endemics is about twice that of the 

 endemic species, so that here again the age and area law is 

 at work. 



Callitris is taken as an example of a Conifer. The genus 

 is endemic in Australia and Tasmania. One species occupies 

 the area covered by the genus, a second group of two species 

 is less widespread, a third group of eight species is still more 

 local, while there are seven species which occupy very small 

 areas. This is as near to the age and area phenomenon as is 

 to be expected in a single genus of less than twenty species. 



Two examples are given of the distribution of ferns. The 

 endemic ferns of New Zealand cover on an average a smaller 

 area than the non-endemic ferns, but the difference is not so 

 marked as in the Angiosperms. A very interesting point in 

 view of the recognised relative antiquity of the ferns is that 

 the endemic ferns occupy on the average about twice the area 

 occupied by the endemic Angiosperms. The endemic ferns 

 also take no notice of Cook's Strait, while the endemic Angio- 

 sperms are markedly divided into two groups by that Strait. 

 The deduction is made that Cook's Strait originated between 

 the dispersal of the endemic ferns and the dispersal of the 



