CH. Ill] EXDEMISM, AGE AND AREA 29 



them at points in general south of the middle of the Indian 

 peninsula, and would on the whole be younger in Ceylon than the 

 wides, and therefore occupy lesser areas on the average. The 

 Ceylon endemics would arise from the wides (or Ceylon-South 

 Indians) in Ceylon, and would be the youngest, and on the 

 average occupy the least areas. All the figures of course must be 

 worked in averages, for an endemic of one group might be 

 occupying a large area when the first wide of another arrived. 



Confirmatory evidence was soon obtained from the floras of 

 New Zealand, Jamaica, Australia, and the Hawaiian Islands. 

 The figures for New Zealand are as follows: 



* Largely undoubted introductions of recent years. 



Facts like these, which are universal, cannot be the result of a 

 selection, but must have some more mechanical explanation. The 

 only one that to the writer seemed at all satisfactory was simply 

 age, as was explained in Age and Area, though of course age in 

 itself was not exactly a factor in distribution. There are very 

 many factors that may affect dispersal, but if one suppose 

 factor a to produce an effect in distribution in a long time x that 

 may be represented by 1, one may reasonably expect that in 

 time 2x it will produce an effect 2. If the effects of all the factors 

 be added up, the total effect in time x may be represented by m, 

 and in time 2x by 2m. Obviously there will be great individual 

 differences between species, so the proviso was made that com- 

 parison (with a view to determining questions relating to age) 

 must only be made between allied forms, which were most likely 

 to behave in an approximately similar way under similar circum- 

 stances. The quickly reproducing, herbaceous Compositae must 

 only be compared with other Compositae, not with the slowly 

 reproducing trees of the Dipterocarpaceae or the Conifers; and 

 so on. One form might even occupy in a decade what might take 

 the other several centuries to occupy. And not only must this 

 precaution be taken, but closely allied species, even, must be 

 taken in tens, to allow of averaging the effects of the many factors 

 that might take part in their distribution. But, bearing these 

 things in mind, one might say that large area of distribution 



