132 AGE AND AREA, AND SIZE AND SPACE, [pt. ir 



positae can, therefore, be said to demonstrate in some consider- 

 able detail the action of the Age and Area law as far as relative 

 age is concerned in a group, the evolutionary history of Avhich 

 in time can be confirmed by many other lines of evidence. 



Size and Space. Dr Willis has also forwarded another pre- 

 diction to the effect that, "on the whole the area occupied by a 

 genus (taking a great many, say ten allies at least) varies in the 

 same sense as the number of species it contains." This also has 

 been worked out for Compositae, using the number of species 

 recorded for each genus by Bentham in the Genera Plantaruni, 

 in order to avoid the inequalities of modern species-splitting 

 and specialisation in particular genera. The results which are 

 given in Table III, when analysed properly, form a remarkable 

 verification of this prediction. The average number of species 

 per genus does follow on the whole the same series as the average 

 generic area. 



TABLE III 



Species/Genera 



