114 SIZE AND SPACE [pt. ii 



Table showing {in the horizontal lines) the average rmmber of vice- 

 counties in Britain reached by the most ividely distributed species 

 in each genus of different sizes, and by the second, third, fourth, 

 and fifth, most widely distributed species in each genus. 



show a steady diminution from top to bottom, whether the first, 

 second, third^ fourth, or fifth species be taken ; and examination 

 of the remaining figures shows that the rule holds equally well 

 for the sixth, seventh, eighth, ninth, and tenth most widely 

 distributed species. The most widely dispersed species of a large 

 genus {i.e. a genus with many species in Britain) is (on the a\erage) 

 more widely dispersed than that of a genus with 6-10 species, 

 this than that of a genus with five, and so on right down the 

 scale, and the same thing shows with the second, third, fourth, 

 and fifth to tenth most widely distributed species. Nothing but 

 a mechanical explanation can explain such mechanical regu- 

 larity. If the vital, climatic, or ecological factors had many 

 differences, other than purely local, in their action, one would 

 expect some breaks in the regularity, but there are none. The 

 genera occupy areas in Britain in proportion to their numbers 

 of species there, and age has ])een the overwhelming factor in 

 their distribution. 



As the species of those genera with one species each average 

 50 vice-counties, and those with two 73 and 33, one may imagine 

 that on the average one species in the latter genera arrived before 

 the solitary one of the former. In the same way (as indicated by 

 the vertical lines in the table) two species in the genera with 

 four or five, three in those with 6-10, and at least five in the 

 larger genera, probably did so. 



Such results as this, which could be easily multiplied, go to 

 show that in a given country the area occupied by a genus 

 increases (on the average of considerable numbers) with the 



