IX] GEOGRAPHICAL DISTRIBUTION 143 



is only possible by unaided locomotion through con- 

 tinuous land areas, the fact that but few gaps occur 

 in the range of a particular sub-family or lesser group 

 seems to indicate that no great time has elapsed since 

 the specialisation of these different forms. The 

 dependence of earthworms upon vegetable mould also 

 points in the same direction and furnishes an argu- 

 ment for the belief that these animals only greatly 

 increased on the advent of abundant dicotyledonous 

 plants, and perhaps indeed were actually contem- 

 poraneous with them. 



