VIII] MOVEMENT AND MIGRATION 119 



of this inference it is clear that it will influence many 

 other propositions connected with the relative age 

 of the families of these worms and with many 

 problems of geographical distribution. It appears 

 to us that this simple explanation is the correct one. 

 But to show this it will be necessary to eliminate 

 other possible explanations. It might be urged that 

 the wider range of the genus Dichogaster and the 

 still wider range of the genus Allolohophora (shown 

 by community of species in widely distant localities) 

 was evidence merely of relative age, that the older 

 groups have had more time to travel and that the 

 newer groups have not had so long a time to spread 

 themselves over their habitat. On this hypothesis 

 the genera of Eudrilidae would be geologically much 

 newer than the genus Dichogaster and similar state- 

 ments might be made for the other forms here 

 under consideration. As already explained we cannot 

 attempt to answer this question in the only way in 

 which it can be really satisfactorily answered, by a 

 reference to fossil forms ; for there are no fossils to 

 refer to. So far as comparative anatomy enables us 

 to arrive towards a solution of the (question, it would 

 appear that the genus Dichogaster belongs to a more 

 ancient race than cither of the other two groups 

 considered, and that of these latter the Lumbricidae 

 are the most modern. Moreover we associate not 

 only a wide, but also a discontinuous, distribution 



