144 



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Fig. 10. Distribution of the Ocnerodrilinae genus Ilyogenia and of two genera of 

 Phreodrilidae (freshwater Oligochaeta), on Wegener's Mesozoic map. 



solved what Michaelsen (1911) called the great puzzle of earthworm bio- 

 geography. Naturally, the explanation based on Wegener's theory does 

 raise important collateral problems, and this has often been reiterated. The 

 main objection is that, if the continental drift theory is correct, there should be 

 a greater uniformity in the distribution of the zoological groups, especially in 

 the Southern Hemisphere. 



This objection can be answered by stating that the genera of earthworms are 

 extremely ancient and have evolved very slowly, whereas other systematic 

 groups have become so greatly differentiated that the ancient resemblance of 

 the faunas has become almost obsolete. 



The great tardiness in the evolution of terricolous Ohgochaetes is docu- 

 mented by innumerable circumstances. One of these, I think, must be apparent 

 to whoever has followed me up to now : the distribution of single genera of 

 earthworms corresponds generally to the distribution of superior taxonomic 

 categories, families and superfamilies, of vertebrates or arthropods. Other 

 indirect data on the antiquity of the Ohgochaetes can be gathered from the 

 study of various fossil types of soil. It is a fact that no fossil remains of 

 earthworms are known but fossil remains of soil types produced by the work 

 of earthworms are well known : gyttja and mull. We know also that these 

 soil types evolved during the Mesozoic in correspondence with the evolution 

 of the modern Spermatophyta. 



