86 THE LESSON OF EVOLUTION 



This can be the only explanation of the sudden ap- 

 pearance of eutherian mammals in the lower Eocene 

 rocks of North America and Europe ; and of the 

 graptolites in the lower Ordovician of Europe. The 

 appearance of dicotyledonous plants in the lower 

 Cretaceous of North America and Europe is more 

 difficult to explain. This flora is thought by some 

 to have originated in the arctic regions, and to have 

 spread southwards ; but, however, this may be with 

 the deciduous trees, a varied evergreen dicotyle- 

 donous flora existed in New Zealand with 

 Belemnites and marine Saurians (Cimoliosaurus , 

 Liodon, &c.), which can hardly be younger, and may 

 perhaps be older, than the cretaceous flora of 

 Greenland. 



