STATISTICS OF METASPERMAE. 



625 



It is possible also to pursue a line of investigation in regard 

 to these 407 genera precisely similar to that which was followed 

 out for the 106 families that are represented in the valley of 

 the Minnesota. As before, in order to analyse the general 

 ranges, we may group the genera under the same six divisions 

 that were established for the families, namely: 



A. Cosmopolitan genera. 



B. Extratropical genera. 



C. Tropical and subtropical genera. 



D. Northern extratropical genera. 



E. Western Hemisphere genera. 



F. North American genera. 



The grouping of the genera under these heads is with the 

 same reservations as in the case of the families. As before, it 

 may be necessary to enter the same genus under more than one 

 head . The number of species developed in any genus is con- 

 sidered to furnish the best index of its relative preponderance 

 in one locality rather than another. Where, then, the species 

 are particularly numerous in extratropical regions and fewer 

 in tropical regions, the genus is in general set down as extra- 

 tropical, and similarly in the other cases. To indicate the 

 distinctive range -of the genera represented in the Minnesota 

 valley is the purpose of the following table: 



Oenera Represented in the Minnesota Valley. 



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