CH. II] INTO NEW AREAS 23 



the vegetative covering of any given area shows that the pos- 

 session of a good dispersal mechanism is only rarely of much 

 value. A general comparison of the flora of the world shows that 

 more than half of the most cosmopolitan genera have little or 

 no mechanism for dispersal, nor is such Avell marked, on the 

 whole, in the largest and most widely distributed families. It 

 also goes to show that in most cases where there are now wide 

 separations by seas, etc., the floras are too large to have been 

 able to arrive across such formidable barriers. 



