CH. XVI] ENDEMISM AND DISTRIBUTION: GENERA 181 



able, and after looking at them it is difficult any longer to main- 

 tain the position that endemic genera in general are survivals of 

 old floras. Of course there are many single examples that are 

 such, but they are quite lost in the crowd when one deals with 

 large numbers. Survivals Avould never, so far as one can conceive, 

 be graduated like this. 



The four columns of percentnges in the table above, plotted as curves. 

 Vertical readings are the percentages, horizontal the number of the group 

 of ten families. (By courtesy of the Editor, Annals oj Botany.) 



Confirmatory evidence may be obtained in various ways. 

 Families that have been long enough upon islands to give rise 

 to endemic genera must be very old, and so must families that 

 have reached both Old and New Worlds. One will therefore 

 expect these two lists to coincide to a large extent, and in fact 

 one finds that 90 per cent, of the island families that contain 

 island endemic genera also reach both worlds. Or again, one 

 will expect that the oldest families Avill have reached most 

 islands, and should contain the most endemic genera by reason 

 of their age. This is easily found to be the case; the West Indies 



