194 THE MONOTYPIC GENERA [pt. ii, ch. xvii 



each commencing with a great number of monotypes, and giving 

 a hollow curve. 



In the smaller areas of the world, like the single continents, 

 one finds the proportion of monotypes very high, while that of 

 dit3^pes is lower, and it falls off steadily to an insignificant figure 

 for the larger genera. In the genera found in both worlds, on 

 the other hand, the exact reverse is the case, and intermediate 

 phenomena show in intermediate areas. 



The area occupied by a monotj^pe may vary enormously, but 

 in general is small; only 66 of them occur in both Old and New 

 Worlds. 



The greatest percentage of monotypes is in the larger families, 

 and it diminishes steadil)^ with the lessening sizes of the families, 

 when these are taken in groups of 40. 



This marked arithmetical relationship of the monotypes to 

 other genera shows that the usual explanations — that they are 

 relics, or that they are special adaptations — are in general in- 

 applicable, and that the explanation offered by Age and Area, 

 with its implications, that as a rule they are young beginners, 

 and probably descended from the larger genera, must in all 

 probability be correct for the great majorit}'. 



