CHAPTER XVII 



THE MONOTYPIC GENERA, AND GENERA 

 OF LARGER SIZE 



r ASSiNG on now to deal with monotypic genera, or genera with 

 one species only, one soon notices that they show the same 

 phenomena that we have already seen in the endemic genera. 

 This is what we should expect upon the hypothesis of Age and 

 Area, as expanded by Size and Space, implj'ing as they do that 

 small genera, endemic or not, are on the whole younger than, 

 and occupy less territory than, the larger genera in the same 

 circles of affinity. 



Few people, perhaps, have realised how numerous the mono- 

 types are. No less than 4853 out of the 12,571 genera of flower- 

 ing plants in my Dictionary (4th ed.) are monotypic, and are 

 usually so restricted in area that most people would call them 

 endemics, A number will doubtless proA'c to have more than one 

 species when we finally know the flora of the world, but new ones 

 are frequently discovered, or created by the splitting of other 

 genera, and there is little likelihood that the percentage will fall 

 much below its present figure of 38-6 per cent, of the total. The 

 ditypes, or genera of two species each, are also very numerous, 

 and include 1632 genera, or 12-9 per cent. In other words, the 

 monotypes and ditypes alone include more than half the genera 

 at present existing, or 51-5 per cent., while the tritypes include a 

 further 921, bringing the total to 58-9 per cent. The monotypes 

 are approximately three times as numerous as the ditypes, and 

 these almost twice as numerous as the tritypes. Beyond ten 

 species the figure for number of genera goes below 500, and at 

 twenty-five species below 200, tapering out in an enormously 

 long tail to the final genera Senecio (1450 species) and Astragalus 

 (1600). 



We have already seen many instances of the hollow curve, and 

 when the genera of the world are plotted by numbers containing 

 1, 2, 3, etc., species, one gets a beautiful example of it. It is idle 

 to suggest that further work will alter the form of this curN-e. 

 The monotypes exceed the ditypes by 3221, and the ditypes 

 exceed the tritypes by 711, and so on right through the list. 



One may even go beyond the genera, and find that the families 



