GEOGRAPHICAL DISTRIBUTION 



241 



there is a definite mechanical relationship between them. If we 

 imagine existing genera to give rise to new genera, as they give 

 rise to new species, by mutations at intervals, we shall then 

 expect that genera as a whole will follow the law of compound 

 interest. But if this be the case, then it follows that whilst the 

 number of genera plotted to the numbers of s{>ecies that they 

 contain vnW give a hollow curve like those on p. 237, the loga- 

 rithm of the number of genera plotted to the logarithm of the 

 numbers of species that they contain will give a straight line^. 



Number of species 

 I 5 10 20 30 40 50 100 200 



10 12 1-4 



log (N9 of species) 



Logarithm curve for all Flowering Plants (from Willis, Diciionary). 

 (By courtesy of the Editor of Nature.) 



That this is in fact very close to the actual truth when con- 

 siderable numbers are dealt with is shown by the figures on 

 pp. 241, 242, which give the logarithmic curves for all flowering 

 plants, for the Rubiaceae, and for the Chrysomelid beetles. This 

 subject must also be left for further consideration in a later book. 

 Suffice it to say for the present that the evidence is decidedly in 

 favour of the origin of new species and genera from old by 

 mutation, which in the long run has followed a very definite 



1 For this deduction I am indebted to my friend Mr G. Udny Yul 

 C.B.E., F.R.S. 



W.A. 16 



