Periodic Mutations 687 



of the mutability of the evening-primrose lies 

 in its usefulness as a guide for further work. 

 The view that it might be an isolated ease, lying 

 outside of the usual procedure of nature, can 

 hardly be sustained. On such a supposition it 

 would be far too rare to be disclosed by the 

 investigation of a small number of plants from 

 a limited area. Its appearance within the lim- 

 ited field of inquiry of a single man would have 

 been almost a miracle. 



The assumption seems justified that analo- 

 gous cases will be met with, perhaps even in 

 larger numbers, when similar methods of ob- 

 servation are used in the investigation of plants 

 of other regions. The mutable condition may inot 

 be predicated of the evening-primroses alone. 

 It must be a universal phenomenon, although 

 affecting a small proportion of the inhabitants 

 of any region at one time : perhaps not more 

 than one in a hundred species, or perhaps not 

 more than one in a thousand, or even fewer may 

 be expected to exhibit it. The exact proportion 

 is immaterial, because the number of mutable 

 instances among the many thousands of species 

 in existence must be far too large for all of 

 them to be submitted to close scrutiny. 



It is evident from the above discussion that 

 next in importance to the discovery of the pro- 

 totype of mutation is the formulation of meth- 



