694 Mutations 



Many facts plead in favor of the constancy 

 of species. This principle has always been 

 recognized by systematists. Temporarily the 

 current form of the theory of natural selec- 

 tion has assumed species to be inconstant, ever 

 changing and continuously being improved and 

 adapted to the requirements of the life-condi- 

 tions. The followers of the theory of descent 

 believed that this conclusion was unavoidable, 

 and were induced to deny the manifest fact that 

 species are constant entities. The mutation 

 theory gives a clew to the final combination of 

 the two contending ideas. Reducing the change- 

 ability of the species to distinct and probably 

 short periods, it at once explains how the 

 stability of species perfectly agrees with the 

 principle of descent through modification. 



On the other hand, the hypothesis of imitative 

 periods is by no means irreconcilable with the 

 observed facts of constancy. Such casual 

 changes can be proved by observations such as 

 those upon the evening-primrose, but it is ob- 

 vious that a disproof can never be given. The 

 principle grants the present constancy of the 

 vast majority of living forms, and only claims 

 the exceptional occurrence of definite changes. 



Proofs of the constancy of species have been 

 given in different ways. The high degree of 

 similarity of the individuals of most of' our 



