ASPECTS OF KINETIC EVOLUTION 3 I I 



it tends to disappear when crossed with unrelated groups. Such 

 variations could not spread or propagate themselves in a nor- 

 mally symbasic species ; each would need to be carefully iso- 

 lated in order to be preserved. In the second place, very few, 

 if any, of the thousands of mutations which have come under 

 the eyes of planters and experimenters have proved to be more 

 fertile, in the true reproductive sense, than the parental types. 

 Nearly all of them are conspicuously deficient in this respect, 

 and would thus struggle under a fatal selective handicap in 

 competing with the parent form, if they were not at once wiped 

 out by interbreeding. Mutations have very great agricultural 

 importance, but their practical value will not be enhanced by 

 overlooking this fact of deficient fertility which is fatal to the 

 view that they represent a genuine condition of progressive 

 evolution. 



Mutations arise sideways, as Professor De Vries explains, 

 but it does not follow that new species are formed in this 

 manner. Mutations are frequent in domesticated plants because 

 varieties in cultivation are separated by inbreeding from the 

 normal forward progress of the whole interbreeding species. 

 Each species when once formed is supposed, under the mutation 

 theory, to remain stationary so that progress can be made only 

 when new varieties become segregated from the mass. 



There is, however, another and very different way in which 

 variations can contribute to evolutionary progress. Instead of 

 being recessive mutations, the variations which have practical 

 evolutionary significance are prepotent, and can work one change 

 after another in the gradually advancing group. The true evo- 

 lutionary significance of mutations is not that species arise by 

 mutation, but that the progressive steps, by which the evolution 

 of species is gradually accomplished, are not imperceptibly 

 small. There may be a very appreciable advance between two 

 successive individuals. 



Very acute selection or some other way of separating a new 

 mutation from its unmodified parent stock must be imagined in 

 order to account for its preservation, but plants and animals 

 abound in characters which could scarcely have been perpetu- 

 ated in this way. With self-fertilized plants a single individual 

 Proc. Wash. Acad. Sci., January, 1907. 



