Variation and Heredity 287 



establish itself, either because it is adapted to live under con- 

 ditions somewhat different from the parent form, so that the 

 dangers of intercrossing are lessened, or because the new- 

 form may absorb the old one. It is also clear, from what 

 has gone before, that the new form can only cease to be fer- 

 tile with the parent form, or with its sister forms, after it has 

 undergone such a number of changes that it is no longer 

 able to combine the differences in a new individual. This 

 result will depend both on the kinds of the new characters, as 

 well as the amounts of their difference. This brings us to 

 a consideration of the results of De Vries, who has studied 

 the first steps in the formation of new species in the " muta- 

 tions " of the evening primrose. 



The Mutation Theory of De Vries 



De Vries defines the mutation theory as the conception 

 that " the characters of the organism are made up of elements 

 (' Einheiten ') that are sharply separated from each other. 

 These elements can be combined in groups, and in related 

 species the same combinations of elements recur. Transi- 

 tional forms like those that are so common in the external 

 features of animals and plants do not exist between the 

 elements themselves, any more than they do between the 

 elements of the chemist." 



This principle leads, De Vries says, in the domain of the 

 descent theory to the conception that species have arisen 

 from each other, not continuously, but by steps. Each new 

 step results from a new combination as compared with the old 

 one, and the new forms are thereby completely and sharply 

 separated from the species from which they have come. The 

 new species is all at once there ; it has arisen from the parent 

 form without visible preparation and without transitional steps. 



The mutation theory stands in sharp contrast to the selec- 

 tion theory. The latter uses as its starting-point the com- 





