134 THE THEORY OF MUTATION 



hand, is apparent in the great majority of the indi- 

 viduals of a race. If, now, a change from latency to 

 activity occurs suddenly, this is a form of mutation. 

 The reverse case, too, may occur a character pre- 

 viously active may become latent ; the character then 

 appears to be lost, and the mutation is said to be 

 retrogressive. De Vries regards the great variety of 

 allied species which is to be found in many groups as 

 being to a large extent the result of retrogressive muta- 

 tion. This type of mutation is also frequent among 

 cultivated plants. Thus, the appearance of a white 

 variety of a species previously only known to produce 

 coloured flowers may constitute a good example of a 

 retrogressive change. Many of the phenomena some- 

 what vaguely described by de Vries as cases of latency 

 have now received a more precise interpretation in 

 terms of the interaction of invisible factors in perfectly 

 definite ways ; these are more fully described in the 

 chapters on Mendelism. Finally, new and distinct 

 types may arise by the intercrossing of separate 

 species, but this is not regarded by de Vries as being 

 an important source of permanent new forms. 



Without following de Vries into all the niceties of 

 his theory as to the particular kinds and methods of 

 mutations, we must admit that his experiments go 

 far to establish the doctrine, in support of which a 

 considerable amount of evidence had previously been 

 accumulated, especially by Bateson, that the origin of 

 species in Nature is generally a definite process, and 

 takes place by steps of considerable amplitude. What, 

 then, is the meaning of individual differences, of that 





