THE THEORY OF MUTATION 145 



seen to be characteristic of half-races ; indeed, a half- 

 race might have been denned as a strain in which the 

 character of the complete race is usually latent, and 

 only rarely appears. An active character, on the other 

 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. Mutations may also be atavistic, 

 consisting in what is known as a " throw-back " to a 

 previous ancestor. In the most usual form of this 

 phenomenon an ancestral character which had pre- 

 viously become latent shows itself once more in the 

 active condition. 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 



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