154 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 
