THE THEORY OF MUTATION 153 



Bioadly speaking, species arise by mutation, by a 

 sudden step in which either a single character or a 

 whole set of characters together become changed. In 

 the former case a new variety in the strict sense of the 

 word is the result ; in the latter a new species (accord- 

 ing to Jordan's definition) is produced. 



But mutation may be of several kinds. In the first 

 place, an entirely new character or set of characters 

 may make its appearance. To such a phenomenon 

 de Vries applies the term of progressive mutation, and 

 it is by steps of this kind that he believes the main 

 divisions of the vegetable kingdom to have been built 

 up. In the case of such mutations the new character 

 is supposed to come into existence first in a latent or 

 hidden condition, and it may be only after many 

 generations that it makes its appearance visibly. On 

 this view the period of mutation is preceded by a 

 premutation period, during which the appearance of 

 the new character is being prepared for. 



A second method of species formation, entitled 

 by de Vries degressive mutation, is indicated when a 

 change takes place in the partial latency of a character. 

 A completely latent character is, indeed, unrecognisable 

 as such. But characters may also be only partially 

 laterit, and in these cases they exhibit themselves from 

 time to time in rare individuals in the form of sports 

 or abnormalities a phenomenon which we have already 

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

 race might have been defined as a strain in which the 

 character of the complete race is usually latent, and 

 only rarely appears. An active character, on the other 



