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 
latent, 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 havealready 
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 
