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REVIEWS. 
Draba verna. Linnean species arise, as Darwin taught, by the 
extermination of intermediate sub-species. The problem therefore 
is, how do Jordan’s species arise? The answer previously given 
to this question was that they arose by natural selection acting upon 
continuous variation ; and the principal evidence for this view was 
that analogous forms arise in cultivation owing to selection by man. 
Before giving his own view as to how species ( i.e ., Jordan’s species) 
afise, De Vries proceeds to demolish this opinion. The only new 
forms actually produced by man’s selection, e.g ., improved races of 
cereals, of sugar beet, and other plants, are not analogous to new 
species, inasmuch as when selection ceases the improved condition 
of the characters selected dies away and there is regression to the 
parental type. 
The varieties of the gardener, on the other hand, are analogous 
to Jordan’s species; their tendency to regress is not to the original 
form, but to a type of their own. But garden varieties are not 
produced by selection. All that selection can do is to separate one of 
two or more forms already existing together and afterwards to 
improve it beyond its own type or modal value. The important 
process is the isolation of the new race. 
What then is the meaning of individual differences, of continuous 
variability ? De Vries points out that for no two plants are the 
conditions of life exactly alike ; a considerable diversity among the 
plants themselves is therefore advantageous. Upon continuous 
variability depend local races, forms adapted to wetter and drier 
situations, highland and lowland races, and the like, but none of these 
are permanent. Beyond the effect of sexual reproduction, which 
combines the tendency to vary of two parents, De Vries believes that 
individual variability depends entirely upon nutrition, under which 
head may be included practically the whole environment of the plant— 
light, space, soil, moisture, and the like. Characters acquired in a 
similar way by previous generations are inherited, and the effect, 
of conditions upon the developing seed while still contained in the 
parent plant may be considerable. The puzzling question of the 
inheritance or non-inheritance of acquired characters is thus disposed 
of : acquired characters are inherited ; they are not of any importance 
in the origin of species. 
Some account must now be given of the way in which, according to 
De Vries, species do arise. Broadly speaking, they 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 
Jordan’s species 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. This is the progressive method of formation of 
new species ; and it is by a series of steps of this kind that De Vries 
