HEREDITY, AND THE ORIGIN OF SPECIES. 19 
sudden appearance of sports and forms of the aspect of 
species which fully support all of the conclusions drawn 
from the observations on the evening-primroses. An ex- 
amination of the facts easily brought together allows us 
to see that certain general principles in the organization 
of the plant, and in its behavior in these breaks or salta- 
tions in heredity may be made out. 
The first and most important of these is one which was 
advanced by De Vries speculatively, before he began his 
experiments in heredity, namely that the plant is essen- 
tially a complex group of indivisible unit - characters. 
These unit-characters may not always be expressed, or 
recognizable in external anatomical characters, since they 
may be in a latent condition, or totally inactive, or ex- 
ternal taxonomic characters may really consist of several 
elementary qualities, but these are not shown in any inter- 
mediate stage although they may be modified within the 
limits of fluctuating variability. 
Any plant, supposedly, includes thousands of unit-char- 
acters, and as they are essentially qualities, or capacities, 
they do not usually coincide with the characters ordinarily 
used in taxonomic descriptions. As an illustration, the 
phases of geotropic sensibility of an organ may be con- 
sidered as a unit-character. Thus a branch is either apo- 
geotropic, directing its tip directly upward, or it may be 
diageotropic placing its axis in a horizontal plane, at right 
angles to the action of gravitation, or it may undergo a 
mutation and “weep” or direct its tips directly downward. 
In any case, however, it possesses one of these three forms 
of reaction. It does not follow, however, that all branches 
are actually in one of these three positions, for other forces 
to which it reacts may operate to place the axes in various 
Planes, and the position of the branch may express just 
such concurrence of elementary characters as alluded to 
above, or indeed the geotropic unit-character may be latent, 
