288 Evolution and Adaptation 
mon form of variability known as individual or fluctuating 
variation ; but according to the mutation theory there are two 
kinds of variation that are entirely different from each other. 
“The fluctuating variation can, as I hope to show, not over- 
step the bounds of the species, even after the most prolonged 
selection, — much less can this kind of variation lead to the 
production of new, constant characters.” Each peculiarity 
of the organism has arisen from a preceding one, not 
through the common form of variation, but through a sudden 
change that may be quite small but is perfectly definite. 
This kind of variability that produces new species, De Vries 
calls mutability ; the change itself he calls a mutation. The 
best-known examples of mutations are those which Darwin 
called “single variations” or “sports.” 
De Vries recognizes the following kinds of variation : — 
First, the polymorphic forms of the systematists. The 
ordinary groups which, following Linnzeus, we call species, 
are according to De Vries collective groups, which are the 
outcome of mutations. Many such Linnzean species include 
small series of related forms, and sometimes even large num- 
bers of such forms. These are as distinctly and completely 
separated from each other as are the best species. Generally 
these small groups are called varieties, or subspecies, — va- 
rieties when they are separated by a single striking char- 
acter, subspecies when they differ in the totality of their 
characters, in the so-called habitus. 
These groups have already been recognized by some 
investigators as elementary species, and have been given cor- 
responding binary names. Thus there are recognized two 
hundred elementary species of the form formerly called Draba 
verna. 
When brought under cultivation these elementary species 
are constant in character and transmit their peculiarities 
truly. They are not local races in the sense that they are 
the outcome in each generation of special external conditions. 
