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 Linnaeus, we call species, 

 are according to De Vries collective groups, which are the 

 outcome of mutations. Many such Linnsean 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. 



