The Various Forms of Variability. 45 



3. The differences, in individuals and organs, which 

 follow Ouetelet's law. 



4. The so-called spontaneous variations. 



The special problem which the mutation theory seeks 

 to explain is the manifold diversity of specific forms; 

 spontaneous variations are the facts on which this ex- 

 planation is based. The truth of this explanation will then 

 be tested by its application to hybrids; and, if possible, 

 proved. Individual variability however will be shown 

 to be of only secondary importance. 



It will be convenient to deal with these groups one 

 by one. 



1. Systematic Polyinorphism and its Supposed Causes. 



Linnean species are aggregate species. They include 

 sometimes a small but often a large series of forms which 

 are as sharply and completely distinguished from one 

 another as are the best species. These lower-rank forms 

 are usually called varieties or subspecies; varieties, if 

 they are characterized by a single striking character, but 

 subspecies if they are distinguishable by the sum of their 

 characters, by their so-called habit. But on this point 

 there is a great diversity of opinion. Some authors re- 

 gard all these special forms as elementary species and 

 consequently give them double names, thereby breaking 

 up the Linnean species. It is well known that in this 

 way Draha verna^ and Viola tricolor- and many other 

 old species have been broken up (in the case of Draha 

 verna into 200) smaller groups of perfectly distinct and 

 usually local elementary species. By experiment and 

 culture these forms prove constant, they do not change 

 into one another, nor do they reproduce the typical or 

 general form of the species. The majority of varieties 



* See Fig. 3 on page 22. ' See Fig. 4 on page 2;^. 



