96 HEREDITY AND VARIATION 



cases like the striped larkspur, which for centuries has gone on 

 producing unstriped as well as striped flowers. " Its changes 

 are limited to a rather narrow circle, and this circle is as constant 

 as the peculiarities of any other constant species or variety. But 

 within this circle it is always changing, from small stripes to 

 broad streaks, and from them to pure colours. Here the vari- 

 ability is a thing of absolute constancy, while the constancy 

 consists in eternal changes ! " Plants with variegated leaves, 

 with double flowers, with fasciated branches, with peloric flowers, 

 and so on, often illustrate the " ever-sporting " tendency. The 

 common snapdragon {Antirrhinum majus) is a very good case, — 

 the striped variety, for instance, cannot be fixed. There is some 

 inherent instability in the combination of unit-characters in 

 these ever-sporting varieties. 



Fluctuations. — De Vries applies this term to the continually 

 occurring individual variations. "It is normal for organisms 

 to fluctuate to and fro, oscillating around an average type. 

 Fluctuations are linear, amplifying or lessening the existing 

 qualities, but not really changing their nature. They are not 

 observed to produce anything quite new ; they always oscillate 

 around an average, and if removed from this for a time, they 

 show a tendency to return to it." They are inadequate ever to 

 make a single step along the great lines of evolution, whether 

 progressively or retrogressively. They do not form the raw 

 material of evolution, as has often been supposed. But, we 

 submit, it is difficult with our present knowledge to discriminate 

 between a fairly large fluctuation and a small mutation. 



Mutations. — -"In contrast to the ever-recurring variability, 

 never absent in any large group of individuals, and determining 

 the differences which are always to be seen between parents 

 and their children, or between the children themselves, we 

 have to rank the so-called sports or single varieties, not rarely 

 denominated spontaneous variations, for which I propose to 

 use the term ' mutations.' They are of very rare occurrence. 



