The Various Forms of Variability. 53 



4. Spontaneous changes. We have long been familiar 

 in practical horticulture with the phenomenon of the sud- 

 den and unexpected appearance of varieties from time 

 to time. Darwin calls these sudden transitions single 

 variations. 



The finest examples are the so-called bud variations. 

 The new form arises as a bud or twig on an individual 

 of the old form and often remains a long time united 

 with it. In a case like this there can be no doubt as to the 

 mutual genetic relationship, and the fact that the transi- 

 tion is discontinuous is at once evident. But even in this 

 sphere there is great uncertainty because bud variations 

 are often born by hybrids and the hybrid nature of an 

 individual is sometimes even betrayed only by such varia- 

 tions. Moreover bud variations are very common on 

 varieties with incompletely fixed (mixed) characters, as 

 in many forms with striped flowers (Antirrhinum, Del- 

 phinium, Aquilegia, Dahlia, Fig. 14, etc.) 



5. On the Magnitude of Mutations. We often hear 

 of spontaneous changes described as sports or as sport- 

 like variations. This term is not a happy one. Natura 

 non facit saltus, said Linnaeus. But we are not told 

 what we are to regard as a jump. There is much more 

 point in describing the individual transitions as jerks 

 and to speak of jerky variability.^ The jerks may only 

 induce quite small changes, but each jerk represents a 

 distinct unit. 



Galton has illustrated the difference between jerk- 

 ing and ordinary variability in a very beautiful way. 

 Imagine a polyhedron which can roll on a flat surface.^ 

 Every time that it comes to rest on a fresh side it takes 



^ "Variation par secousses" of some French writers. 

 F. Galton, Hereditary Genius, 1869, p. 369. 



