Variation and Heredity 289 



Many other Linnaean species are in this respect like Draba 

 vcrna, and most varieties, De Vries thinks, are really element- 

 ary species. 



Second, the polymorphism due to intercrossing is the 

 outcome of different combinations of hereditary qualities. 

 There are here, De Vries says, two important classes of facts 

 to be kept strictly apart, — scientific experiment, and the 

 results of the gardener and of the cultivator. The experi- 

 menter chooses for crossing, species as little variable as pos- 

 sible ; the gardener and cultivator on the other hand prefer 

 to cross forms of which one at least is variable, because the 

 variations may be transmitted to the hybrid, and in this way 

 a new form be produced. 



New elementary characters arise in experiments in crossing 

 only through variability, not through crossing itself. 



Third, variability in the ordinary sense, that is, individual 

 variability, includes those differences between the individual 

 organs that follow Quetelet's theory of chance. This kind 

 of variability is characterized by its presence at all times, in 

 all groups of individuals. 



De Vries recalls Galton's apt comparison between variability 

 and a polyhedron which can roll from one face to another. 

 When it comes to rest on any particular face, it is in stable 

 equilibrium. Small vibrations or disturbances may make it 

 oscillate, but it returns always to the same face. These 

 oscillations are like the fluctuating variations. A greater 

 disturbance may cause the polyhedron to roll over on to a 

 new face, where it comes to rest again, only showing the 

 ever present fluctuations around its new centre. The new 

 position corresponds to a mutation. It may appear from our 

 familiarity with the great changes that we associate with the 

 idea of discontinuous variability, that a mutation must also 

 involve a considerable change. Such, however, De Vries says, 

 is not the case. In fact, numerous mutations are smaller 

 than the extremes of fluctuating variation. For example, the 



