The Various Forms of Variability. 55 



up a new position of equilibrium. Little shocks make 

 it totter ; it oscillates round its position of equilibrium 

 and finally returns to it. A slightly stronger push how- 

 ever can make it go so far that it comes to lie on a new 

 side. The oscillations round a position of equilibrium 

 are the fluctuations, the transitions from one position 

 of equilibrium to another correspond to the mutations. 

 The track left behind by the rolling polyhedron can be 

 regarded as the line of descent of the species ; each sub- 

 division of this track, corresponding to a side of the 

 polyhedron, representing a particular elementary species ; 

 each transitional movement to a new position a muta- 

 tion. 



The more numerous one imagines the sides of such 

 a polyhedron to be, the smaller, of course, are the muta- 

 tions. But this illustration gives no insight into the 

 causes which effect the changes in position. 



The observation of many single variations has intro- 

 duced the view that mutations must always be consider- 

 able changes and especially that they should be greater 

 than variations. But this is by no means the case, and 

 it appears that many mutations are smaller than the 

 differences between extreme variants. This is imme- 

 diately clear if one compares e. g. Draha vcrna or TypJia 

 angitstifoUa and latifoUa. The single species of Draha 

 verna (discriminated by Jordan, De Bary, Rosen and 

 others), which have been shown by repeated sowing to 

 be constant, differ less from each other than extreme 

 variations in the same characters (form and size of the 

 leaves, petals, pods, etc. ) usually do in other plants ; as 

 can be seen best by comparing them with the partial 

 variations of the leaves of our trees, that is, with the 

 differences between the various leaves of one and the 



