Mutation-Periods 261 



pages I have always taken for granted that the species 

 and varieties are in their ordinary and unchanging state. 

 But this is by no means always the case. The origination 

 of new species and varieties demands that their immutabil- 

 ity should not be absolute, or at least should be suspended 

 from time to time. Experience confirms this by showing 

 that there are periods in the life of species, during which 

 they are, so to speak, especially inclined to produce new 

 types. At that time they produce the new varieties and 

 species, not only once but repeatedly, and not only a single 

 one, but frequently a considerable number. Genera rich 

 in species, such as the pansies and the rock-roses,'^ are the 

 remains of such periods of variability, and everywhere in 

 nature we meet with similar ones. In garden-plants we 

 see, from time to time, periods during which certain 

 varieties occur by preference, as the double dahlia of 

 about the middle of the last century, the forms of toma- 

 toes in recent decades, and numerous other instances 

 teach us. On its first appearance the gardeners call the 

 new form a conquest, the later appearances are only repe- 

 titions, and are therefore of only very secondary practical 

 value. 



The power of reproducing one or more new species 

 indicates a condition of unstable equilibrium of the given 

 internal units. In the nuclei the new characteristic is al- 

 ready invisibly present, but inactive.' Certain causes, un- 

 known to us, can transform this into a permanent condi- 

 tion. This state of unstable equilibrium may be main- 

 tained in the great majority of individuals, through a 

 series of generations, as is the case with my Oenotheras. 

 But from time to time, sometimes in individual cases 

 every year, there is a shock, and the equilibrium becomes 



"^Sonnenrdschen (H elianthemum) . Tr. 



