X Preface to the First Voliune. 



varies in the degree of its manifestation according to the 

 same laws as those to which the elements of the species, 

 already existing, conform. 



The properties of these units can be studied far more 

 conveniently by means of experiments in hybridization than 

 by merely observing or rather waiting for their origin. On 

 the basis of this principle the most complicated phenomena 

 of hybridization must be exj^lained by means of the results 

 of the simplest crosses. For, by a combination of these 



^_j,.xx v^i cpv^x^ics luc sLuuy oi tnese processes may 



warrant. 



A knowledge of the laws of mutation must sooner or 

 later lead to the possibility of inducing mutations at will 

 and so of originating perfectly new characters in animals 

 and plants. And just as the process of selection has en- 

 abled us to produce improved races, greater in value and in 

 beauty, so a control of the mutative process will, it is hoped, 

 place in our hands the power of originating permanently 

 improved species of animals and plants. 



Hugo de Vries. 

 Amsterdam, August 1901. 



