X Preface to the First Volume. 



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 explained by means of the results 

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

 simpler processes we may expect to arrive at an elucidation 

 of the laws to which the phenomena of hybridization con- 

 form, and ultimately be in a position to predict the result 

 in special cases. In this way the application of the theory 

 of mutation to the elucidation of the phenomena of hybridi- 

 zation will enable us to ascertain what conclusions relating 

 to the origin of species the study of these 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. 



