PREFACE 



Under the influence of the work of Nilsson, Burbank, 

 and others, the principle of selection has, of late, changed 

 its meaning in practice in the same sense in which it is chang- 

 ing its significance in science by the adoption of the theory 

 of an origin of species by means of sudden mutations. The 

 method of slow improvement of agricultural varieties by re- 

 peated selection is losing its reliability and is being supplant- 

 ed by the discovery of the high practical value of the ele- 

 mentary species, which may be isolated by a single choice. 

 The appreciation of this principle will, no doubt, soon change 

 the whole aspect of agricultural plant breeding. 



Hybridization is the scientific and arbitrary combination 

 of definite characters. It does not produce new unit-char- 

 acters; it is only the combination of such that are new. 

 From this point of view the results of Burbank and others 

 wholly agree with the theory of mutation, which is found- 

 ed on the principle of the unit-characters. 



This far-reaching agreement between science and prac- 

 tice is to become a basis for the further development of 

 practical breeding as well as of the doctrine of evolution. 

 To give proof of this assertion is the main aim of these Essays. 



Some of them have been made use of in the delivering of 

 lectures at the universities of California and of Chicago 

 during the summer of 1906 and of addresses before various 

 audiences during my visit to the United States on that oc- 

 casion. In one of them (II. D.), the main contents have 



