y' 



150 DE VRIES— ELEMENTARY SPECIES IN AGRICULTURE. [April i8 



considered the breeding of races as a slow process of gradual 

 amelioration, he proposed the same slow and almost imperceptible 

 changes as the source of evolution in nature. Since his time experi- 

 ence and theory have made very manifest advances. Especially 

 the principle of the unit-characters, which is the basis of the theory 

 of origin of species by mutation, leads us to the acceptation of salta- 

 tory changes or so-called sports as the most probable way of nature 

 to produce new forms. According to this theory species are not 

 changed into one another, but new forms arise sideways from the 

 old ones. The whole strain continues unchanged and only produces 

 from time to time single aberrant individuals. These are the real 

 sources of all progress, and experience has shown that in the main 

 their new characters are hereditary, and that their progeny remains 

 true to their new types even from its first appearance. 



In agricultural breeding-practice the production of new races is 

 an intricate problem. In many cases their relation to the theoretical 

 conceptions is quite clear, in others it is still surrounded with doubt. 

 In my book on the mutation theory I have explained how the obvious 

 facts agree with that idea, but it was at that moment impossible to 

 remove all doubts and so I purposed to return to these questions 

 another time (Mut. Th., I, p. 82). Five years have since elapsed . 

 ( far more complete analysis of the agricultural breeding processes, 

 and new discoveries have been published which enable us to give a;:S> 

 Especially at the agricultural experiment station in southern Sweden 

 quite unsuspected facts relating to the variability of agricultural 

 crops have been brought to light. They are of a nature to throw over 

 all the old ideas concerning race amelioration and give proof that 

 the methods now generally in use in Europe are faulty as well from 

 a practical as from a scientific point of view. The director of that 

 station, Dr. Hjalmar Nilsson, has discovered that most of our 

 ordinary agricultural crops are not only composed of elementary 

 species, as was long known before him, but that each cultural 

 variety contains hundreds of sharply definite types. These are 

 widely distinct from one another as well in botanical characters as 

 in those properties which decide on their utility from the breeder's 

 point of view. Moreover, they differ so widely from one another 



