92 PLANT-BREEDING 



the prevailing belief of the most renowned agriculturists, who 

 considered the breeding of races as a slow process of gradual 

 improvement, he proposed the same slow and almost imper- 

 ceptible changes as the source of evolution in nature. Since 

 his time experience and theory have made very manifest 

 progress. Especially the principle of the unit-characters, 

 which is the foundation of the theory of the origin of species 

 by mutation, leads us to the acceptance of saltatory changes 

 or so-called sports as most probably Nature's way of pro- 

 ducing new forms. 



According to this theory species are not changed into one 

 another, but new forms arise laterally from the old stems. 

 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. 



On another occasion I have tried to show that in horti- 

 culture experience complies almost wholly with this concep- 

 tion, and the historical researches of Korschinsky give 

 proof of the accuracy of this conclusion. In agricultural 

 breeding-practice the production of new races is a more intri- 

 cate problem. In many cases their relation to the theoreti- 

 cal conceptions is quite clear, in others it is still surrounded 

 with doubt. In my book on the Mutation theory I have ex- 

 plained how the obvious facts agree with the idea, but it was 

 at that moment impossible to remove all doubts, and so I pro- 

 posed to return to these questions another time. (Mut. Th. 

 I, p. 82.) Five years have since elapsed and new discoveries 

 have been published which enable us to give a far more com- 

 plete analysis of the agricultural breeding processes. Espe- 

 cially at the Agricultural Experiment Station in Southern 

 Sweden quite unsuspected facts relating to the variability of 



