16 PLANT-BREEDING 



facts will be dealt with in our next chapter, and it may be 

 s ^cient, here, merely to have indicated them. 



The principle of mutation is conducive to the assump- 

 tion of distinct units in the characters of plants and animals. 

 Even as chemistry has reached its present high develop 

 ment chiefly through the assumption of atoms and mole- 

 cules as definite units, the qualities of which would be meas- 

 urable and could be expressed in figures, in the same way 

 systematic botany and the allied comparative studies are 

 in need of a basis for measurement and calculations. The 

 determination of the degree of affinity now largely depends 

 upon vague estimates and personal views; while, on the 

 basis of the theory of mutations, the relationship is meas- 

 ured by the number of the mutations which have made the 

 forms under consideration different from their common 

 ancestors. The mutations themselves have evidently oc- 

 curred in previous times and cannot be counted now. But 

 if it were possible to count their products, the characters, 

 the same aim could be reached. 



The study of these unit-characters may be undertaken 

 in three different ways: first, by the production of hybrids; 

 secondly, by the investigation of associated characters; and 

 in the third place, by the direct observation of mutations 

 producing such units. In hybrids the characters of the par- 

 ents may be combined in different ways, but the unit-charac- 

 ters connot be split or divided. This follows directly from 

 their definition. Thus the different combinations may lead 

 to the distinction of the constituents of the mixture. In my 

 third chapter I shall deal with this question on the ground of 

 the experiments of Luther Burbank. They afford a suffi- 

 cient source of evidence to discuss this question and are well 

 deserving of a separate treatment. On the other hand, their 

 methods and scientific results are the same as tHose of the 

 European horticultural breeders, as described elsewhere. 



