Influence of Environment 21 



characteristics forms the rule, while the combined varia- 

 tion of several of them is the exception. It is true that 

 in most cases it cannot be decided whether the given at- 

 tribute is determined by a single hereditary character or 

 by a small group of them. 



On the other hand an accumulation of several varia- 

 tions in one race can easily be accomplished, and it occurs 

 quite commonly in cultures as well as in nature. But the 

 cases which were sufficiently well controlled and de- 

 scribed, usually show that the single variations have not 

 evolved simultaneously, but one after another, and this 

 is sufficient to prove their independence. 



Such an hereditary character, isolated from the rest, 

 can now become the object of experimental treatment. 

 Through suitable selection it may be gradually strength- 

 ened or weakened, and at the will of the breeder it may 

 be brought into a certain relation to the other unchanged 

 characters. The red color of the copper-beech has been 

 so much intensified that even the cell-sap in the living 

 cells of the wood became intensely red. The doubling of 

 flowers frequently leads to a complete disappearance of 

 the sexual organs. And in numerous instances only those 

 organs change which are subjected to selection while the 

 others remain unaffected by it. The adaptation of the 

 cultivated plants of agriculture to the needs of man, and 

 of the horticultural ones to his aesthetic sense, demon- 

 strates this to us in the clearest manner. 



Experimental treatment further leads to the study of 

 the influence of external circumstances on the unfolding 

 of hereditary characters. Here again these prove them- 

 selves to be factors of which each may vary independ- 

 ently from the others. Young varieties especially are 

 objects for study, and all those which have not as yet 



