72 HEREDITY AND EVOLUTION IN PLANTS 



quality, which bred true for resistance. The water- 

 melon, in the southern states, is subject to a very de- 

 structive disease which causes a wilting of the vines and 

 consequent loss of fruit. By crossing the ordinary non- 

 resistant watermelon with the closely related common 

 citron, which is wilt resistant, W, A. Orton, of the United 

 States Department of Agriculture, produced a water- 

 melon resistant to this disease. Numerous other illus- 

 trations might be given. This is becoming one of the 

 common and successful methods of combating plant 

 disease. 



63. Unsolved Problems. — ^Like all truly great con^ 

 tributions to science, Mendel's discoveries have raised 

 more questions than they have answered. Therein hes, 

 in part, their great value. So, also, the most important 

 effect of Darwin's work was that it set men to asking 

 questions. The history of botany, as of all natural science 

 since 1859, is chiefly the attempts of men to answer the 

 questions raised by Darwin, or stimulated in their own 

 minds by his books. So with Mendel and de Vries; 

 biological science, since igoo, has been largely occupied 

 in trying to answer the questions raised by these men. 



What are these questions? There is not space here 

 even to ask them all, much less to endeavor to answer 

 them even briefly; but they include the following large 

 problems : 



I. Are acquired characters inherited? In other words, 

 do characteristics acquired after birth by the body or 

 mind of the parent, either by its own activity or as a re- 

 sult of the immediate effects of environment, influence 

 the germ-cells so as to alter the inheritance which they 

 transmit? Some say yes, others say no; others say, only 



