136 WHAT EVOLUTION IS 



dominance. In this way particular 

 examples may show individual dif- 

 ferences and thus illustrate the extent 

 to which the general principles are 

 open to readjustment. 



De Vries, having discovered inde- 

 pendently much that was afterwards 

 found in the writings of Mendel and 

 having come to many of the same 

 conclusions that this writer had ar- 

 rived at, turned to the problem of 

 evolution in the hope that the new 

 ideas on heredity would be helpful in 

 understanding descent with modifi- 

 cation. In 1901 he published the first 

 general account of his mutation 

 theory. According to this theory, the 

 characters by which we distinguish 

 different plants and animals are made 

 up of units which are sharply sep- 

 arated from one another and are 

 without intergrades. They are repre- 

 sented by such features as black and 



