The Origin of Species 315 



genera Hieracium and Rubus have been split up 

 into very many species by some botanists, while 

 others reduce them to a small fraction of these, re- 

 garding the differences as not sufficiently definite or 

 constant to warrant raising these varying forms to 

 specific rank. 



In America, at the present time, there is a strong 

 tendency to treat some of our native genera in 

 much the same manner. The Thorns (Cratsegus) 

 offer a notorious example of this. In the latest 

 edition of " Gray's Manual " sixty-five species are 

 described, while in the edition of 1868 but nine 

 species were recognized. Unfortunately it is im- 

 possible to trace the pedigree of any of these forms, 

 and whether they have arisen by the accumulation 

 of small differences, or whether they are the results 

 of discontinuous variations or mutations, we are not 

 in a position to say. 



Artificial Selection. Perhaps the strongest evi- 

 dence offered by plants for the truth of the -Dar- 

 winian theory of natural selection, is the origin of 

 new forms under cultivation by selection alone. In 

 this way, for instance, the amount of sugar in the 

 root of the sugar beet has been doubled, and the 

 percentage of quinine in the bark of certain varie- 

 ties of Cinchona has been very largely augmented. 

 Many varieties of flowers and vegetables have also 

 resulted from pure selection. Among Burbank's 

 many interesting results is one which illustrates the 

 origin of a new form by selection alone. From 



