IMPROVEMENT OF PLANTS AND ANIMALS 9 



because of their development for just such conditions, 

 are able to live. Possibly, in succeeding centuries we may 

 have more of our native plants added to the list of "worst 

 weeds." 



12. De Candolle's Law. By the process of natural selec- 

 tion, plants have become adapted to the climate in which 

 they live, and have thereby become ill adapted to climates 

 farther north or south. Seed of box elder (Acer Negundo) 

 grown at St. Louis is not hardy in northern Iowa, although 

 no botanical differences are observable. The American 

 plum (Prunus Americana) is hardy in Nebraska; but 

 when these trees are taken to Texas they winter-kill, 

 because they start too early in the spring. Red cedars 

 (Juniperus communis) grow from North Dakota to 

 Tennessee; but when seed from either region is taken to 

 the other the trees produced are not hardy. De Candolle, 

 who made a careful study of the matter, concluded that 

 native forms are not hardy when taken one hundred 

 miles north or south of their source. 



This adaptation to climate deserves more attention 

 than is often given to it. Northern varieties of apples, 

 grapes, peaches, oats, and corn, are not usually adapted 

 to southern conditions, nor are the southern varieties 

 usually desirable in the North. The Baldwin apple, which 

 constitutes the greater part of the New York apple or- 

 chards, is not profitable in Delaware. The Ben Davis, 

 which is the most important apple in Missouri, is not 

 desirable $o far north as New York. Alfalfa seed from 

 southern Europe is not hardy in northern United States, 

 but much of this seed is sold in New York. Corn does 

 not mature properly when seed is obtained from a hun- 



