352 PLANT-BREEDING 



may sketch the development of the spurs of the orchids in 

 connection with the length of the proboscis of bees and 

 butterflies, but we cannot directly observe the changes which, 

 we assume, are brought about by such influences. In all those 

 cases it is equally possible, and in some even probable, that 

 they have not been originated in the way in which the 

 plants are now using them. The higher the degree of 

 differentiation, the more probable our mode of explanation 

 may be, but in the more simple and ordinary cases, includ- 

 ing the desert plants and many similar instances, the environ- 

 ment has only selected the suitable forms from among the 

 throng, and has no relation whatever to their origin. 



Present distribution is the effect of migration, and 

 migration is governed and directed by the given characters 

 of the species. It produces the intimate relationship of the 

 organisms to their environment, to climate and soil as well 

 as to all their vegetable and animal competitors. But in 

 this the qualities of the -organisms are the causes, and the 

 distribution is the effect. 



