PRACTICAL POLLINATION 241 



sive precisely because of their independence. 

 The method of cross-fertiHzation that they have 

 adopted does indeed enable some of them to 

 blend the strains of different individual plants; 

 but in every instance the parents must be grow- 

 ing in the immediate vicinity of each other. 



Except by the accidental and most unusual 

 transfer of a plant through the agency of a 

 passing animal, there is hardly the remotest 

 chance of effecting cross-fertilization between 

 individual mosses or lichens or ferns growing in 

 widely separated regions. 



But we have already seen that it is precisely 

 this blending of traits brought from parents 

 growing imder different environing conditions 

 that is chiefly responsible for making plants vary 

 and furnishing the materials for evolutionary 

 progress. So it goes without saying that the 

 plants that are restricted in the choice of possible 

 mates to individuals growing under the same 

 conditions to which they themselves are sub- 

 jected, cannot expect to change rapidly and 

 therefore do not evolve in any such ratio as plants 

 having the other habit. 



And in point of fact we iBnd that the plants 

 that retain this primitive custom of fertilization 

 with the aid of motile germ cells, acting quite 

 independently of insect or wind, are plants of a 



