PLANT AFFINITIES 



The struggle for existence is always keen, and 

 the individual organism that lacks ever so little 

 of equaling its fellows in vitality and responsive- 

 ness to its environment must inevitably perish. 



Nevertheless, the experiment of producing 

 new forms through the hybridizing of old ones 

 is perpetually being made, and must continue to 

 be made, if existing forms are to remain plastic, 

 ready to take advantage of the changed conditions 

 of environment; ready, that is to say, to evolve 

 in future as they have evolved in the past. 



But there are limits beyond which this per- 

 petual experimentation with new nascent species 

 could not advantageously be carried, and so 

 nature puts a sharp limitation upon the extent to 

 which the experiment may be undertaken. 

 AFFINITY FOUNDED ON COUSINSHIP 



And this is done by the simple procedure of 

 making it increasingly difficult for species to 

 interbreed in proportion as the species become 

 divergent in character. 



Tarweeds, for example, may interbreed among 

 themselves, and various species of mint may 

 similarly interbreed, but no species of tarweed 

 could hybridize with a species of mint. One 

 member of the rose family may hybridize with 

 another blackberry with raspberry, let us say, 

 or quince with apple; and in the same way dif- 



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