290 LT'TIIKR BURl^ ^ 



tional if a cumbination between natur } p< ( i. s 

 ninrircs beyond the first or second generation. 



ITbe struggle for existence is alwtys keen, and 

 the indiTidual organism that hu;ks erer 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 combination 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 condi- 

 tions of environment; that is to say, ready 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 successfully car- 

 ried, and so nature puts a sharp limitation upon 

 the extent to which the experiment may be 

 undertaken. 



AFFINmr FOUXDED ON COUSINSHIF 



And this is done by the simple procedure of 

 making it increasingly difficult for species to 

 interbreed in proportion as the species l)ecome 

 divergent in character. 



Tarweeds, for example, may interbreed among 

 themselves, and various species of mint may 

 similarly interbreed, but no species of tarweed 



