LUTHER BURBANK 



on the parent forms; and (2) that limits are 

 imposed by the relative lack of affinity of one 

 species for another. 



As to the first point, it must be recalled that 

 each existing species has been produced only after 

 long generations of struggling against adverse 

 conditions. Constantly there is a tendency to 

 variation within certain limits even in the case 

 of the most fixed species. Such variations con- 

 stitute tests of the fitness of the species to live 

 in the environment in which it finds itself. Favor- 

 able variations are preserved by natural selection, 

 simply because they have the capacity to outgrow 

 the original form, or outlast it in times of drought 

 or other hardship. 



And so every existing wild species proves by 

 the very fact of its existence that it has a large 

 measure of adaptability to the existing envir- 

 onment. 



It is always improbable, then, in the nature 

 of the case, that any new intermediate form such 

 as would arise from hybridizing two allied species, 

 will be better adapted to survive than the parent 

 form. Such cases do arise, else we should have 

 no new species, but in general the rule holds. So 

 we may fairly count it exceptional if a hybrid 

 between natural species survives beyond the first 

 or second generation. 



[38], 



