PLAXT AFFINITIES 219 



because they are not an improvement, from the 

 standpoint of adaptation to their environment, 

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

 posed 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 ad- 

 verse 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. 

 Favorable 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 

 environment. 



It is always improbable, then, in the nature 

 of the case, that any new intermediate form, such 

 as would arise from the combination of two al- 

 lied 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 excep- 



