Double Adaptations 435 



out any relation to adaptation. This however 

 is partly due to our lack of knowledge, 

 and partly to the general rule that in nature 

 only such sports as are useful are spared by 

 natural selection, and what is useful we ordi- 

 narily term adaptive. 



Another side of the question remains to be 

 considered. The word variety, as is now be- 

 coming generally recognized, has no special 

 meaning whatever. But here it is assumed in 

 the clearly defined sense of a systematic va- 

 riety, which includes all subdivisions of spe- 

 cies. Such subdivisions may be, from a 

 biological point of view, elementary species and 

 also be eversporting varieties. They may be 

 retrograde varieties, and the two alternating 

 types may be described as separate varieties. 



It is readily granted that many writers 

 would not willingly accept this conclusion. But 

 it is simply impossible to avoid it. The two 

 forms of the water-persicaria must remain 

 varieties, though they are only types of the dif- 

 ferent branches of a single plant. 



If not, hundreds and perhaps thousands of 

 analogous cases are at once exposed to doubt, 

 and the whole conception of systematic varie- 

 ties would have to be thrown over. Biologists 

 of course would have no objection to this, but 

 the student of the flora of any given country 



