92 Selection Docs Not Lead to Origin of Species. 



their separation was brought about by selection in the 

 field. But that obviously does not settle the question 

 as to whether these pairs have arisen by a process of 

 gradual change or of sudden convulsion. These most 

 important results considered in relation to the subject 

 under discussion not only do not enable us to decide, for 

 this particular case, between the mutation and the selec- 

 tion theory, but they also leave undecided the question 

 whether (supposing the latter is true) the change was 

 completed in a few generations, or was attained in the 

 course of centuries. 



§ 10. ACCLIMATIZATION. 



Acclimatization is perhaps the best touchstone that 

 can be used for testing the efficacy of the doctrine of 

 selection. Selective breeding, which so often has to 

 overcome natural selection, in this case works in accord 

 with it. Moreover there is a strong agreement between 

 artificial and natural acclimatization, whether this latter 

 be the result of migrations of organisms or of essential 

 changes in their climatic or cecological environment. 



It is here therefore that we have most chance of find- 

 ing out how much natural selection is capable of doing. 



But the harvest turns out ver}' scanty — so scanty in- 

 deed that the upholders of the doctrine of selection are 

 loath to assign it a very prominent position among their 

 arguments. 



In practice the process of acclimatization is extra- 

 ordinarily complex. In most cases we are only concerned 

 with finding out whether such and such a species can 

 grow in a new locality or not. And we find that either 

 the difference between the old and the new localitv has 



