THE ORIGIN AND THE EXTINCTION OF SPECIES 363 



Similar examples might be cited in regard to the Alpine-Arctic 

 bntterflies. 



It is intelligible enough that we are still very far from l)eing able 

 to give a precise account of the main changes in the plant and animal 

 world during the history of the earth in regard to the special causes 

 which have produced them. Possibly the future will throw more 

 light upon this subject by extending our knowledge of the fossil 

 remains of all countries. But so much at least we can say at present, 

 that there is no reason to refer the djdng out of the earlier forms 

 to anything else than the changes in the conditions of life, the 

 struggle for existence, and the limitation of the power of transformation 

 and adaptation due to the organization which the species has already 

 attained ; there is no trace of any such thing as a phyletic principh- 

 of life in the vitalistic sense, as far as the decadence of species is 

 concerned. 



