XI. THE SUMMARY OF PART III 217 



clinging to the habit of cumbrous host change which was indispensable 

 for the rejuvenescence at their viral stage but which has lost its 

 essential significance now that the organisms have acquired the ability 

 to perform the sexual reproduction, the better method for the rejuve- 

 nation. This shows how difficult is for the organisms the abandon- 

 ment of their long abiding habitat. 



