62 THE THEORIES OF EVOLUTION 



in the animal kingdom: atrophied eyes of ani- 

 mals living under ground, hindlegs of cetaceans, the 

 vermiform appendix and caudal vertebrae of man, 

 etc. Those organs are vestiges of more developed 

 organs which were once useful to the ancestors of the 

 animal and have now become useless or harmful. 

 How did they become attenuated? Cases may dif- 

 \ fer. If a once useful organ, for instance, should be- 



^ come harmful, it could be attenuated through a 

 reversion of the regular processes of natural selection : 

 those individuals alone in which that organ would be 

 least developed would survive and transmit that 

 character to their offspring. When, on the other 

 hand, the organ is merely useless and its disappearance 

 does not present serious advantages, we must find 

 some other explanation for its atrophy. 



The explanation given by Weismann is a cessation 

 of selection as far as the organ is concerned. Not only 



^does selection develop an organ, but it maintains it at 

 the height of its development. When its action 

 ceases, the animals that possess the organ as well as 

 those that do not possess it, have an equal chance to 

 survive and to beget offspring. Hence the word 

 panmixia. The average development of the organ 

 is lower in every new generation until the organ be- 

 comes atrophied or disappears completely. 



The theory of panmixia was the first subsidiary 

 theory which Weismann formulated in order to ap- 



