Theory of Change of Function 



to use the words of Milnes Marshall, " a closed 

 sac lying just underneath the vertebral column. 

 In many fish it acquires a connection by a duct 

 with some part of the alimentary canal. It then 

 becomes an accessory breathing organ, especially 

 in those fish which are capable of living out of 

 water for a time, e.g. the Protopterus of America. 

 An interesting series of modifications exists con- 

 necting the air-bladder with the lung of the 

 higher vertebrates, which is undoubtedly the 

 same organ." 



This theory, however, does not seem adequate 

 to explain the origin of all organs. It does 

 not explain, for example, how limbs developed 

 in a limbless organism. Wallace tried to 

 avoid the difficulty by asserting that it is un- 

 reasonable to ask a new theory that it shall 

 reveal to us exactly what took place in remote 

 geological ages and how it took place. To this 

 the obvious reply is, firstly, that we ought not 

 to give unqualified acceptance to any theory of 

 evolution until it does afford us such explana- 

 tions, and, secondly, that the theory of the origin 

 of species by means of natural selection is no 

 longer a new r one. 



Latterly, however, Wallace appears to have 

 given up all hope of being able to account for 

 the origin of new organs by means of natural 

 selection, for he states on page 431 of the issue 

 of the Fortnightly Review for March 1909 : 



37 



