58 THE THEORY OF CHANGE OF FUNCTION 



by pointing out that an organ which had already 

 attained considerable size and complexity might, 

 owing to some change in the habits or surroundings 

 of the animal possessing it, have not its structure 

 but its function changed i.e., that an organ already 

 in use for one purpose might become employed for 

 some other purpose, might gradually lose its original 

 function and undergo further modifications fitting it 

 better for its adopted function. 



This idea, which clearly affords us a possible 

 mode of getting over the difficulty, is only briefly 

 alluded to by Darwin, but was taken up and 

 developed by Dr. Dohrn who, in a pamphlet 

 published in 1875, discussed it at considerable 

 length. 



A very good example of the principle in question 

 is afforded by the swimming bladder of fishes. 

 This, in most fish, is 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 Africa. An interesting 

 series of modifications exists connecting the air 

 bladder with the lung of the higher Vertebrates, 

 which is undoubtedly the same organ. The air 

 bladder of fishes is in fact a very good example of 

 change of function, inasmuch as it is used originally 

 for purposes of flotation and afterwards it is pre- 

 served as a lung. This example is instructive also 

 because the evidence of embryology, on which we 



