1 66 THE DARWINIAN THEORY 



stages be useless for the purpose which it ultimately 

 fulfils. 



This very important objection applies to a great 

 number of cases, of which the origin of the wing is 

 a typical one. 



2. The Origin of the Lung is another example of 

 the difficulty we are considering. The lung develops 

 as a small saccular outgrowth from the throat, and 

 it is quite unintelligible that a slight depression at 

 the back of the mouth should have been preserved 

 because it was useful for breathing air directly. 



The explanations in these cases are good in- 

 stances of many which are afforded by an important 

 theory. 



The Theory of Change of Function. — This 



theory was suggested by Darwin, and afterwards 



developed more fully by Dohrn, as affording a 



possible solution of difficulties such as those we are 



considering. The principle is, that an organ may 



[lose its original function, and yet persist because it 



lis useful for another purpose — i.e., that an organ 



/ may be used for two or more different purposes, one 



\ predominating at one time, another at another time ; 



and, further, that structural modifications may ensue 



fitting the organ better for its adopted function. 



This theory offers an explanation of the first 

 commencement of the lung, which is shown to have 

 arisen from the swimming bladder of fishes through 

 change of function, and a series of forms is known 

 to exist connecting the air bladder of fishes with the 

 lung of the higher Vertebrates, which is undoubtedly 

 the same organ. 



