THE THEORY OF CHANGE OF FUNCTION 59 



are accustomed to rely, fails utterly here. The lung 

 develops as a pit-like depression in the floor of the 

 oesophagus. Now this could not have been the 

 earliest origin of the lung, for it would be utterly 

 useless as such for the simple reason that food 

 would always be falling into it. 



In developing the theory of Change of Function 

 Dr. Dohrn points out that it is a very common 

 thing, if not the general rule, for an organ to have 

 not one function alone but a number of different 

 functions to perform, of which functions one at any 

 given time is predominant and the rest subordinate ; 

 but that it is quite conceivable that some slight 

 change of circumstance might cause the relative 

 importance to be changed, and one of the subor- 

 dinate functions to become the primary one, or to 

 put it in his own words : " Each function is a 

 resultant of several components of which one is 

 the principal or primary function, the other 

 secondary. Diminution of the primary function 

 and increase of a secondary function alters the 

 total function ; the secondary gradually becomes 

 the primary, the total function is changed and the 

 issue of the whole process is the transformation of 

 the organ." 



For instance, the primary function of the stomach 

 is undoubtedly the secretion of gastric juice ; a 

 secondary function is the movement of its muscular 

 walls aiding the action of the gastric juice by 

 bringing the contents into closer contact with 

 it. Now in no animal are the glands abso- 

 lutely uniformly dispersed over the stomach walls, 



