58 EXPERIMENTAL RESEARCHES 



fowls. Therefore, that part of the receptors 

 which deals with the nutritive substances in 

 the fowl's organism is not in use during the 

 passage through the pigeon. In view of their 

 great instability it is possible that this portion 

 of the receptors may become atrophied, whilst 

 the receptors specific for pigeon's substances 

 become correspondingly increased. Thus the 

 micro-organism would lose its power of assimi- 

 lating certain substances of the fowl's organism. 

 If such a parasite were transferred back to the 

 fowl supposing one of the specific constituents 

 of fowls to be necessary for its proliferation it 

 would no more be able to grow in the fowl. 



This is, therefore, a case of the loss of certain 

 receptors which are absolutely necessary for 

 nutrition. That such a change, having been 

 once acquired, should be permanently trans- 

 mitted from one generation to another, is 

 hardly surprising in view of my recent obser- 

 vations on the modifications of arsenic-resistant 

 trypanosomata. Obviously, the total loss of the 

 chemical structures in question must render 

 such modifications irreparable. Such charac- 

 teristic instances as those in which a single pas- 

 sage produces a permanent loss of virulence for 

 a certain animal species, are not very frequent. 



