270 LECTURE VI. 



assign no cause for the production of such 

 structures. Those who think that irrita- 

 biUty depends upon nerves, must do so by 

 a violation of their own principles, for they 

 must suppose the hydatid to have nerves, 

 though none can be demonstrated, even by 

 the aid of the microscope. Yet if they 

 choose to contend that the phasnomena of 

 irritability in detached parts which possess 

 nerves are maintained by their presence, it 

 is impossible to refute them, for we cannot 

 remove the nerves without destroying the 

 irritable substance. The arguments which 

 invalidate their supposition are, that irrita- 

 bility exists where we have no evidence of 

 nerves, and that it bears no proportion to 

 the magnitude of the nervous system. 



From the perplexities and obscurity of 

 other opinions, let me turn your attention 

 for a moment to contemplate the simplicity 

 and clearness of Mr. Hunter's Theory of 

 Life. In all its functions he recognized 

 an active principle, capable of producing 

 motions in matter that would not move 

 without it, and of controlling motions to 

 which, from other causes, such matter has a 



