2 E VOL UTION AND DISEASE. 



in the course of a criminal trial to ask a medical witness, 

 when the plea of insanity is urged on a prisoner's behalf, 

 either to define insanity, or to state his opinion where 

 sanity ends and insanity begins. The judge knows full 

 well the difficulty, indeed the impossibility of even a 

 skilled witness making a satisfactory reply to such a 

 question. 



As with mental so with bodily conditions, it is im- 

 possible to state definitely the borderland between health 

 and disease, either in relation with functional aberration 

 or textural alteration. And in many instances we shall 

 find conditions which we regard as abnormal in man, 

 presenting themselves as normal states in other animals. 



If it be difficult to define disease when our remarks 

 are restricted to the human family, it becomes obviously 

 more difficult when we attempt to investigate disease on 

 a broad zoological basis. As the great barrier which 

 exists between man and those members of his class most 

 closely allied to him consists, not in structural characters, 

 but in mental power, it necessarily follows that there 

 should be a similarity in the structural alterations in- 

 duced by diseased conditions in all kinds of animals, 

 allowing, of course, for the differences in environment. 

 This we now know to be the case, and it is clear that as 

 there has been a gradual evolution of complex from 

 simple organisms, it necessarily follows that the principles 

 of evolution ought to apply to diseased conditions if 

 they hold good for the normal, or healthy, states of 

 organisms : in plain words there has been an evolution 

 of disease pari passu with evolution of animal forms. 

 For a long time it has been customary to talk of physio- 



