2 -EVOLUTION 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 
otadrs 
and disease, either in relation with functional aberration 
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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. 
PSE Sey 
If it be difficult to define disease when our remarks 
are restricted to the human family, it becomes obviously 
pre eatin 
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 part passu with evolution of animal forms. 
For a long time it has been customary to talk of physio- 
