INTRODUCTION. 3 
logical types of diseased tissues, and my earlier efforts 
were directed in searching among animals for the 
purpose of detecting in them the occurrence of tissues, 
which in man are only found under abnormal conditions. 
The search was of great value to me, for the statement 
proved to be true in only a limited sense; at the same 
time the truth of an opinion held by nearly all thoughtful 
physicians, that disease may in many instances be re- 
garded as exaggerated function, was forcibly illustrated, 
and I quickly saw that the manifestations of disease 
were regulated by the same laws which govern physio- 
logical processes in general, and that many conditions 
regarded as pathological in one animal are natural in 
another. It will be useful to illustrate this by some 
concrete examples. To take a simple case. The inside 
of our cheeks has a soft lining known as mucous mem- 
brane. In very rare instances children have been born 
with tufts of hair growing in this situation. Such a 
condition is truly abnormal. A physiological type for 
such a phenomenon is found in the mouths of rodent 
mammals ; the inside of the cheeks of rabbits, hares, 
porcupines, and the like, present naturally patches of 
hairy skin. Pigment is widely diffused in animal bodies, 
both under natural and unnatural conditions, using the 
term unnatural as equivalent to disease ; this explana- 
tion is necessary, for disease being controlled by 
natural conditions cannot logically be regarded as un- 
natural. 
In the dace (fig. 1) we notice sundry collections of 
black pigment dotted among the scales. When ex- 
amined critically the centre of each dot contains a white 
