INTRO D UCTION. 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 sdarch 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. i) we notice sundry collections of 

 black pigment dotted among the scales. When ex- 

 amined critically the centre of each dot contains a white 



