268 E VOL UTION AND DISEASE. 



scopical characters agreeing with those formed in seba- 

 ceous cysts, may arise from the transformation of warts, 

 and so closely do the two forms mimic each other that 

 it is not easy to distinguish between them. Thus to 

 all outward appearance the horn of the mouse in fig. 

 132 is identical in its structure and naked-eye characters 

 to that growing from the back of the mouse ; but I 



FIG. 132. The head of a Mouse with a cutaneous horn 

 arising from a wart. (Mus. Royal College of Sur- 

 geons. ) 



have had ample opportunity of demonstrating its origin 

 in a wart. These specimens illustrate how necessary it 

 is to exercise care in discriminating between patho- 

 logical productions apparently resembling one another. 

 Cutaneous horns are very common in man, and have 

 been known to attain a length of fifteen centimetres ; 

 they were formerly regarded by ignorant persons as 



