268 EVOLUTION 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 
Fic. 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 
ina 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 
