270 EVOLUTION AND DISEASE. 
it measures in length nearly one metre, and is twenty-— 
eight centimetres in circumference at the base. — 
In birds cutaneous horns grow very rapidly, and are 
shed with each moult: in the course of three months 
such horns have been known to attain a length of seven 
FIG, 134.—The head and leg of a Thrush: the horns on the 
leg arise from warts, that on the head from a sebaceous 
cyst. 
and even ten centimetres, and to be reproduced for 
several consecutive years. An excellent specimen of this 
form is shown on the thrush (fig. 134). This bird was 
observed by Mr. Roger Williams : it had three wart horns 
on the legs, and upon its head a sebaceous cyst with a 
