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 ot 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 



