24 EVOLUTION AND DISEASE. 
same way that the hairy tuft may be accounted for in 
the back of those with spina bifida occulta, These 
fowls are extremely uncertain in their gait, given to 
performing circular movements, and walking sideways if 
excited, as though they possessed an unstable nervous 
system. Darwin was assured that we had here to deal 
with a character first acquired and transmitted by the 
hen.! 
Fic. 13.—The head of a Polish fowl to show the feathery tuft. 
(After Darwin.) 
A somewhat similar condition is seen in ducks. Pre- 
served in the museum of the Royal College of Surgeons 
is a duckling with a small tumour projecting from the 
top of its head ; hanging from the side of the tumour is 
a miniature but well-developed foot (fig. 14). The 
swelling is connected with the duckling’s brain by means 
of a small rounded hole in the summit of the cranium. 
* © Animals and Plants under Domestication.” 
