26 EVOLUTION AND DISEASE. 
Tiedemann! in 1831 described and figured the skull 
of a duck with a foot growing from its occiput. There 
is a fancy breed of ducks in which the distinguishing 
feature is the presence on the occiput of a rounded knob 
or swelling covered with feathers. 
The acquisition and transmission of such characters 
shed light on some rather puzzling conditions. Ab- 
normal growth of hair induced by contact with irritating 
substances may explain the presence of hair in such a 
curious situation as the stomach of a crayfish and the 
hairs which form the remarkable plug around the pyloric 
orifice of the darter’s stomach (fig. 15). This bird feeds 
on fish, and as Garrod, in his excellent account of the 
anatomy of the darter’s stomach, puts it, “ This peculiar 
hairy mat acts as an excellent sieve to prevent the 
entrance of solid particles, fish-bones, &c., into the 
narrow intestines.”2 From what we know concerning 
the effects of irritation upon the skin it is quite con- 
ceivable that the contact of fish-bones and scales would 
act as irritants and induce a crop of hairs which, being 
advantageous to the bird, have been inherited. It in no 
way invalidates the argument by urging that skin, not 
mucous membrane, is furnished with hairs. Even the 
complex intestinal mucous membrane may, under ex- 
ceptional circumstances, become converted into pilose 
skin. Such abnormal skin is more likely to possess hair 
if it be irritated. Abnormal growth of hair from irrita- 
tion is paralleled by the elongation of the cutaneous 
papillz under similar circumstances. This may be studied 
* “ Zeitschrift fir Physiologie,” Bd. iv. p. 121. 
* “ Proceedings of the Zoological Society,” 1876, p. 335. 
