2 6 E VOL UTION AND DISEASE. 



Tiedemann r 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 

 papillae under similar circumstances. This may be studied 



1 "Zeitschrift fur Physiologic," Bd. iv. p. 121. 



2 " Proceedings of the Zoological Society," 1876, p. 335. 



