476 Johnson, A Four-ioinged Wild Duck. [bet. 



a thickened fibro-cartiiaginous ligament (Fig. 3, Fl.) about 6 mm. 

 in length by 2 mm. in width, which strongly suggests the humero- 

 ulnar pulley of the normal wing. The ligament is followed by a 

 flattened tendon of origin and this, at about the second third of the 

 forearm, passes into the muscular portion which has its insertion 

 direct upon the entire posterior border of the ulnar bone of the car- 

 pus. Some fibers of the muscle arise from the inner surface of the 

 enveloping tendinous sheath. This muscle occupies a position 

 corresponding to that of the M. flexor carpi ulnaris of the normal 

 wing. 



Viewing the muscles of the abnormal wing as a whole, one may 

 fancy the arrangement as an attempt to dispose of the muscles 

 formed, in a manner as closely approaching the normal plan as the 

 skeletal conditions of the case and the muscle material available 

 would permit. 



The question of causes. With regard to the causes underlying 

 the formation of supernumerary digits or limbs in nature, it may be 

 said that our knowledge is very meager. That supernumerary 

 structures of this kind may be artificially induced in some of the 

 lower vertebrates, often with constant and predictable results, has 

 long been established. And that such parts occur in nature from 

 causes analogous to those of the experimental laboratory is doubt- 

 less true; but it is also undoubtedly true that a great many cases 

 occur which are entirely independent of such external causes. 



As Barfurth ('95) has pointed out, a number of investigators 

 have held the theory — and he calls this the atavistic theory — that 

 polydactylism represents a "throw-back" (Riickschlag) to an older 

 primitive type of limb which possessed more than five digits. This, 

 because it had been observed that the accessory digit occurred 

 especially in connection with either the first or the last digit of the 

 normal series, and a like supernumerary digit was often known to 

 occur in the same individual on both hand and foot, and was inherit- 

 able. Bardeleben, Wiedersheim and others, for instance, assumed 

 that the primitive mammalian limb was not pentadactyl but hep- 

 tadactyl. Still others pointed further back to the rays of an ances- 

 tral fin type. 



It would indeed seem that if, in an animal where the normal digi- 

 tal condition for its particular group represents a reduction in 



