no 



E VOL VT1ON AND DISEASE. 



atavistic polydactyly can only occur in non-pentadactyl 

 mammals, but even in them, as will be shown in the 

 chapter on Atavism, the number of the digits may be 

 increased by dichotomy. 



There is reason to believe that supernumerary digits 

 and limbs may be produced by cleavage throughout the 

 vertebrate sub-kingdom. Albrecht has figured a mud- 

 fish (Protopterus annectans\ with bifurcation of the right 

 pectoral limb (fig. 58). The specimen is preserved in the 

 museum at Konigsberg. 



Haswell and Howes have 

 shown and admirably illustrated 

 the tendency manifested by the 

 fins of Ceratodus to bifurcate. 

 This is seen not only in the radii 

 supporting the fringe of horny 

 rays, but in the main or support- 

 ing axis. 1 



Among amphibians super- 

 numerary limbs are not rare. 

 An example from the common 

 toad is sketched in fig. 59. The 

 axis of the limb is at right angles to the trunk, and 

 articulates with the ilium by a perfect ball-and-socket 

 joint. 1 - The additional limb has the usual segments of 

 femur, crus, and pes. The muscles were well-developed. 

 This specimen illustrates a condition of frequent 

 occurrence in such limbs ; it is furnished with an ab- 

 normal number of digits the usual number is five, 

 whereas in this case the pes presents seven. 

 1 " Proc. Zool. Society," 1887. 



FIG. 57. A double hand. 

 (After Jardine-Murray. ) 



