IIo EVOLUTION 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. 
shown and admirably illustrated 
' the tendency manifested by the 
fins of Ceratodus to bifurcate. 
supporting the fringe of horny 
ing axis.? 
Among amphibians — super- 
zi numerary limbs are not rare. 
Fic. 57.—A double hand. An example from the ‘common 
(After Jardine-Murray. ) 
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. The additional limb has the usual segments of 
femur, crus, and pes. The muscles were well-developed. 
This specimen illustrates a condition of frequent 
Haswell and Howes have — 
This is seen not only in the radii — 
rays, but in the main or support- — 
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. 
* “ Proc. Zool. Society,” 1887. 
a ha 
