PKENTISS: rOLYDACTYLISiM IN MAN AND DOMESTIC ANIMALS. 303 



The least answerable of the arguments against the general occurrence 

 of reversionary poly dactyl ism is the fact that more than five digits are 

 found in certain cases of polydactylism (man and cat), and that in other 

 cases the extra digits, though of vestigial origin, are exceedingly vari- 

 able, and often duplicated (swine and pes of Carnivora). Some factor 

 other than reversion must enter here, unless we assume with Albrecht 

 ('86) that the tendency to digital duplication is reversion to the bifid 

 fin-rays of elasmobranch fishes, or with Bardeleben ('86) that the sixth 

 and seventh digits represent reversions to a hypothetical six-toed or seven- 

 toed ancestor. Albrecht's assumption seems absurd, for we know that 

 such duplications are of common occurrence in the development of other 

 structures to which his explanation of reversion cannot apply. Likewise, 

 it has been clearly shown by various investigators that Bardeleben's " prae- 

 pollex " theory is a mere assumption unsupported by the evidences of 

 anatomy, embryology, or palaeontology. For (1) the "prae-pollex " rudi- 

 ments never develop into digits and are not located in the region where 

 the supernumerary digits appear in man (Forster, '61; Gegenbaur, '88; 

 Zander, '9l). (2) They are not the vestigial remains of a degenerating 

 digit, but secondary developments, or neomorphs (Tornier, '89 ; Carlsson, 

 '90 ; Wiedersheim, :02). (3) The most primitive reptilian fossils (the 

 Ichthyopterygia) possess only five digits (Baur,'87). The "prae-pollex " 

 theory is thus rightly rejected by such eminent anatomists as Gegenbaur 

 and Wiedersheim. With it, as a consequence, must go the assumption 

 that polydactylism in pentadactyle extremities is a reversion to a hepta- 

 dactyle type. 



In comparing the skeletal parts of the polydactylous manus shown in 

 Figure 13 (Plate 5)' and in Figure K with the normal and fossil condi- 

 tions (Figs. F and G), no one can doubt that reversion is the true cause 

 of such abnormalities. The same- conclusion holds true for a fully 

 formed hallux in the dog and for the cases of vestigial polydactylism in 

 the liorse and ruminants. It seems probable, however, from the varia- 

 tions which we have described in swine, that the character of digits pro- 

 duced by reversion is not firmly fixed in the germ, and that on crossing with 

 normal animals, the abnormal character, since it is dominant in Mendel's 

 sense of the word, is transmitted to the offspring, but in diflerent de- 

 grees of variation and duplication. Experimental breeding may settle 

 this question, but at present we can only argue from analogy with other 

 forms. Thus, Bateson found that the extra digits of the fowl varied 

 greatly on crossbreeding. But in the case of the fowl the extra digits are 

 sports, not palingenetic structures. 



