PRENTISS: POLYDACTYLISM IN MAN AND DOMESTIC ANIMALS. 503 
The least answerable of the arguments against the general occurrence 
of reversionary polydactylism 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 
fiu-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, 91). (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 A with the normal and fossil condi- 
tions (Figs. # 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 horse 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, thé abnormal character, since it is dominant in Mendel’s 
sense of the word, is transmitted to the offspring, but in different 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, 
