PRENTISS: POLYDACTYLISM IN MAN AND DOMESTIC ANIMALS. 307 
the hands polydactyle. In either case, however, they would be capable 
of producing other D& offspring, if married to normal individuals, and 
these offspring might themselves be normal or polydactyle ; should they 
marry with recessive individuals like themselves, pure J’s would be pro- 
duced as well as #D’s, and such individuals again would be polydactyle 
on both hands and feet. Wilson’s theory of nutritive variation is thus 
rendered unnecessary, as Mendel’s law explains how all cases of polydac- 
tylism, not due to external causes, may be the result of inheritance. 
All such inherited types of polydactylism are thus ancestral. But 
only those forms in which the extra digits develop directly from rudi- 
ments and vestiges may be attributed to palingenetic reversion. In 
those cases in which digital rudiments and vestiges are duplicated, rever- 
sion and germinal variation may occur together; but the duplications of 
functional digits are probably caused by germinal variation alone. As 
to the cause of these germinal variations, or sports, we know little or 
nothing. 
IX. Summary. 
ne Polydactylism consists in an excess in the number of digits pos- 
sessed by the individual over the number peculiar to the species. 
2. The supernumerary digits generally occur symmetrically placed on 
the right and left extremities, either in the manus, in the pes, or in both ; 
they are found most frequently in the manus. 
3. The extra digits are formed most frequently in connection with the 
fifth and first digit in man; with the first digit in the fowl, Carnivora, 
and swine ; with the second digit in ruminants and the Equidae. In 
general, polydactylism may be said to affect digits which are normally 
much reduced or modified. 
4. Cases of polydactylism in which more than five digits occur cannot 
be attributed to reversion alone (a heptadactyle ancestor is hypothetical, 
the so-called prae-pollex and post-minimus are rudiments of secondary 
development, and they have never been known to produce functional 
digits). 
5. Palingenetic polydactylism is limited to those forms in which — 
the number of functional digits being normally reduced to fewer than five 
— the digital rudiments develop and reproduce, more or less completely, 
the structure of homologous digits typical of some ancestral form. The 
evidences of comparative anatomy, embryology, and palaeontology show 
this to be the case in the horse, ruminants, and swine; possibly in the 
pes of Carnivora. 
6. This eventual dominance of a digital character, which has been 
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