PRENTISS: POLYDACTYLISM IN MAN AND DOMESTIC ANIMALS. 307 



the hands polydactyle. In either case, however, they would be capable 

 of producing other DR offspring, if married to normal individuals, and 

 these offspring might themselves be normal or polydactyle ; should they 

 marry with recessive individuals like themselves, pure Z^'s wouhl be pro- 

 duced as well as RD'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 tlie cause of these germinal variations, or sports, we know little or 



nothing. 



IX. Summary. 



1. 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 

 tlie 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 ; witli the second digit in ruminants and the Equidae. In 

 general, polydactylism may he said to aff'ect digits which are normally 

 much reduced or modijied. 



4. Cases of polydactylism in which more than five digits occur cannot 

 be attributed to reversion alone (a heptadactyle ancestop is hypothetical, 

 the so-called prae-poUex and post-minimus are rudiments of secondary 

 development, and they have never been known to produce functional 

 digits). 



5. PaHngenetic 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 sti'ucture 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. Tliis eventual dominance of a digital character, which has been 



VOL. XL. — NO. G 5 



