308 BULLETIN : MUSEUM OF COMPAllATIVE ZOOLOGY. 



transmitted in a recessive condition through many generations, is in strict 

 accordance with Mendel's law of heredity. 



7. Neogenetic and palingenetic forms of polydactylism are, like other 

 new characters, extremely variable ; as they are hereditary, we may con- 

 clude that duplications of both functional and vestigial digits are due to 

 variations in the gametes. 



8. The poly dactyl e abnormalities of man and the domestic animals 

 may be classified as follows : 



I. Teralological polydactylism includes those cases of digital duplica- 

 tion and malformation which are produced by external influences ; it occurs 

 rarely in all animals, often in correlation with other monstrosities. 



II. Neogenetic polydactylism includes those digital variations, or 

 sports, which are produced by some internal cause, presumably germinal 

 variation, 



a. Duplication of unmodijied functional digits occurs occasionally in 

 all animals and is transmissible. 



h. Variation of modijied but functional digits is the ordinary form 

 of polydactylism in man, the cat, and the fowl (pes), and it also is 

 transmissible. 



III. Palingenetic polydactylism includes those cases in which digital 

 rudiments, or vestiges, develop into extra digits. 



a. The extra digits reproduce more or less completely the structure of 

 the homologous functional digits of related fossil ancestors; this condi- 

 tion is found in the horse, ruminants, swine, and the pes of the dog. 



h. The extra digits arise as variations or duplications of rudiments, or 

 vestiges ; they are neogenetic in so far as they do not reproduce ancestral 

 conditions. Examples are the hallux and pollex having three phalanges 

 and the various duplications of these digits found in the manus of swine 

 and the pes of Carnivora. 



