VARIATION. Xll 



animals, and is often so excessive in them that the whole 

 manus or pes becomes doubled and usually exhibits the 

 appearance of a right and left manus or pes united together. 

 This condition is excessively rare in the Unguiculata ; one 

 case only is recorded in man *. 



Another kind of polydactylism, confined to animals with 

 less than five toes, arises from the redevelopment of some of 

 the absent digits : No. 302 exhibits this condition in the 

 Horsef. A similar condition perhaps occurs in the domestic 

 Dog, where the inner toe of the pes is frequently developed : 

 it appears, however, to be abnormal in having three phalanges 

 instead of two ; at least if there are only two the metatarsal 

 bone is removed from its normal and very constant position. 

 In the St. Bernard Dog this inner toe is double, that is, six 

 toes characterize the pes ; so that in this case the supernu- 

 merary toes may probably be looked upon as entirely addi- 

 tional, as they are in the Dorking Fowl, where such digits 

 characterize the breed (see Nos. 294 and 295). Lastly, 

 sometimes a digit or a whole limb exhibits unmistakable 

 dichotomy : No. 307 presents this condition, and the 

 Musee Dupuytren contains a similar human foot. It is 

 not possible, however, to distinguish dichotomy of a digit in 

 all cases from mere addition ; it is probable, however, that 

 the conditions are really very distinct. The theory that 

 such additional parts originate from the fusion of twin 

 embryos may be looked upon as entirely superseded J. 



3. Variations in the lobulation of the spleen. The num- 

 ber of transitional states between an entire spleen and one 

 much lobulated or divided render it probable that the most 

 marked cases of lobulation and division of that organ are 

 properly considered as mere varieties. 



4. Variations in the form and arrangement of muscles ; 

 reversions to anterior types in the muscular and osseous 

 systems. 



* J. J. Murray, Med.-Chir. Trans, vol. xlvi. p. 29. 

 t Wood-Mason, Proc. Asiat. Soc. Beng. 1871, p. 24. 

 J Schultze, Virchow's Archiv, Bd. xvii. p. 484. 



