of Anat. and Physiology,' 1872, p. 56, for a case given by Dr. Handyside,
in which two brothers exhibited this peculiarity; see also a paper by Dr.
Bartels, in 'Reichert's and du Bois-Reymond's Archiv.,' 1872, p. 304. In
one of the cases alluded to by Dr. Bartels, a man bore five mammae, one
being medial and placed above the navel; Meckel von Hemsbach thinks that
this latter case is illustrated by a medial mamma occurring in certain
Cheiroptera. On the whole, we may well doubt if additional mammae would
ever have been developed in both sexes of mankind, had not his early
progenitors been provided with more than a single pair.

In the above work (vol. ii. p. 12), I also attributed, though with much
hesitation, the frequent cases of polydactylism in men and various animals
to reversion. I was partly led to this through Prof. Owen's statement,
that some of the Ichthyopterygia possess more than five digits, and
therefore, as I supposed, had retained a primordial condition; but Prof.
Gegenbaur ('Jenaischen Zeitschrift,' B. v. Heft 3, s. 341), disputes Owen's
conclusion. On the other hand, according to the opinion lately advanced by
Dr. Gunther, on the paddle of Ceratodus, which is provided with articulated
bony rays on both sides of a central chain of bones, there seems no great
difficulty in admitting that six or more digits on one side, or on both
sides, might reappear through reversion. I am informed by Dr. Zouteveen
that there is a case on record of a man having twenty-four fingers and
twenty-four toes! I was chiefly led to the conclusion that the presence of
supernumerary digits might be due to reversion from the fact that such
digits, not only are strongly inherited, but, as I then believed, had the
power of regrowth after amputation, like the normal digits of the lower
vertebrata. But I have explained in the second edition of my Variation
under Domestication why I now place little reliance on the recorded cases
of such regrowth. Nevertheless it deserves notice, inasmuch as arrested
development and reversion are intimately related processes; that various