cases. It is well known that some Lemurs normally have two pairs of
mammae on the breast. Five cases have been recorded of the presence of
more than a pair of mammae (of course rudimentary) in the male sex of
mankind; see Journal 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 structures in an embryonic or arrested condition, such as a
cleft palate, bifid uterus, etc., are frequently accompanied by
polydactylism. This has been strongly insisted on by Meckel and Isidore
Geoffroy St.-Hilaire. But at present it is the safest course to give up
altogether the idea that there is any relation between the development
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