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Rh more strictly under our present head of reversion. Certain structures, regularly occurring in the lower members of the group symmetrically on the breast; and more especially from one case, in which a single efficient mamma occurred in the inguinal region of a woman, the daughter of another woman with supernumerary mammæ. But I now find (see, for instance, Prof. Preyer, 'Der Kampf um das Dasein,' 1869, s. 45) that mammæ erraticæ, occur in other situations, as on the back, in the armpit, and on the thigh; the mammæ in this latter instance having given so much milk that the child was thus nourished. The probability that the additional mammæ are due to reversion is thus much weakened; nevertheless, it still seems to me probable, because two pairs are often found symmetrically on the breast; and of this I myself have received information in several cases. It is well known that some Lemurs normally have two pairs of mammæ on the breast. Five cases have been recorded of the presence of more than a pair of mammæ (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 mammæ, 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 mammæ 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 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 ('Jenaische Zeitschrift,' B. v., Heft 3, s. 341), disputes Owen's conclusion. On the other hand, according to the opinion lately advanced by Dr. Günther 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 double the number 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 is due to reversion from the fact that such digits not only are strongly inherited, but have the power of regrowth after amputation, like the normal digits of the lower vertebrata. This fact of their regrowth remains inexplicable, if the belief in reversion to some extremely remote progenitor is rejected. Arrested development and reversion are intimately connected, and thus the belief in reversion in the present case is to a certain extent supported by the frequent, or almost constant, coincidence insisted on by Meckel and I. Geoffroy St.-Hilaire, between various arrests of development, such as cleft-palate, bifid uterus, cyclopean state of the eyes, &c., and the presence of additional digits (see, on this head, M. A. Ronjou, 'Types Primitifs des Mammifères,' p. 61; and M. Bertillon, 'Valeur Phil. Hyp. du Transformisme'). It is, on the other hand, no real objection to the view here