THE DESCENT OR ORIGIN OF MAN 59 



Reversion. — Many of the cases to be here given might 

 have been introduced under the last heading. When a 

 structure is arrested in its development, but still continues 

 growing, until it closely resembles a corresponding structure 

 in some lower and adult member of the same group, it may 

 in one sense be considered as a case of reversion. The lower 

 members in a group give us some idea how the common pro- 

 genitor was probably constructed; and it is hardly credible 

 that a complex part, arrested at an early phase of embryonic 

 development, should go on growing so as ultimately to per- 

 form its proper function, unless it had acquired such power 

 during some earlier state of existence, when the present 

 exceptional or arrested structure was normal. The simple 

 brain of a microcephalous idiot, in as far as it resembles 

 that of an ape, may in this sense be said to offer a case of 

 reversion. °' There are other cases which come more strictly 



Dumb," 2d edit., 1810, p. 10) has often observed the imbecile smelling their 

 food. See, on this same subject, and on the hairiness of idiots, Dr. Maudsley, 

 "Body and Mind," 1870, pp. 46-51. Pinel has also given a striking case of 

 hairiness in an idiot. 



^ In my "Variation of Animals under Domestication" (vol. ii. p. 51) I 

 attributed the not very rare cases of supernumerary mamm^ in women to rever- 

 sion. I was led to this as a probable conclusion, by the additional mammse 

 being generally placed 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 mammse. But 

 I now find (see, for instance. Prof. Preyer, "Der Kampf um das Dasein," 1859, 

 s. 45) that mammce erraticx occur in other situations, as on the back, in the 

 armpit, and on the thigh; the mamms in this latter instance having given so 

 much milk that the child was thus nourished. The probability that the addi- 

 tional mammse are due to reversion is thus much weakened; nevertheless it stiU 

 seems to me probable, because two pairs are often found synametrically 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 mammse on the breast. 

 Kve cases have been recorded of the presence of more than a pair of mammro 

 (of course rudimentary) in the male sex of mankind ; see ' 'Journal of Anat. and 

 Physiology," 18*72, p. 56, for a case given by Dr. Handyside, in which two 

 brothers exhibited this peculiarity; see also a paper by Dr. Bartels in "Eeich- 

 ert's and du Bois-Eeymond's Archiv.," 1812, p. 304. In one of the cases al- 

 luded to by Dr. Bartels, a man bore five mammse, one being medial and placed 

 above the navel ; Meckel von Hemsbaeh thinks that this latter case is illustrated 

 by a medial mamma occurring in certain Cheiroptera. On the whole we may 

 well doubt if additional mammse 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 hesita- 



