42 THE DESCENT OF MAN. 



which come more strictly under our present head of rever- 

 sion. Certain structures, regularly occurring in the lower 

 members of the group to which man belongs, occasionally 

 make their appearance in him, though not found in the 

 normal human embryo ; or, if normally present in the 

 human embryo, they become abnormally developed, al- 

 though in a manner which is normal in the lower members 

 of the group. These remarks will be rendered clearer by 

 the following illustrations. 



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 Anal antl 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 ; Meek el von Hemsbach thinks that this latter case is illus- 

 trated by a medial mammae 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 progeni- 

 tors 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 poly dactyl ism in men and various animals to rever- 

 sion. I was partly led to this through Prof. Owen's statement, that 

 some of the Ichthyopterygia possess more than five digits, and there- 

 fore, 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. Giinther, 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 verte- 

 brata. 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 pro- 

 cesses ; 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 Geoff roy St.-Hilaire. But at present it is the safest course to 

 give up altogether the idea that there is any relation bet ween the de- 

 velopment of supernumerary digits and reversion to some lowly or- 

 ganized progenitor of 



