30 THE DESCENT OF MAN. 



present head of reversion. Certain structures, regularly occurring 

 in the lower members of the group to which man belongs, occa- 

 sionally 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, although in a manner 

 which is normal in the lower members of the group. These re- 

 marks will be rendered clearer by the following illustrations. 



In various mammals the uterus graduates from a double organ, 

 with two distinct orifices and two passages, as in the marsupials, 

 into a single organ, which is in no way double, except from 

 having a slight internal fold, as in the higher apes and man. 



mae (of course rudimentary) in the male sex of mankind; see 'Journal 

 of Anat. and Pliysiology," 1S72, p. 56, for a case g-iven 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 Archlv.,' 1872, p. 304. 

 In one of the cases alluded to by Dr. Bartels, a man bore Ave mam- 

 mae, one being medial and placed above the navel; Meckel von Hems- 

 bach 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 ani- 

 mals to reversion. I was partly led to this through Prof. Owen's state- 

 ment, that some of the Ichthyopterygia possess more than five digits, 

 and therefore, as I supposed, had retained a primordial condition; but 

 Prof. Gegenbaur ('Jeaiaischen Zeitschrift,' B. v. Heft 3, s. 341), dis- 

 putes 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 difHculty 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 alttr 

 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, in as much as arrested development 

 and reversion are intimately related processes; that various structures 

 m an embryonic or arrested condition, such as a cleft palate, bifid 

 uterus, &c., are frequently accompanied by polydactylism. This has 

 been strongly Insisted on by Meckel and Isidore Geoffrey St. Hilaire. 

 But at present it is the safest course to give up altogether the Idea 

 that there is any relation between the development of supernumerary 

 digits and reversion to some lowly organized progenitor of man. 



