120 THE DESCENT OF MAN. [Part I. 



progenitor of man must have possessed this bone nor- 

 mally divided into two portions, which subsequently be- 

 came fused together. In man the frontal bone consists of 

 a single piece, but in the embryo and in children, and in 

 almost all the lower mammals, it consists of two pieces sepa- 

 rated by a distinct suture. This suture occasionally per- 

 sists, more or less distinctly, in man after maturity, and 

 more frequently in ancient than in recent crania, especially 

 as Canestrini has observed in those exhumed from the Drift 

 and belonging to the brachycephalic type. Here again he 

 comes to the same conclusion as in the analogous case of 

 the malar bones. In this and other instances presently to 

 be given, the cause of ancient races approaching the lower 

 animals in certain characters more frequently than do the 

 modern races, appears to be that the latter stand at a 

 somewhat greater distance in the long line of descent 

 from their early semi-human progenitors. 



Various other anomalies in man, more or less anal- 

 ogous with the foregoing, have been advanced by dif- 

 ferent authors 37 as cases of reversion ; but these seem not 

 a little doubtful, for we have to descend extremely low in 

 the mammalian series before we find such structures nor- 

 mally present. 38 



37 A whole series of cases is given by Isid. Geoffroy St.-Hilaire, 

 1 Hist, des Anomalies,' torn. iii. p. 437. 



38 In my ' Variation of Animals under Domestication ' (vol. ii. p. 57) 

 I attributed the not very rare cases of supernumerary mammas in women 

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

 mammae 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 mammae. But Prof. Preyer (' Der Kampf um das Dasein,' 

 1869, s. 45) states that mammce erraticce have been known to occur iu 

 other situations, even on the back ; so that the force of my argument is 

 greatly weakened or perhaps quite destroyed. 



With much hesitation I, in the same work (vol. ii., p. 12), attributed 

 the frequent cases of polydactylism in men to reversion. I was partly led 



