ON VARIATION. 61 



shown 1 that most of the Indo-European race, together 

 with the Esquimaux, present a reversion to a lemu- 

 rine form in the second and third superior molars, and 

 sometimes, in the case of the Esquimaux, in the first 

 superior molar also. I have also shown, 2 after a study 

 of the dentition of the extinct Mammalia, that the more 

 complex molars of later placental orders, have been 

 derived from a tritubercular type, which prevailed 

 throughout the earth just before the opening of the 

 Eocene period. In the line of human and quadruma- 

 nous phylogeny, the lemurs of the Eocene period pre- 

 sented this type of molar in the upper jaw, and mostly 

 continue to do so to the present time. The true mon- 

 keys, however, added the fourth tubercle or hypocone, 

 in accordance with the developmental law in Mamma- 

 lia generally, and the apes and men of the lower races 

 present the same characteristic. Now, in the yellow 

 race the hypocone of the last molar is generally want- 

 ing, while in the white race it is usual to find it absent 

 from both the second and third molars. In this we 

 have a case of reversion. 



The reduction of the third (last) superior molar, 

 and of the inferior as well, has gone further in the 

 white race, since the tooth is frequently abnormally 

 small, abortive, or totally wanting. The external su- 

 perior incisor has a similar history, although its reduc- 

 tion and loss is not nearly so frequent as that of 

 the last molars. These losses from the dental series 

 are not of the nature of reversions, since the number 

 of teeth is more and more numerous as we recede in 

 time along the line of human ancestry. It is, on the 

 contrary, the continuation of a process which has been, 



1 American Journal of Morphology, 1888, p. 7. 



% American Naturalist, 1884, p. 350 ; Origin of the Fittest, 1887, p. 347. 



