468 The American Naturalist. [June, 



difficult to conceive of reversion to such a remote period, yet it 

 is supported by other evidence. An embryonic third incisor 

 has, I believe, been discovered. As long ago as 1863 Sedg- 

 wick 1 recorded a case of six upper and lower incisors in both 

 jaws, and appearing in both the milk and permanent denti- 

 tions; this anomaly was inherited from a grandparent, a 

 striking instance of hereditary reversional tendency. We 

 might consider that these cases of supernumerary teeth 

 belonged in the same category as polydactylism or additional 

 fingers, which are not atavistic, but for the fact that they do 

 not exceed the typical ancestral number, whereas the fingers 

 do. 



We owe to Windle 1 a careful review of the incisor rever- 

 sions in which he shows that the lost incisors reappear more 

 frequently in the upper than the lower jaw (coinciding with 

 the fact that the lower teeth were the first to disappear in the 

 race) ; he considers that the lost tooth was the one originally 

 next the canine, and concludes by adding our present upper 

 outer incisor to the long list of degenerating organs. 3 He sup- 

 ports this statement by measurements and by citing cases in 

 which it has been found absent. Yet the reduction of the 

 jaws is apparently outstripping that of the teeth, if we can 

 judge from the frequent practice among American dentists of 

 relieving the crowded jaw by extraction. 



We now turn to the arches and limbs. Flower has pointed 

 out that the base of the scapula is widening in the higher 

 races, so that the " index," or ratio of length to breadth is 

 quite distinctive. Gegenbaur associates this with the develop- 

 ment of the scapulo-humeral muscles and the greater play of 

 the humerus as a prehensile organ. 



In general, the arm increases in interest as we descend 

 toward the hand, both in the skeleton and musculature, 

 because here we meet with the first glimpses of facts which 

 enable us to form some estimate of the rate of human evolu- 

 tion. The well-known humeral torsion (connected with 



British and Foreign Medico-Chirurgical Review, 1863. 



