KINETOGENESIS. 357 



while it is pressed against the pad which replaces the 

 superior incisors. Why the superior incisors should 

 have disappeared in this group is not yet clear to my 

 mind. 



In this connection Dr. Allen (/. c.') reminds us that 

 in hypertrophy of the tongue in man, the inferior in- 

 cisors are thrown forward and are widely separated 

 from each other. He considers it reasonable to infer 

 that in lower animals where the tongue is used for pre- 

 hension, the similar change which takes place in the 

 teeth, from a vertical to a horizontal position, is induced 

 by this cause. 



vi. HOMOPLASSY IN MAMMALIA. 



The direct evidence in favor of kinetogenesis above 

 adduced is greatly strengthened by corroborative tes- 

 timony presented by distinct phyla of animals. Re- 

 stricting myself here to Mammalia, I will enumerate a 

 number of cases where the same structures have ap- 

 peared in distinct lines of descent under similar me- 

 chanical conditions, a phenomenon already referred to 

 on page 72 under the name of Homoplassy. 



Before reviewing the subject, I cite what is the 

 most remarkable example of homoplassy in the Mam- 

 malia which has yet come to the knowledge of paleon- 

 tologists. Ameghino has discovered in the Cenozoic 

 formations of Argentina a group of Ungulata which he 

 calls the Litopterna, and which 1 regard as a suborder 

 of the Taxeopoda, allied to the Condylarthra (p. 128). 

 Ameghino placed the group under the Perissodactyla, 

 but the tarsus and carpus are of a totally different char- 

 acter, and indicate an origin from the Condylarthra 

 quite independently of that division. The carpal and 

 tarsal bones are in linear series, or if they overlap, it is in 



