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 j 

 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 j 

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

 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 I 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 



