LIFE HABITS AND DENTITION 93 



orous group this tooth apparently takes on a secondary stage 

 of development. In the Opossums, the most primitive of all 

 Marsupials, this tooth is well developed. It is much reduced 

 in Phascologale, vestigial in Thylacinus and absent in Sarcophi- 

 lus. Hence it is becoming progressively eliminated though, as 

 may be inferred from the condition in Thylacinus in which the 

 last premolar is well developed and retains its primitive char- 

 acter, the elimination of the milk tooth is by no means neces- 

 sarily paralleled by corresponding reduction in its successional 

 tooth. The milk tooth is also vestigial in the Bandicoots, in 

 the Koala and in the Wombat but in the Phalangers and 

 Kangaroos (Macropodidae) its reduction has been arrested and 

 indeed reversed. Apparently because of the advantage which 

 the tooth subserves in a purely herbivorous dentition it has 

 taken on a secondary growth in these animals and remains in 

 place until the attainment of early adult age. 



Although we have spoken of reversed evolution the expres- 

 sion must not be taken too literally. Evolution is irreversible 

 after a certain stage in so far as actual structure is concerned; 

 for example, a hypocone once lost cannot be restored. On the 

 other hand the formation of a pseudohypocone on the last 

 molar can result from the division of the metacone as in the 

 Koala and the Kangaroos. Evolution is reversible in the pos- 

 sibility of adaptation to environment or to function. The de- 

 ciduous molar becomes secondarily enlarged in the Kangaroos 

 so as to simulate its primitive appearance in the Opossums, 

 although the parts actually eliminated cannot themselves be 

 regained. Further in consideration of adaptation to environ- 

 ment and function it must not be forgotten that the loss of a 

 tooth, as of a digit, necessarily diminishes the plasticity of the 

 organism as a whole in future possible adaptations. 



In the adaptive radiation of the Marsupials various other 

 principles of evolution are to be noted. 



Divergent evolution is seen in the production of the molars 

 which are bunodont in the Phalangers, selenodont or crescent- 



