PHYLOGENY OF MAMMALS 135 



example in the case of the mammary-pouch 

 theory. For, assuming that the mammary 

 organs of the Marsupials were directly derived 

 from those of the Monotremes (especially 

 Echidna), and, again, that the mammary 

 organs of the Placentals came immediately 

 from those of the Marsupials, it followed 

 that these orders must have succeeded each 

 other in a linear fashion : 



Placentalia. 



f 



I 



Marsupialia. 



t 



Monotremata. 



But the possibility of such a genealogical 

 tree has for more than thirty years been raised 

 beyond all discussion, as Huxley showed in his 

 famous essay, " On the Application of the 

 Laws of Evolution to the Arrangement of the 

 Vertebrata and more particularly of the Mam- 

 malia" (1880). 



Since that time three contrasting views as to 

 the phylogeny of the Mammals have been put 

 forward. 



Huxley himself founded the first of these 

 views by postulating the existence of three 



