THE MONOTREMA, OB CLOACAL ANIMALS. 91 



the outer milk apparatus does not appear to have 

 been directly transmitted only to the Marsupials ; 

 thus the various formations of teats, which can be 

 traced, from those first beginnings, amid all sorts 

 of modifications, through the whole series of the 

 higher Mammals, also corroborate the inductive 

 proof of the relationship of the whole mammal 

 world including Man. 1 And even in this case the 

 history of the development of the living repre- 

 sentative of an animal group, supplements the want 

 of observations on conditions and processes that 

 have played a part in the remote period of the 

 earth's history. 



A separate origin for the Monotrerna cannot be 

 unconditionally rejected, but is exceedingly im- 

 probable, if only on account of the perfect agree- 

 ment of the embryonal mammary apparatus of the 

 Marsupials and of the other orders, with the mar- 

 supial pouch of the Echidnas. The suppositions 

 favouring the possible independence of the Mono- 

 trerna do not, of course, possess any actual 

 foundation as long as we do not know the 

 amphibio-reptile forms where the mammal charac- 

 ter shows the first signs of incoming and of 



1 Klaatsch, ' Zur Morphologie der Saugethierzitzen ' (Mor- 

 pholog. Jahrbuch, ix. 1883). 



