28 THE DESCENT OF THE PRIMATES 



gone to Australia purposely to study the develop- 

 ment of the Ornithodelphia, an inquiry which 

 many years ago had been commenced, but never 

 been terminated, by an English embryologist. 

 And the principal reason why so much interest 

 is felt in the development of these very animals 

 is, that they and not any others are expected to 

 give us the clue to many points at pr-esent insuf- 

 ficiently understood in the embryonic history of 

 the placental mammals. 



In this respect I would wish to choose a posi- 

 tion more or less diametrically opposed to that of 

 Semon.^ The Monotremes cannot reasonably be 

 expected to teach us anything concerning the ear- 

 liest phases through which the Placentals have 

 passed. And too long has the conclusion re- 

 mained unchallenged that, because the Mono- 

 tremes have been shown to lay eggs of the 

 Sauropsidan type, the ancestors of the Placentals 

 must have passed through a stage in which they 

 necessarily reproduced themselves in the same 

 manner, certain details of the embryonic sac of 

 the Mammalia undoubtedly favoring this view. 

 A reptilian ancestor to the Mammalia has thus 

 found more favor than an amphibian one ; and 



1 See Semon's Lecture in the Report of the Leyden Interna- 

 tional Zoological Congress of 1895, p. 295, footnote. 



