THE THREE MAMMALIAN GROUPS. 145 



mistakably indicated by Comparative Anatomy and Onto- 

 geny. 



These show that the whole Mammalian class is divisible 

 into three main groups, or sub-classes, corresponding to 

 three successive stages of phylogenetic evolution. These 

 three stages, which consequently represent three important 

 ancestral stages in the human pedigree, were first dis- 

 tinguished in the year 1816 by the celebrated French 

 zoologist, Blainville, who named them, according to the 

 different structure of the female organs of reproduction, 

 Ornithodelpkia, Didelphia, and Monodelphia (&A0ic, 

 which, being interpreted, is uterus). It is not, however, 

 only in the varied structure of the sexual organs that these 

 three classes differ from one another, but in many other 

 respects also, so that we x;an safely maintain the important 

 phylogenetic statement: The Monodelphia, or Placental 

 Animals, have descended from the Didelphia, or Pouched 

 Animals; and the latter, again, have descended from the 

 Cloacal Animals, or Ornithodelphia. 



Accordingly we have now to consider, as the sixteenth 

 ancestral stage in the human pedigree, the oldest and lowest 

 main group of Mammals; the sub-class of the Cloacal 

 Animals (Monotremata, or Ornithodelphia}. They are so 

 named in consequence of the cloaca, which they have in 

 common with the other lower Vertebrates. This so-called 

 cloaca is the common excretory channel for the excrement, 

 the urine, and the sexual products (Fig. 327). For, in 

 these Cloacal Animals, the urinary duct and the sexual 

 canals yet open into the posterior parts of the intestine, 

 while in all other Mammals they are wholly separated from 

 the rectum and anus, and open by a special orifice 



