250 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



trematic organization. What is certain is, that 

 as early as the end of the Trias two groups of the 

 lower Mammals were already sharply differentiated. 

 Different palaeontologists, Owen, Seeley, Osborn, 

 etc., have attempted, with great skill, to show 

 the affinities which the class of Mammals presents, 

 from the osteological point of view, with the Therio- 

 dont Reptiles of the South African Trias. Without 

 ignoring the curious resemblance of the molars of 

 the Tritylodon with those of the Multituberculate 

 Mammals, and the resemblance of the bones of the 

 limbs of the Theriodesmus with those of the Lemu- 

 rians and Carnivora, one could no way dream of 

 deriving the Triassic Mammals directly from any 

 of the known forms of these African Reptiles, who 

 are animals of relatively great dimensions. The 

 law of increase in size demands, in fact, for small 

 animals like the Dromatherium or the Microlestes, 

 ancestors still more tiny, which should be sought 

 for in Primary strata. We are logically brought to 

 foresee the presence of true mammals at perhaps 

 very remote epochs of Palaeozoic times. 



If we enter into the details of the Mammalian 

 groups, we shall have to register similar pushings 

 back in the case of many branches. It was the rule 

 a few years ago to affirm that the Placental Mammals 

 only commenced in the Tertiary epoch. This was in 

 truth a very improbable proposition, since in the 

 very lowest Eocene both of Europe and North 

 America, Placentals belonging to several already 

 well-differentiated orders, Insectivores, Creodonts, 

 Condylarthra, Amblypods, Tillodonts, and even 

 Lemurians, have been discovered. The presence 



