LIFE IN TERTIARY TIMES 289 



series it is highly probable that the placenta was at first discoidal, 

 then became zonary, and finally diffuse or cotyledonary. 

 To the Insectivora with discoidal placentas succeeded the 

 Carnivora with zonary placentas, and here the evolution 

 was arrested. The Rodents correspond to the initial condition 

 in the herbivorous animals, Elephants and Hyrax to the 

 second stage ; Pachydermata and Ruminants to the last. 

 In support of this way of looking at it, it may be pointed out 

 that the young of animals with a discoidal or zonary placenta 

 are born incapable of feeding themselves or of walking, and 

 that the Herbivora which have a diffuse or cotyledonary 

 placenta are born in a fairly advanced state of development, 

 and are capable of walking and running. It is in these animals, 

 moreover, that the limbs are most highly differentiated. 

 The Metatheria are confined to-day to South America and 

 Australia, land-areas which were united only during the 

 existence of the Gondwana continent, and we must con- 

 sequently put their origin back to that period. At one time, 

 however, they were cosmopolitan. The Eutheria appeared 

 later, probably outside the regions to which the Metatheria 

 have been driven back to-day — at any rate, outside Australia — 

 where the Metatherians constituted the entire Mammalian 

 fauna prior to the European occupation. 



In common with Reptiles the primitive Mammals had 

 uniform teeth, and four limbs constructed on the same plan 

 each ending in five digits. Living under the same conditions 

 they would necessarily have evolved in analogous fashion 

 if these conditions actually counted in their evolution. Like the 

 Theropod Reptiles of the Triassic Period, all Mammals 

 masticated their food, and their teeth were appropriated 

 in the same way to the various functions that this habit 

 requires ; they were divided into cutting teeth or incisors, 

 tearing teeth or canines, grinding teeth or molars. Except 

 that the molars, instead of remaining simple and being modi- 

 fied only by a broadening of the crown, as in nearly all the 

 Theropods, were made up like those of the Ceratopsidse, 

 by the union of several teeth, whose roots generally remained 

 separate but whose crowns became one. Efforts have been 

 made to determine the number of the teeth thus united from 

 the number of tubercles possessed by the crown, and the 

 following adage has even been formulated : tot numeramus 



u 



