LIFE IN TERTIARY TIMES 291 



An identity of diet has occasionally produced in Marsupials 

 and Placental Mammals some astonishing resemblances in 

 detail over and above the general resemblances just enumer- 

 ated. Diprotodon, for example, which lived in Australia at 

 the beginning of the present era, was about as large as a 

 Rhinoceros and had a dentition almost identical with that of 

 the Rodents. Its upper jaw, having no canines, became 

 elongated into a snout bearing two enormous incisors separated 

 from the molars by a wide space, and concealed immediately 

 behind these large incisors w r ere smaller ones similar to those 

 of a Rabbit, except that instead of one there were two, one 

 behind the other. 



The limbs underwent slight modifications only, and in 

 a particular direction. The anterior limbs, being frequently 

 used for prehension, retained their five digits ; but the hind 

 legs, in species belonging to numerous genera living on insects 

 or fruits, have the second and third toes united and are 

 relatively slender. This arrangement recalls that to be 

 observed in the Kingfisher, Hornbill, and other syndactylous 

 Birds, and is due to the same cause. These Marsupials live 

 on trees whose branches they are obliged to seize, and there- 

 fore the longest digit plays the principal role ; the others press 

 against it, unite with it, and partially atrophy. This arrange- 

 ment is preserved and exaggerated in the jumping Kangaroos, 

 whose median digit is very large, the hallux absent, and the 

 other digits astonishingly slender and joined together by skin. 

 Nothing in the Kangaroo's present mode of life demands such an 

 arrangement. But it is at once explained if we regard these 

 animals as descendants of climbing Marsupials, a supposition con- 

 firmed by the existence of arboreal Kangaroos, the Dendrolagi. 

 The Marsupials are, moreover, far removed from the Placentals 

 with regard to their place in the scheme of Nature. 

 From the beginning of the Tertiary Epoch, the Placental 

 Mammals, being endowed with a method of reproduction 

 far superior to that of the Marsupials, have everywhere had the 

 advantage of them, have multiplied very rapidly, and adapted 

 themselves to the most varied conditions of life ; and, living 

 in security and amid plenty, they have frequently been able to 

 increase their size from one generation to another. They 

 thus played the same part that the Reptiles filled in the 

 Secondary Epoch, without attaining to their dimensions, 



