52 THE MAMMALIA. 



America was for a long time i)arted off from North America by 

 the sea, and probably this circumstance gave these low mamma- 

 lian forms a chance in the struggle for existence. The higher 

 forms of mammals were i)revented from penetrating to the 

 Southern Continent during the ages when the Edentata were 

 taking possession of the country. 



The Rodents have diminished both in number of species and 

 in size. A gigantic dormouse has been found in the Maltese 

 Post-Pliocene, described by Falconer as being " as big in compa- 

 rison to a living dormouse as the Bandicoot rat is to a mouse." 

 The great beaver of the Cave deposits of Europe {Trogoniheriimi) 

 differed little from the existing beaver except in its greater size. 

 The large extinct beaver of Ohio {Castoroides Ohioensis) attained 

 a comparatively gigantic size, reaching a length of about five feet. 

 Though beaver-like in form, Castoroides was nearly allied to the 

 Capyharas, and was the largest of all known Rodents. The 

 Rodents are found fully specialised in the Eocene ; therefore, 

 although they show certain affinities with the Marsupials, they 

 must have differentiated from the Marsupial stock before the 

 Tertiary period. 



Especially amongst the extinct genera of Rodents we find 

 some remarkable suggestions of Marsupial structure. Fseudosciic- 

 rus and Sciuridon, in the character of their teeth, approach the 

 Australian Koala, and Sciuroides recalls the phalangers and kan- 

 garoo rats. And, on the other hand, one group of the Marsupials, 

 of which the wombat is the only living representative, had 

 incisors growing from persistent pulps, like the incisors of 

 Rodents. In Tertiary times, these Marsupials {Fossorid) pos- 

 sessed forms rivalling the tapir in size. 



The origin of the Insectivora is still buried in obscurity. Of 

 some of the higher orders of Mammals we can trace the gradual 

 evolution. We can follow up the Ruminants, the Solipedes, the 

 Carnivora, and the Quadrumana to more generalised forms. But 

 the Insectivora belong to one of those old primitive orders whose 

 representatives are found already speciahsed in the Eocene. 

 Moles and hedgehogs are already found, at this remote period, in 

 a Mammalian pedigree. Not only were they of the true Insecti- 

 vorous type, but the mole and the hedgehog had learned, even in 



