Introduction 



In the ages since then one type of mammal after another 

 has arisen, some being modified step by step into the forms 

 that inhabit the earth to-day while others have been entirely 

 exterminated. 



In some cases the series of fossil remains are so complete 

 that we can easily trace the ancestry of several of our modern 

 mammals, as, for instance, the horse, which is shown to be 

 originally descended from a five-toed beast, while successive ages 

 show the specialization of the feet, first with four toes and then 

 with three, until finally we have the existing horse with his one 

 large toe or hoof on each foot. 



At the present time the great bulk of mammals belong to 

 one group known as the EttHieria modern mammals though 

 we have remnants of two other more primitive groups which 

 were much more extensively developed in the past. These are 

 now almost entirely restricted to Australia and the neighbouring 

 islands where they have been cut off from their mainland rela- 

 tives at the time that Australia became separated from the Asia- 

 tic continent, and have there been preserved to the present day, 

 free from the inroad of the higher forms of mammals which 

 spread over the continents and, being better adapted to existing 

 conditions, crowded the earlier forms out o f existence. 



The most primitive of the older mammals are the Protothen'a 



early mammals comprising the duck bill and spiny ant-eater of 



Australia, animals which resemble in skeletal characters the earliest 



known fossil mammals, and which lay eggs somewhat like 



those of the reptiles. 



The second group, the Marsupialia pouched mammals in- 

 cludes a large number of species in Australia and the opossums 

 of America. One of the leading peculiarities of these animals is 

 that their young are born at a very early stage of development 

 in a perfectly helpless condition and are then placed in an ex- 

 ternal pouch on the belly of the female where they continue 

 their development. 



The modern mammals Entheria comprise a number of dis- 

 tinct types the relationship of which is not always clear, though 

 they are all derived from a common origin and are more closely 

 related to one another than to either of the preceding groups. 



The aquatic whales and manatees, while not closely related to 

 one another, differ so much from the land mammals that it is very 



XV] 



