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 Eutheria—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 of existence. 
The most primitive of the older mammals are the Profotheria 
—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—Eutheria—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 
xvi 
