34 THE INTERNATIONAL MONTHLY. 
arably connected with these factors. However, as this paper is 
to be a genealogical study, an attempt to set forth the present 
state of knowledge concerning the evolution of the Mammalia, 
these wider and more general problems must be passed by. 
In the third place, palaeontology holds the key which unlocks 
the mysteries of the geographical distribution of animals. Any 
rational theory of evolution must explain not only the 
origin and history of animal groups, but also why they happen 
to occur in certain regions and not in others. The problem 
is not to be dismissed by assigning everything to the causal effici- 
ency of climate. Climate is a very important factor, it is true, 
in controlling distribution, but it is only one factor. While a 
given animal cannot exist where the climate is unfavorable to it, 
yet a favorable climate cannot, of itself, produce the animal, as is 
plainly seen when an animal is artificially established in a new 
region, where it flourishes inordinately and may even become a 
dangerous pest, like the rabbits introduced into Australia. The 
fauna of any region is the outcome of its whole past history, and 
that history is recorded in palaeontology. 
Each of the great land masses of the globe has been the scene 
of an evolution of forms more or less peculiar to itself, or to use 
Osborn's expressive phrase, they have each been the centre of an 
"adaptive radiation of types." Of these centres the principal 
ones are Eurasia, North America, South America, Africa, and 
Australia. Each of these systems of adaptively radiating types 
of animals possesses members more or less resembling those of 
other systems, a fact which has brought great confusion into the 
schemes of classification, but the resemblances are deceptive and 
are due to the independent acquisition of similar characters, not 
to any close relationship. Those types which have been evolved 
through a long line of ancestry in a given region constitute the 
strictly indigenous element in the fauna of that region, but in 
addition every continent contains a larger or smaller element 
which has migrated thither from other regions. For the migra- 
tions of land animals it is essential that a land connection between 
the regions should persist for a longer or shorter period. Hence 
