PREFACE. Vii 
used to illustrate the distribution of various groups of animals. 
These maps are not confined to groups of any fixed rank, but 
are devoted to a selection of groups of various grades. Some 
show the range of single species of a genus—as the lion, the 
tiger, the puma, and a species of fox; others are devoted to 
sections of genera,—as the true wolves ; others to genera,—as the 
hyznas, and the bears; others to portions of families,—as the 
flying squirrels, and the oxen with the bisons; others to families, 
—as the Mustelide, and the Hystricide; and others to groups 
of families or to orders,—as the Insectivora, and the opossums 
with the kangaroos. But in no one grade are all the groups 
treated alike. Many genera are wholly unnoticed, while several 
families are only treated in combination with others, or are 
represented by some of the more important genera. 
In making these observations I. by no means intend to 
criticise Mr. Murray’s book, but merely to illustrate by an 
example, the method which has been hitherto employed, and 
which seems to me not well adapted to enable us to establish 
the foundations of the science of distribution on a secure basis. 
To do this, uniformity of treatment appeared to me essential, 
both as a matter of principle, and to avoid all imputation of a 
partial selection of facts, which may be made to prove anything. 
I determined, therefore, to take in succession every well-estab- 
lished family of terrestrial vertebrates, and to give an account of 
the distribution of all its component genera, as far as materials 
were available. Species, as such, are systematically disregarded, 
—firstly, because they are so numerous as to be unmanageable ; 
and, secondly, because they represent the most recent modifica- 
tions of form, due to a variety of often unknown causes, and 
are therefore not so clearly connected with geographical changes 
as are the natural groups of species termed genera ; which may 
be considered to represent the average and more permanent 
