FKEFACE. 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 

 hyaenas, and the bears ; others to portions of families, — as the 

 flying squirrels, and the oxen with the bisons ; others to families, 

 — as the Mustelidse, and the Hystricidse ; 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 



