12 DISTRIBUTION OF MONOTREMATA. [CH. I 



as higher than families are naturally not common. The 

 most striking perhaps is that of the Rhynchocephalia ; this 

 group has but one species, the well-known Hatteria, which 

 is only found in a few small islands off the coast of New 

 Zealand. Not quite so striking from some points of view, 

 but perhaps more so in another, in that the group is a 

 higher one, is the case of the Monotremata; the two or 

 three genera are limited to certain parts of the Australian 

 region. 



The facts just enumerated lead us to one of the 

 axioms of the science of Zoogeography, which was for- 

 mulated by Mr Sclater 1 in the following words. " Every 

 species occupies a definite area on the world's surface ; 

 and in like manner every genus and family, or other 

 higher assemblage of species, occupies a definite area on 

 the earth's surface ; or more shortly, locality or existence in 

 a certain spot is quite as much an attribute of animals as 

 structure or the possession of a certain form or shape." 



The meaning of a restricted distribution. 



It does not, however, by any means follow that this 

 area is now as it always has been. To study Zoogeography 

 properly a knowledge of the extinct forms of life is not 

 only desirable but necessary. By the aid of Palaeontology 

 various facts, at first sight dark and meaningless, become 

 clear, or at least clearer. We must imagine each species 

 setting out from its centre of origin and gradually ex- 



1 "The Geographical Distribution of Mammals," Manchester Science 

 Lectures, 1875. 



