130 EARTHWORMS AND THEIR ALLIES [CH. 



parts of America and of the East. In this way the 

 entire globe may be mapped out into regions charac- 

 terised by their inhabitants and these regions may 

 also be further subdivided. The commonly accepted 

 regions were originally devised by Mr Sclater and are 

 known as the Palaearctic, Nearctic, Neotropic, Ethio- 

 pian, Oriental (Mr Sclater's name was ' Indian '), and 

 Australian. These regions were originally formed 

 to convey the facts relative to the distribution of 

 Passerine birds only ; but it is generally held that 

 they apply also to the distribution of vertebrates 

 generally. The science of zoogeography does not 

 however end with the display of maps conveying 

 graphically the mere facts of distribution of this group 

 and that. Its business is also to enquire into the 

 causes of the affinities between the faunas of different 

 regions or the varying degree of remoteness which 

 those faunas may show. On the one hand the varying 

 powers of dispersal and the means of extending their 

 range possessed by different animals have to be con- 

 sidered, and on the other hand geological changes in 

 the relative position of land masses have to be taken 

 into account. 



The specific identity between the earthworms of 

 Great Britain and the adjacent part of the continent' 

 of Europe would be very difficult to understand were 

 we only acquainted with the fact that salt water is 

 fatal to these animals. But we also know from 



