INTRODUCTION 19 



direction increases in intensity southward. This 

 is well shown in the east to west direction, (d map 

 5), by examining a series of Rufus-breasted 

 Whistler (Pachycephala rufiventris). The moun- 

 tain chain divides the darker race from the paler 

 race, the latter being subject to less humidity. 



A long stretch of country between the great rift 

 valley and Eucla is composed of limestone, covered 

 by salt bush and blue bush. This is a feeble 

 connecting link, when compared with an earlier 

 geological age, between the birds of South and 

 West Australia. The Nullabor Plain, as the 

 country is known, is the present connecting link 

 of two avifaunal areas. 



The distribution of certain species goes far 

 beyond Australia. A few are found east in 

 Polynesia, others north in the Phillipines group, 

 and a further number, with their close relatives, 

 in India (a, b, c, map 6). Shown as follows our 

 birds are : — 

 Land : A. Internal. The thousand species and 



races truly Australian A^ Forest, A^ Grassed 



Plain, A^ Partial Desert. 



B. External. Extra Australian migratory 

 birds. 

 Sea : C. Vast colonies of petrel and gull families 



that rest on the mainland islands and live at sea. 



Apart from distribution we have on a large scale, 

 migration ; the coming in from Siberia in Septem- 



