INTRODUCTION 17 



h — d different in their colour though similar in their 

 colour scheme. These are parallel lines of plumage 

 tint, one of which is subject to the darkening 

 influence of the coastal ranges, and the other to 

 the tendency directly or by natural selection to 

 take out colour. 



This is the main influence of the salt bush and 

 other scrub lands between the Cloncurry and 

 Wakool tracts. The tendency to lose colour from 

 e to h is checked at j. Here we have the rapidly 

 changed conditions of a mangrove coast : a portion 

 of tidal vegetation extending four thousand miles 

 along the northern fringes (map 2). 



This brackish-mud feature is the outer line of 

 other types of country. In the west, between the 

 Ashburton and Gascoyne districts, there is an 8 

 inch annual rainfall, the dry belt extending inland 

 a thousand miles. This is the barrier to the per- 

 manent northern species passing into south-west 

 Australia, and for those of the south going north 

 to the Kimberley country ; rich in grass and insects. 

 Such a bird as the bush lark (Mirafra h^ h^ h^, map 

 5) will hold its own in a parched province by 

 agreeing in colour with its earth. The severe and 

 varying food conditions of the northern country 

 between the Ord and Flinders Rivers reacts on the 

 frame of the bird by reducing it (b in map 5). As 

 the same species is found south, the body is of 

 larger proportion. The colour acting in an opposite 



