CH. l] DISTRIBUTION OF RHEA. 19 



has its own particular habitat, and does not interfere with 

 its neighbours. In the map which Mr Wallace gives in 

 illustration of the facts of distribution of this genus it is 

 seen that two species overlap just at the confines of 

 Europe and Asia, while the former of these, the European 

 Garrulus glandarius, is also overlapped by one species in 

 the south-east of Europe and by another in Algeria. It 

 is far more usual for species to occupy in common a given 

 area, than for a division of the territory to have taken 

 place. Nevertheless the example just quoted is by no 

 means unique, even among birds whose powers of flight 

 set ordinary barriers at defiance. But a rigid partition of 

 the area of a genus is more commonly met with among 

 animals which have not these exceptional means of 

 disposal. This will now r be illustrated by three examples. 



Distribution of Rhea. 



The distribution of the species of Rhea 1 illustrates the 

 limitation of the species of the same genus each to its 

 own particular tract. The genus itself occupies a con- 

 siderable area of S. America, to which continent it is 

 absolutely confined. The three species of the genus have 

 been lately subjected to a careful comparison by Dr 

 Gadow, who has plainly differentiated the three recognised 

 species, viz. Rh. americana, Rh. macrorhyncha and Rh. 

 darwini. The anatomical characters which distinguish 

 them are not of course very marked, but they are amply 



1 See Gadow, "Ou the Anatomical Differences in the three species of 

 Rhea," P.Z.S. 1885, p. 308. 



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