AtR. T. II. POTTS ON KER01>IA CnASSinOSTRIsJ 505 



otherwise be limited in their range and variable in their type. 

 Natural Selection is as efficient in producing permanence of type 

 in some cases as in accelerating variations in other cases. 



If we suppose separation without a difference of external cir- 

 cumstances is a condition sufficient to ensure variation, it renders 

 intelligible the fact that, in nearly allied forms on the same island, 

 the degree of divergence in type is in proportion to the distance 

 in space by which they are separated. The difference between 

 two miles and ten miles makes no change in climate ; but it is 

 easy to believe that it is the measure of a coi-responding difference 

 in the time of separation. In forms that differ more essentially, 

 the separation may have been as complete and as long-continued 

 in the case of those which now inhabit one valley as in the case 

 of those which are separated by the length of an island. "When a 

 wide degree of divergence has been established, hybridation would 

 be precluded. We accordingly find that the difference between 

 species of different genera or subgenera is in most instances 

 equally great whether we take for comparison those from the same 

 or from different valleys. 



If, on the other hand, we suppose that a difference in the 

 external conditions is necessary to the evolution of distinct forms, 

 these and other similar facts remain unexplained. 



Notes on Keropia crassirostris, G-ml. (" Piopio "), 

 By Thomas II. Potts, Esq., P.L.S. 



[Read November 7, 1872.] 



In vrriting on the natural history of our birds, the bewailment of 

 their lessened numbers has come to be a matter of course. The 

 rapid settlement of the colony, in the case of the Thrush, has 

 limited its range greatly ; few birds have retreated with so much 

 haste before the efforts of the cultivator. 



Let us take a section of this island, say a hundred miles in 

 width (including Banks's Peninsula) and stretching from the 

 eastern to the western shore ; this will afford some information as 

 to its present habitat. 



Within this given range at one time the Piopio might be found 

 in any bushy place not too far from water, where belts of shrubs 

 afforded shelter and abundance of seeds ; ten years at least have 

 passed since we heard of its occurrence in this neighbourhood 



