The Bell-birds of America. 



407 



It may be easily imagined tliat 0. priscus finding a country 

 suitable to its habits and- mode of life, gradually extended its 

 range northwards and southwards, until, instead of its original 

 "specific, area" (a, b, c, cl, Fig. 1), it became occupant of the 

 larger area (A, B, C, D, Fig. 2), i.e., the whole of the present 

 "generic area" of Ghasmorhynclius. Now, so long as the area 

 A, B, 0, D, remained united and homogeneous it is probabie 



Fig. 1. Fig. 2. Fig. 3. Fig. 4. 



G. prisons. 



D 



A 



B_ 



G. prisms. 



G. prise lis. 



G. prisms. 



G. prisons. 



A 



C. tricar unculatus. 















C. variegatus. 













C. niveus. 











3 



C. nudicollis. 



T) 



that, in spite of the innate tendency to perpetual variation, 

 there would be but slight modifications in the structure of our 

 0. priscus, as the different varieties would to a certain extent 

 neutralise each other by interbreeding. At all events the 

 changes introduced by progressive variation would be similar 

 or nearly similar over the whole area, and would be such as 

 would, perfiaps, enable its to distinguish the extreme northern 

 and southeim forms from one another, as is usually the case with 

 species spread over a very extended area, but not of sufficient 

 importance to necessitate the recognition of more than one 

 species. 



But now let us suppose that the slow course of secular 

 variation which is constantly altering every part of the sur- 

 face of our globe, operates upon the area A, B, 0, D, where the 

 vacant intervals are placed in Fig. 3, and renders these portions 

 unfit for the habitation of G. priscus. This might take place 

 either by the submergence of the vacant tracts, and division 

 of the extended area into four islands, or by their elevation, 

 or simply by the failure of certain trees on the fruit of which 

 Gliasmorhynchits subsists within the vacant areas. Ghas- 

 morhynchns priscus would, after this event took place, occupy 

 four different areas, completely cut off from one another, as 

 shown in Fig. 3. The tendency to variation would now act 



