CHAP. IV EVOLUTION THE KEY TO DISTRIBUTION 67 



The European cmd Japanese Jays. — Another case some- 

 what resembling that of the marsh tit is afforded by the 

 European and Japanese ja^^s {Gamdus glandarius and G. 

 japonietcs). Our common jay inhabits the whole of Europe 

 except the extreme north,, but is not known to extend any- 

 where into Asia, where it is represented by several quite 

 distinct species. (See Map, Frontispiece.) But the great 

 central island of Japan is inhabited by a jay (G. japonicus) 

 which is very like ours, and was formerly classed as a sub- 

 species only, in which case our jay would be considered to 

 have a discontinuous distribution. But the specific 

 distinctness of the Japanese bird is now universally 

 admitted, and it is certainly a very remarkable fact that 

 among the twelve species of jays which together range 

 over all temperate Europe and Asia, one which is so closely 

 allied to our English bird should be found at the remotest 

 possible point from it. Looking at the map exhibiting the 

 distribution of the several species, we can hardly avoid the 

 conclusion that a bird very like our jay once occupied the 

 whole area of the genus, that in various parts of Asia it 

 became gradually modified into a variety of distinct species 

 in the manner already explained, a remnant of the original 

 type being preserved almost unchanged in Japan, owing 

 probably to favourable conditions of climate and protection 

 from competing forms. 



Supposecl Examples of Discontinuity among North 

 American Birds.— In North America, the eastern and 

 western provinces are so different in climate and vegetation, 

 and are besides separated by such remarkable physical 

 barriers — the arid central plains and the vast ranges of the 

 Rocky Mountains and Sierra Nevada, that we can hardly 

 expect to find species wdiose areas may be divided 

 maintaining their identity. Towards the north however 

 the above-named barriers disappear, the forests being 

 almost continuous from east to west, while the mountain 

 range is broken up by passes and valleys. It thus happens 

 that most species of birds which inhabit both the eastern 

 and western coasts of the North American continent 

 have maintained tlieir continuity towards the north, 

 while even when differentiated into two or more allied 



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