l8o EVOLUTION OF TO-DAY. 



jays furnishes again a good illustration. One 

 could begin in the British isles, travel eastward 

 to the Pacific Ocean, and then, turning south, 

 return west again, south of the Himalayas, through 

 Western Asia and Northern Africa to Spain, and 

 be able to find jays in all of this region, except 

 a few interrupted areas which do not form suit- 

 able habitat for the genus. The species has 

 changed many times, no less than twelve dis- 

 tinct species being found in this region ; but the 

 specific areas either touch each other or overlap. 

 And in America another genus of jays is found, 

 with its appropriate distribution. Exceptions to 

 this rule are rather more numerous than in the 

 former case, and this, of course, we would expect, 

 for evolution looks upon a genus as a highly devel 

 oped species, in which the varieties have become 

 distinct enough to rank as species. If this is true, 

 genera must be older than species, and although 

 originally they must have had a common, central 

 starting-point, they have had more time and oppor- 

 tunity for their dispersal and extinction than species; 

 and examples of discontinuity do occur, although 

 they are very few compared with the abundance of 

 instances of continuity in generic distribution. One 

 species of magpie is confined to Spain and Por- 

 tugal, while another appears in Northeastern Asia 

 and Japan. One species of a mole (Ulotrichus) in- 

 habits Japan and another British Columbia. Two 

 species of thrush-tit live in the Himalayas, while 

 a third is confined to Java. The genus Ritta 

 (ground-thrush) ranges from Hindostan to Aus- 



