22 GEOGRAPHICAL DISTRIBUTION. 



lands of Central Europe, in the Pyrenees, Alps, and the Bavarian 

 Highlands, and again in the Caucasus, the last region isolated by 

 fully one thousand miles of non-inbabited country. Equally strik- 

 ing examples were supposed to be alforded by the fresh-water seals 

 of Lake Baikal and the brackish-water species of the Caspian, which 

 were considered to be identical with the northern Phoca fcEtida and 

 P. vitulina respectively, but more careful study has shown this 

 identification to be erroneous." The critical studies made by Mr. 

 Seebohm of the Central and East Asiatic faunas have disclosed 

 a number of extraordinary instances of discontinuous habitation 

 among birds. One of these is exemplified in the'case of a South 

 European variety of the common marsh-tit (Parus palustris), which 

 reappears in an undistinguishable guise in China, although in an 

 intervening tract of some four thousand miles (east of Asia Minor) 

 the variety is entirely wanting, being replaced by one or more 

 closely related fonns. Ceryle guttata, a spotted king-fisher, appears 

 to be confined to Japan and the Himalaya Mountains, being com- 

 pletely wanting in China ; and the same is tiTie of a species of 

 crested eagle (Spizaetus orientalis), with the exception that its 

 range embraces the Island of Formosa. Similarly, we have two 

 species of birds, the rufous-breasted fly-catcher (Siphia superciliaris), 

 and the Darjeeling wood-pigeon (Palumbus pulchricoUis), which 

 are absolutely confined to the Himalayas and the Island of Formosa. 

 But while individual cases of species inhabiting discontinuous 

 areas do present themselves, they are of comparatively rare occur- 

 rence, and the general law of regional continuity may be recognised. 

 In a region occupied by a given species of animal there is usually 

 an area which is par excellence more thickly inhabited than any 

 other, and which may, consequently, be termed the "metropolis" 

 of that species. From this metropolis there is in most cases a 

 radial distribution of the individuals of the species, with a thinning 

 out towards the periphery. Distinct species of the same genus 

 rarely have coincident geographical distributions; in other words, 

 they rarely occupy precisely the same areas, but more generally 

 these areas, if at all continuous, overlap each other to a greater or 

 less extent. This fact is beautifully exemplified in the case of the 

 American hares, which are represented by some eleven species, and 

 about as many well-marked varieties. Commencing at the far 

 north, we have the polar or variable hare (Lepus variabilis or L. 



