CHAP. IV EVOLUTION THE KEY TO DISTRIBUTION 65 



station for which its organisation adapts it ; so that forest 

 or marsh or mountain animals are of course only found 

 where there are forests, marshes, or mountains. This 

 may be true, and when the separate forests or mountains 

 inhabited by the same species are not far apart there is 

 little that needs explanation; but in one of the cases 

 refeiTed to there was a gap of a thousand miles between 

 two of the areas occupied by the species, and this being 

 too far for the animal to traverse through an uncongenial 

 territory, we are forced to the conclusion that it must 

 at some former period and under different conditions 

 have occupied a considerable portion of the intervening 

 area. 



Among birds such cases of specific discontinuity are 

 very rare and hardly ever quite satisfactory. This may be 

 owing to birds being more rapidly influenced by changed 

 conditions, so that when a species is divided the two 

 portions almost always become modified into varieties 

 or distinct species ; while another reason may be that 

 their powers of flight cause them to occupy on the average 

 wider and less precisely defined areas than do the species 

 of mammalia. It will be interesting therefore to examine 

 the few cases on record, as we shall thereby obtain ad- 

 ditional knowledge of the steps and processes by which 

 the distribution of varieties and species has been brought 

 about. 



Discontinuity of the Area of Farus 2^alustris. — Mr. See- 

 bohm, who has travelled and collected in Europe, Siberia, 

 and India, and possesses extensive and accurate knowledge 

 of Palsearctic birds, has recently called attention to the 

 varieties and sub-species of the marsh tit {Parus j^cihistris), 

 of which he has examined numerous specimens ranging 

 from England to Japan.^ The curious point is that those 

 of Southern Europe and of China are exactly alike, while 

 all over Siberia a very distinct form occurs, forming the 

 sub-species F. borealis.^ In Japan and Kamschatka other 



1 See Ibis, 1879, p. 32. 



2 In Mr. Seebohm's latest work, Bi7xls of the Japanese Emjyirc (1890), 

 he says, " Examples from North. China are indistinguishable from those 

 obtained in Greece " (p. 82). 



F 



