CHAP. II THE ELEMENTARY FACTS OF DISTRIBUTION 



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species extending over a considerable portion of the area 

 occupied by the genus and including the entire areas of 

 some of the other species. So little has been done to 

 work out accurately the limits of species that it is very 

 difficult to give examples. One of the best is to be found 

 in the genus Dendroeca, a group of American wood- warblers. 

 These little birds all migrate in the winter into the tropical 

 regions, but in the summer they come north, each having 

 its particular range. Thus, D. dominica comes as far as 

 the middle Eastern States, D. ccerulea keeps west of the 

 Alleghanies, D. discolor comes to Michigan and New 

 England ; four other species go farther north in Canada, 

 while several extend to the borders of the Arctic zone. 



The Species of Tits as Illustrating Areas of Distribution. 

 — In our own hemisphere the overlapping of allied species 

 may be well illustrated by the various kinds of titmice, 

 constituting the genus Parus, several of which are among 

 our best known English birds. The great titmouse {Parus 

 major) has the widest range of all, extending from the 

 Arctic circle to Algeria, Palestine, and Persia, and from 

 Ireland right across Siberia to the Ochotsk sea, probably 

 following the great northern forest belt. It does not 

 extend into China and Japan, where distinct species are 

 found. Next in extent of range is the coal tit {Parus 

 ater) which inhabits all Europe from the Mediterranean 

 to about 64° N. latitude, in Asia Minor to the Lebanon 

 and Caucasus, and across Siberia to Amoorland and Japan. 

 The marsh tit {Parus jpalustris) inhabits temperate and 

 south Europe from 61° N. latitude in Norway to Poland 

 and South-west Russia, and in the south from Spain to 

 Asia Minor. Closely allied to this — of which it is probably 

 only a variety or sub-species — is the northern marsh tit 

 {Paints horealis), which overlaps the last in Norway and 

 Sweden, and also in South Russia and the Alps, but 

 extends further north into Lapland and North Russia, and 

 thence probably in a south-easterly direction across 

 Central Asia to North China. Yet another closely-allied 

 species {Parus camtschathensis) ranges from North-eastern 

 Russia across Northern Siberia to Lake Baikal and to 

 Hakodadi in Japan, thus overlapping Parus horealis in the 



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