Overlapping areas among the species of a genus is a more common phenomenon, and is almost universal where these species are numerous in the same continent. It is, however, exceedingly irregular, so that we often find one {19} 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. coerulea_ 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 64deg N. latitude, in Asia Minor to the Lebanon and Caucasus, and across Siberia to Amoorland and Japan. The marsh tit (_Parus palustris_) inhabits temperate and south Europe from 61deg 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 (_Parus borealis_), 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 camtschatkensis_) ranges from North-eastern Russia across Northern Siberia to Lake Baikal and to Hakodadi in Japan, thus overlapping _Parus borealis_ in the {20} western portion of its area. Our little favourite, the blue tit (_Parus coeruleus_) ranges over all Europe from the Arctic circle to the Mediterranean, and on to Asia Minor and Persia, but does not seem to pass beyond the Ural mountains. Its lovely eastern ally the azure tit (_Parus cyaneus_) overlaps the range of _P. coeruleus_ in Western Europe as far as St. Petersburg and Austria, rarely straggling to Denmark, while it stretches all across Central Asia between