_Callithea Leprieuri_ is alone found.
Perhaps the most interesting and best known case of a series of allied
species, whose ranges are separate but conterminous, is that of the
beautiful South American wading birds, called trumpeters, and forming the
genus Psophia. There are five species, all found in the Amazon valley, but
each limited to a well-marked district bounded by great rivers. On the
north bank of the Amazon there are two species, one in its lower valley
extending up to the Rio Negro; and the other in the central part of the
valley beyond that river; while to the south of the Amazon there are three,
one above the Madeira, one below it, and a third near Para, probably
separated from the last by the Tocantins river.

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,