240 PALMER, The Maryland YVellow-throat. Te 
At the southern end the form is also a large one because the 
individuals obey the law, that: where a species occupies a range 
one end of which is a low, tropical or island area, then the individ- 
uals occupying this portion are larger than those occupying an 
adjoining dissimilar habitat. The result may be a species or 
subspecies differing from the parent stock according as the 
separation is long, or the environing influences are intense or 
weak. In the case of brachidactyla migration has intensified the 
divergence by compelling a longer wing, a process still in prog- 
ress. The lapse of time since the glacial retreat has not been 
sufficient to make the outer quill the longest but such is evi- 
dently the tendency. In agi/is we have a species which evidently 
occupies largely what was an island area in preglacial or inter- 
glacial times, and the development of the wing was hastened by 
the birds having to migrate over, or by, a Mississippi Sea! In 
tolmiei and philadelphia we have two slightly differing species; 
evidently two divergents from a common stock; divergent 
because they extended their range northward on opposite sides 
of the Mississippi Sea. It would seem also that in ¢vichas and 
all its eastern relatives we have one branch, and in me/anops and 
occidentalis another, long separated by the same cause. The 
retreat of the ice has permitted the forms to be localized where 
we now know them, and as the Mississippi Sea disappeared 
trichas and occidentalis approached each other and have appar- 
ently insensibly intergraded. A similar case is the Meadowlarks 
which have joined habitats during the historical period. 
It will have been noticed that the ranges of the three forms of 
trichas on the Atlantic slope are quite peculiar. Considered lati- 
tudinally the range of roscoe ends where ¢richas is in its prime, 
and in its turn this gives way to drachidactyla. The reason is evi- 
dent, they occupy different faunal areas, the characteristics of 
which are determined by altitude, the slope angle and the charac- 
ter of the drainage. The width of the faunal area and the char- 
1The former presence of this sea is indisputable and its effects on the dis- 
tribution of North American bird life have hardly been noticed, yet it divided 
it into two parts, usually with representatives on each side. The life of the 
parts have approached each otheras the sea disappeared and at the same 
time southern forms have moved northward. ‘ 
