Vol. XVII 
1900 
PALMER, Zhe Maryland Vellow-throat. 241 
acter of the topography, geology and vegetation are important 
factors. The Mississippi Valley bird is like true ¢vzchas because 
it occupies similar altitudinal and-geological territory ; it is unlike 
trichas mainly because its winter habitat is different and is at a 
greater distance. ‘The latter influence is the slightest and per- 
mits us to retain the bird in the same subspecies, but further 
northwards and especially northeastwards, the other influence 
becomes stronger and a different subspecies is the result. The 
best examples of drachidactyla are from the Atlantic coast, be- 
cause there the birds make a sea journey and evidently special- 
ized quicker because of their comparatively narrow habitat during 
the glacial retreat; the broader area of the Mississippi region 
and the lake barriers having delayed the development of the birds 
northward. The migration of drachidactyla through the Bahamas 
proves that this group of islands was formerly less widely sepa- 
rated from the continent. 
In my study of these birds, I have been unable to consider 
temperature as an important factor in producing differential 
characters. The strong psychological characters cannot be 
accepted as caused by temperature, for they are almost entirely 
absent in the females. The physiological characters are neces- 
sarily to be correlated with other more direct influences, many of 
which I have indicated. One effect of temperature has been to 
permit of a larger number of forms in the tropics, but only indi- 
rectly by its influence on other life and in connection with other 
causes in affecting the topography. Northward few species are 
found, but they are strikingly different even in the’ same temper- 
ature areas. Temperature acts principally as a barrier in pre- 
venting, along the northern and higher borders of the ranges of 
these birds, favoring topographical conditions. If our present 
arctic zone was to disappear, these birds would gradually work 
northwards and eventually, if favorable conditions continued for 
a long period, would.break up into various forms under the influ- 
ence of local conditions; but they would never have the short 
rounded wings of their tropical relatives unless their migration 
should wholly cease. 
The genus Geothlypis is purely Nearctic wherever it is found, 
- and undoubtedly developed in North America during preglacial 
