Vol. XVII 
1900 
PALMER, The Maryland Vellow-throat. 239 
acter of the local habitat. These birds inhabit bushy moist 
situations, where they obtain their food near or on the ground, 
so that it is not remarkable that the effects of a generally prevail- 
ing arid climate should counteract purely local pigment effects. 
Thus paleness in this bird is largely a mechanical effect due to 
climate and wearing. There is a constant struggle between the 
effects of a local, rich, moist land diet, which alone produces the 
pigment, and the high and generally arid climate which has many 
months in which to effect a bleaching change. 
CONCLUSIONS. 
The evidence shows that the Yellow-throats of the Eastern 
States can be readily segregated into three well marked sub- 
species, each occupying topographically, geologically and clima- 
tologically distinct faunal areas of enormous extent, and that 
each one is the product of its own peculiar environment plus in 
one case the effects of a long migration. The comparatively 
recent, geologically speaking, retreat of the glacial snow and ice 
has permitted, slowly to be sure but steadily, the evolution of 
brachidactyla from its preglacial short-winged ancestor. This has 
been effected not by the extension of the range of a species or 
even a subspecies but by the slow increase of the more northern 
individuals as the retreating snow and ice opened up a greater 
extent northward of possible breeding range. Concomitant with 
this increase of individuals in one direction occurred also a 
change of structure and character, the change being in an as- 
cending ratio as a more northern point was reached each year or 
cycle of years. The result was a constantly increasing differ- 
ence, the birth of a subspecies. Therefore, like so many other 
North American birds, drachidactyla is a result of a decrease of 
glacial ice and snow. We have then along approximately the 
same longitude three distinct forms of one species with two sets 
of intermediates. At one end the form is large because the 
individuals obey the well known law, that: where a species occu- 
pies a longitudinal or altitudinal range the more boreal individuals 
are larger than their relatives of adjoining lower ground habitat. 
