34 
Beebe, The Early Life of Loon Chicks. 
r Auk 
L Jan. 
so to speak, a graduated scale of regularly arising, cumulative 
differences, but at their points of contact they are more unlike than 
at their geographical extremes. 
We consequently are led to consider the possibility of the Yel- 
low-throats having acquired their present range through some 
such method of progress as the Grackles appear to have followed, 1 
and an earlier stage of which the Loggerhead and Migrant Shrikes 
exhibit. An apparently not dissimilar case is afforded by the Pa- 
rula Warblers, in which the New England form is the same as that 
found in the Mississippi Valley. 
In other words, Yellow-throats may have advanced from Florida 
northward, and also from the Mississippi Valley eastward and 
northward; when, as has been said, the Northern Yellow-throat 
is not a direct geographical offshoot of the southern bird, although 
both doubtless had a common point of origin. Intergradation, 
therefore, is not necessarily climatic but follows actual contact 
occasioned by extension of range. 
Auk, XXIV, 5 1907, p. 
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