Allen on the Instinct of Migration. 153 
become hereditary, and so fixed as to be what we term an in- 
stinct.” 
In reference to this point, let us revert for a moment to the 
geological history of North America. Nothing is doubtless more 
thoroughly established than that a warm-temperate or sub-tropical 
climate prevailed down to the close of the Tertiary epoch, nearly 
to the Northern Pole, and that climate was previously everywhere 
so far equable that the necessity of migration can hardly be supposed 
to have existed. With the later refrigeration of the Northern re- 
gions, bird life must have been crowded thence toward the tropics, 
and the struggle for life thereby greatly intensified. The less yield- 
ing forms may have become extinct ; those less sensitive to climatic 
change would seek to extend the boundaries of their range by a 
slight removal northward during the milder intervals of summer, 
only, however, to be forced back again by the recurrence of winter. 
Such migration must have been at first “ incipient and gradual,” 
extending and strengthening as the cold wave receded and opened 
up a wider area within which existence in summer became possible. 
What was at first a forced migration would become habitual, and 
through the heredity of habit give rise to that wonderful faculty 
we term the instinct of migration. With the development of this 
new instinct, and from the same general cause, undoubtedly origi- 
nated much of the diversity that now characterizes the North Amer- 
ican avifauna. If we consider our present fauna in reference to the 
geographical relation and probable origin of its leading forms, we 
find that a large proportion of the species belong to genera that are 
either nearly cosmopolitan, or which range throughout the colder 
portions of the Northern hemisphere. With the invasion of the 
great cold wave, these with other forms must have been pressed 
southward, and have thus become isolated and subjected to more or 
less changed conditions of environment, under the influence of which 
they became to a greater or less degree differentiated from their 
Old World affines, in some cases merely as geographical races, in 
others specifically, if not even also occasionally generically. The 
orographic changes that marked the same general period would tend, 
in virtue of resulting climatic modifications, to further differentia- 
tion within the different areas of the continent itself. The remain- 
ing species belong to strictly American types, which doubtless 
originated either within or near the present American tropics, since 
the metropolis of nearly all the groups they respectively represent 
