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 Tertiarv^ 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. \Yith 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 foi'ms 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 I'eceded and opened 

 up a wider area within wliich 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 pi'esent 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 neai'ly cosmopolitan, or which range throughout the colder 

 portions of the Northern hemisphere. W^ith 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 degi'ee differentiated from their 

 Old World affiues, 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 difterentia- 

 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 repi'esent 



