Vol.XXI-J Taverner, The Origin of Migfation. ^2^ 



improbable that the birds themselves realize why they migrate, or 

 what benefits are to be thus gained or enemies escaped. When 

 the proper season comes, "the spirit moves them," and they go or 

 come, as the case may be. However instinctive their habit 

 may now be, there must have been a time when migrations were 

 intelligent movements, intended to escape some danger or secure 

 some advantage; and through generations of repetition they have 

 become fixed into hereditary habits, closely with reproduction and 

 reproductive seasons. In time the two habits became so inter- 

 dependent that the awakening of the sexual desires sympathetic- 

 ally affected the migratory instincts and caused restlessness and 

 a desire that was only to be satisfied by the accomplishment of 

 the same long journey that their progenitors had taken for 

 generations. 



Of the many theories that have been advanced to explain this 

 question, I will mention a few that seem the most important and 

 the most generally received. While advancing nothing abso- 

 lutely new, I wish to call attention to one factor in the question 

 that has not, in my estimation, been given its due importance, 

 nor has it been recognized, as far as I am aware, that therein lie 

 possibilities probably capable of producing all the phenomena of 

 migrations as we now see them. Of this, more anon. 



There is a theory extant, supported by W. K. Brooks in his 

 * Foundations of Zoology ' that has received a considerable 

 amount of attention. This ascribes migration to a desire to 

 find nesting sites secure from arboreal Mammalia and Reptilia. 

 This supposes, and perhaps correctly so, that the northern nest- 

 ing stations are safer from these enemies than the tropical ones; 

 though any one familiar with our northern woods, and acquainted 

 with our ubiquitous red squirrel, may have good grounds for 

 doubting the general statement, as far as it relates to mammals, 

 at least. 



There are certain facts of distribution, however, that this theory 

 fails to explain, and which seem, indeed, to be in direct antago- 

 nism to it. Typical instances of this can be seen in the distribu- 

 tion and ranges of the families of Cuckoos and Doves. Also the 

 occurrence of such an elaborate and careful nest builder as the 

 Baltimore Oriole, as far north as the Transition fauna. Surely, 



