28 THE MIGRATIONS OF BIRDS 



Habits and reaction to environment in birds must 

 date back through this same long period of years, so 

 that what we now recognize as definite instincts 

 among them, which are merely reactions to the 

 usual stimuli of everyday life, must have had their 

 beginning back in the same remote periods. Birds 

 sought for food, fought for territory, and produced 

 their kind, were subject to seasonal changes, shifts 

 in environment, ecological successions among plants 

 and trees, and climatic variations, then as now. We 

 must consider the present migratory instinct as an 

 outcome of all the various complex circumstances 

 that have affected birds, and for its origin must look' 

 to remote ages. The actual migrations now found in 

 the northern hemisphere have been influenced pro- 

 foundly by the climatic changes of the Pleistocene, 

 but the cause of migratory flight goes so far back in 

 time that we may discuss it, but may not hope to 

 offer more than guess or supposition as to its actual 

 origin. 



With this statement in mind it will be understood 

 that hypotheses advanced regarding the origin of 

 migration are merely attempts to explain it on the 

 basis of what may have happened, judging from the 

 observations of our limited human experience. 

 There can be no final proof of the validity of the 

 arguments. They stand as scientific guesses at what 

 is supposed to be the truth. 



