INTRODUCTION. 17 



Taking it for granted that all our readers know what is understood by the migration 

 of birds, the regular travel towards the north in spring, and the regular return in 

 fall towards the south, of certain birds, and also what is understood by the term a 

 permanent resident, we will at once remark that there is no fundamental difference 

 between the categories, since perhaps the greater part of the permanent residents 

 travel about more or less extensively during the cold season, and the range of migra- 

 tion of many so-called migrating species is very limited, while not a few are residents 

 in one country, though migrating in other localities, as for instance, the meadow lark, 

 the purple grackle, the bluebird, etc. A moment's reflection will therefore convince us 

 that the migrating state has developed in originally sedentary birds. 



The next thing to take into consideration is the fact that it is not the cold that 

 drives the migratory birds away in fall, since other birds equally equipped stand the 

 climate very well, and remain in the country the migrants left ; the only reason why 

 the latter go is because they are in some way or another deprived of the special food 

 upon which their existence depends. The fact is simply that they have the choice 

 either to go or to starve. It is also clear that they will generally not go farther than 

 is absolutely necessary. The residents, on the other hand, are able to stay, because 

 their principal food is to be had at all seasons in the region where they are born. 



It is furthermore evident, from what daily experience teaches us, that no life-sustain- 

 ing possibility is left unoccupied by nature, so that when she opens a new field where 

 a living can be made, there the invitation to immigrate is at once accepted. Birds 

 organized like those of which we said above that the approaching winter gives them 

 the choice between going away or starving, but which only go so far as barely neces- 

 sary, would be the first ones to avail themselves of the abundance of food in their 

 old quarters with the returning summer. A conjectural case will help to elucidate 

 the above remarks. Suppose, then, that the bluebird originally inhabited a great area 

 having a uniform climate enabling the individuals throughout the range of the species 

 to find their food all the year round, they would then be sedentary over the whole 

 area. Suppose the climate became gradually colder in winter at the northern border, 

 suspending insect life during a part of the year. Those living in that region would 

 have to go or starve, and it cannot be doubted that those going in the right direction 

 viz. southward and they only, would survive, while the rest would be killed. The 

 next year the survivors will return and breed, and again only the travelers going south 

 will save their lives. We can now understand how a migratory habit might originate ; 

 and as we know that habits easily become hereditary when necessary for the preserva- 

 tion of the species, we are compelled to concede that the so-called " instinct of migra- 

 tion is nothing but a hereditary habit forced upon certain kinds of birds by ' natural 

 selection.' " 



But it will be seen that the result may be the same if we reverse our conjecture, 

 and suppose that a bird for example, the nightingale originally inhabited a rather 

 restricted area, which subsequently became extended for a part of the year, the 

 summers of the adjacent territory gradually becoming inhabitable ; the result would 

 be the same. 



The theory, thus far, looks acceptable; the question is now whether sufficient 

 evidence can be had to make it probable that such conditions as those supposed above 

 have actually existed, in answer to which I shall quote the following from Professor 

 J. A. Allen's pen : " In reference to this point, let us revert for a moment to the 

 geological history of North America. Nothing is doubtless more thoroughly estab- 

 VOL. iv. 2 



