328 Taverxer. The Origin of Migration. \ji% 



elementary, into a country just recovering from the rigors of winter, 

 is a very hazardous solution of any problem. Especially must 

 this have been true in the early days of the habit, when the races 

 were much less adequately provided with hereditary experience 

 and structure necessary for its successful conclusion. In this 

 light, it seems highly improbable that anything short of the stern- 

 est necessity would favor the development of a habit so fraught 

 with danger to the individuals of a species ; and that, if any less 

 hazardous solution were possible, it would have been taken advan- 

 tage of. 



The great diversity of food and nesting habits exhibited by 

 closely allied species, shows how easily, comparatively speaking, 

 these habits are modified. Therefore, if any peculiar nesting or 

 food requirements menaced the welfare of tropical residents to the 

 extent that must have been necessary to produce migration, it is 

 reasonable to suppose these habits would have been altered to 

 suit surroundings long before such a dangerous habit as migra- 

 tion could have been adopted. 



The natural inference is that the problem was something that 

 could be solved in no less hazardous way. For it would be much 

 easier for birds to learn to build woven pensile nests at the end of 

 long slender branches, or to adopt food that closely allied species 

 found acceptable, than to create all the elaborate instincts, powers 

 and structures necessary to enable them to traverse great stretches 

 of country unguided, and in the face of meteorological disturb- 

 ances, new enemies, strange foods, and all the dangers attendant 

 upon migration. These grounds, then, alone seem sufficient to 

 discredit any such phenomena as the foregoing, as prime causes 

 in the origination of this habit. 



The one cause that seems adequate to produce such great 

 results, is that one which ultimately rules the whole animate world 

 — the sufficiency of the food supply. Admitting that in the trop- 

 ics there is, at any time, or more especially during the migration 

 seasons, a lack of, or a severe struggle for food, and we have a 

 necessity sufficiently imperative to cause the origin of any habit 

 that it is possible to form. Mr. J. A. Allen, and others, have 

 shown that the usual struggle for existence, always and every- 

 where intensely severe, is sutficient to cause an overflow into an 



