28 J. E. LITTLEBOT THE MIGEATIOIT OF BIEDS. 



by the fear of hunger in the autumn "What we mean 



is, that there is an impulse within him which constrains him at 

 the right time to migrate." The concluding sentence appears to 

 be intended as a key to the preceding propositions. Dr. Weissmann 

 not only attributes northern migration to an impulse that constrains 

 ''birds at the right time to migrate," but he pursues his subject 

 a step further, and endeavours to define the method by which this 

 " impulse," or, as he elsewhere terms it, this " wandering instinct," 

 became engrafted into their nature. 



The argument is ingenious and interesting, but it cannot be 

 considered to furnish a sufiicient answer to the much-debated query, 

 "Why do birds migrate northwards in the spring? The evidence 

 adduced supplies, without doubt, an appreciable motive for the 

 migration of birds from certain defined districts to others more or 

 less contiguous, but it fails to account, in a satisfactory manner, 

 for the enormous geographical area over which the migrations of 

 certain species are known to extend, or for the admitted fact that 

 the desire to migrate is as strong in young birds as in old ones, and 

 even among those that are confined in cages and supplied with 

 every requirement. That the supply of food in equatorial Africa 

 becomes short during the heat of summer is extremely probable, 

 and that a distinct motive for migration from that particular 

 locality is thereby afforded, appears to be a reasonable conclusion. 

 But there is food in abundance south of the tundra of Sibeiia, and 

 why, under such conditions, should the willow-wren, a tiny bird 

 that regularly frequents, as every one knows, the gardens and 

 hedgerows of England, but which migrates southward during the 

 winter, as far or even farther than the arid plains of Africa, leave 

 behind it on the approach of spring — as multitudes of this species 

 do — the aphide- haunted vines of France and Germany, the olive 

 groves of Italy and Greece, or the many attractions of our English 

 counties, to wing its way hundreds and hundreds of miles north- 

 ward, and all for the sake of spending a very short summer in 

 Sibei ia, or northern Eiissia ? 



There is an attractive theory, which was briefly alluded to after 

 the reading of my former paper, and it is necessary that I should 

 now discuss it. It was argued on that occasion that migration north- 

 wards is simply a physiological question, regulated and explained 

 by physiological facts and inferences ; in other words, that the 

 necessities connected with incubation render migration, during the 

 breeding season, to districts where a lower temperature prevails, 

 absolutely imperative. This position was illustrated by reference 

 to the well-known fact that successful incubation among our 

 domestic poultry takes place, for the most part, before the heat of 

 summer has commenced ; but surely it is hardly allowable to build 

 up an argument respecting the habits of birds in their naturally 

 wild condition upon experience gained by observing them in 

 a domesticated and highly artificial state. I will briefly notice 

 a few facts which seem to range themselves in opposition to this 

 hypothesis. («) The range of migration that prevails among several 



