G. GENERAL NATURAL HISTORY AND ZOOLOGY. 341 



It is the more easy to agree with Professor Newton in his 

 criticism on this theory, as the phenomena of migration in 

 North America show, not a paroxysmal impulse, but a long- 

 continued movement, which lasts for weeks and even months, 

 during which the birds make progress in definite lines, usu- 

 ally proceeding in the autumn to well-established wintering 

 grounds, from which they return in the spring to almost the 

 precise spot whence they at first started. In opposing the 

 hypothesis of the Times correspondent Professor Newton at 

 the same time confesses his ignorance in regard to the phe- 

 nomenon, and remarks that the attention of observers should 

 be directed to the following points : 



First, the original cause or causes of migration. In some 

 cases he thinks that scarcity of food is a sufficient and a most 

 obvious cause. As food grows scarce toward the end of 

 summer, in the most northern limits of the ranges of species, 

 the individuals affected thereby seek it in other countries. 

 In doing this they crowd out other individuals, and these, in 

 turn, press upon still another zone, resulting in a stampede 

 of the birds inhabiting a vast extent of country. He, how- 

 ever, does not find that the return movement is to be explain- 

 ed by any such hypothesis, since there is always an abun- 

 dance of food in the winter quarters of the migrants, who 

 leave for the North, where the ground may be still covered 

 with snow, and where they are subjected to great inconve- 

 nience in their search for food. Next, the mode or modes 

 of migration ; not only whether different birds migrate in 

 the same manner, but whether the same species maintain the 

 same peculiarities throughout. The great question, however, 

 is how birds find their way back to their old homes, return- 

 ing after a journey of thousands of miles to the very spot 

 where they were hatched, or where they had nested the pre- 

 vious season, and arriving at a given point on almost the 

 same day in many successive years. 



An hypothesis was presented some years ago by Midden- 

 dorf, an eminent Russian naturalist, who believed that he 

 had found in the magnetic currents circulating between the 

 north and south poles of the earth the cause of the phenom- 

 ena in question. This answers very well for the species 

 which actually make use of a movement in the direction re- 

 ferred to, but fails to explain the case where, as in many 



