32 ZOOLOGY. 



dence that they do more or less fully intergrade. Convenience seems to 

 demand such a course, in order to enable the naturalist to specify what par- 

 ticular variety or race of a species inhabits a given section of country: a 

 method, in fact, already more or less generally practiced." 



With the question of the bearing of these facts of geographical distribu- 

 tion and variation upon the matter of the origin of genera and species, we 

 have at present nothing to do ; but that they form an important element in 

 the solution of the problem is none the less certain. 



With regard to the causes which influence the migration of birds, the 

 celebrated Swedish poet Runeberg, during a long and severe illness at 

 Helsingfors, occupied himself in observing their habits, especially in regard 

 to migration, and ascribes it to an ardent and intense longing for light. 

 This ingenious theory, which is hardly to be considered tenable, he explains 

 as follows : 



" When the days shorten in the north, the birds go south ; but as soon 

 as the long northern nights set in with their luminous long-drawn hours, the 

 wanderers return to their old haunts. It is generally supposed that they 

 move southward to get more abundant food. But why do they leave the 

 rich southern feeding grounds to return northward ! Simply because one 

 thing is richer there, and that is light. The same instinct that makes plants 

 turn toward the light, and stretch their branches to reach it, also works in 

 birds, and compels them to fly after and follow it. The bird of passage is of 

 noble birth ; he bears a motto, and his motto is Lux mca dux." 



But Prof. Alfred Newton, in discussing this theory in " Nature and 

 Science " for January, says : 



" In some cases, scarcity of food would seem to be a sufficient cause; 

 and it is undoubtedly the most obvious one that presents itself to our 

 mind. As food grows scarce toward the end of summer in the most 

 northern limits of the range of a species, the individuals affected thereby 

 seek it in other countries. Thus doing, they press upon the haunt of 

 other individuals ; these in like manner upon that of yet others ; and so 

 on, until the movement, which began in the far north, is communicated to 

 the individuals occupying the extreme southern range of the species at that 

 season ; though, but for such an invasion, these last might be content to 



