104 HALF-HOURS IN THE GREEN LANES. 



Our native birds may be roughly grouped into 

 the migratory and non-migratory species. Of the 

 former we have two kinds those that come to us 

 in the early summer, and leave in the late autumn 

 for more southern climes ; and those birds which, 

 on the contrary, visit us in the late autumn and 

 remain during the winter, leaving us again on the 

 approach of spring. The origin of these migratory 

 habits has not yet been philosophically settled. 

 That, some time in the history of the past, it has 

 been developed through changing physical circum- 

 stances, few naturalists will doubt. No geological 

 period throws light on the possibility of such a 

 habit being formed, except the Glacial epoch the 

 last of any extent that affected the northern hemi- 

 sphere. The slow but sure increase of cold which 

 then affected these latitudes must have largely 

 influenced the habits of British birds. Those in- 

 capable of standing against cold would be driven 

 each winter further and further to the south, whilst 

 those habits of attachment to localities which many 

 birds possess, would cause them to return to their 

 original homes whenever circumstances allowed 

 them, that is, in the summer months. It is of the 

 migratory birds principally that so many anecdotes 

 are told of their returning every year to build in 

 the same place as they did the year before, showing 

 how strongly developed is what phrenologists would 

 call their "locality." It is a geological fact that 

 during the long continuance of the Glacial period. 



