THE PROCESS OF DISPERSAL, AND CLIMATE 

 AND CHANGES. 



Coming now to the progress made by advancing species. I have 

 elsewhere expressed the opinion that such advances are often 

 dependent upon the acquaintance made by these species of the 

 countries over which they have been accustomed to pass, first during 

 an autumn migration, and next upon a return spring passage. This 

 result may, however, be only after repeated experiences — it may be 

 after only a long series of years — but in the case of others there may 

 be a much more rapid development. 



I have also argued that not until certain occupied areas — whether 

 oversea, or in the more restricted areas of our island of Great Britain 

 — have become congested, will such developments take place. 



I have also hinted that temporary — or even in some cases per- 

 manent — shifting of breeding grounds may take place, due to 

 abnormal change in meteorological conditions, especially should 

 such continue for a series of seasons in succession. 



But there is another cause within our own country which I think 

 is of a somewhat different nature. That, I believe, is simply the first 

 stage of congestion of certain species which ornithologists have been 

 accustomed to look upon as resident species, but which from their 

 local haunts becoming filled up press away in any direction of least 

 resistance — let me say, in a northward direction. Such species 

 could hardly be considered to have made prior acquaintance with 

 suitable localities, because they had not migrated by these new routes 

 before. They simply " flew outwards " because the area hitherto 

 sufficient to hold their numbers, could hold their increase no 

 longer. 



Lines of so-called "least resistance" may be selected from several 

 causes. The influence may be initiated by the conformation of the 

 land, or other topographical peculiarities favourable to the well-being 



Ixxv 



