ro COLIN CLOUT'S CALENDAR. 



the spring-time. It is not surprising, therefore, that 

 early naturalists should have taken refuge in the hypo- 

 thesis of a special instinct implanted in the swallows, 

 independently of experience, and prompting them to 

 seek the appropriate climate by some unknown ' sense 

 of direction ' at the proper times of year. But, with our 

 existing knowledge as to the past history of European 

 geography and meteorology, no such cutting of the 

 Gordian knot is now necessary. 



We know that the climate of England in compara- 

 tively recent times was apparently as warm as that of 

 North Africa ; and we know that at the same period the 

 beds of the Mediterranean and the English Channel were 

 dry land. Hence it was then at least as easy for the 

 swifts and swallows to range from Scotland to Sahara as 

 it now is in America for the hardier humming-birds to 

 range from Canada to Mexico. But when the change 

 of ' cosmical weather ' made England by slow degrees 

 too cold in winter for flowers and midges to flourish all 

 the year round, the swallows would begin gradually to 

 fly a little to the south, as each autumn came on, and 

 remove a little to the north again as spring returned. 

 At first, no doubt, they would only have to shift their 

 quarters very slightly in search of more plentiful food, 

 without themselves being conscious of any special 

 migration. In course of time, however, as the difference 

 in climate became more and more marked, the birds 

 would have to fly further and further south with each 

 successive autumn, and would be enticed further and 

 further north again to their original homes with each 

 successive spring. Thus at last the practice of migration 

 would become engrained in the nervous system, and 



