THE DEPARTURE OF THE SWIFTS. 177 



really date later than civilisation, as one may suspect of 

 many other creatures, both weeds and house parasites, 

 like geckos and crickets : for after all, even civilisation 

 is old enoucrh to have exercised some minor transforming 

 influence upon the outer shapes of organic beings, as it 

 undoubtedly has upon their habits and instincts. 



Long ago, Gilbert White was much puzzled with the 

 difficulty, suggested to him by the swifts, as to what 

 became of the annual increase which must take place 

 even among such small breeders as these ; for though 

 they lay but two eggs at a time, and sit but once each 

 summer, instead of twice like the other swallows, yet 

 that must give a constant increment of population at 

 the rate of about double every year, even after allowing 

 for normal deaths of old birds. What becomes of such 

 increase ? That was the question that puzzled the 

 naturalist of Selborne ; and if he had been a Darwin, or 

 even a Malthus, it might have led him gradually on to 

 the great discovery of the principle of natural selection 

 which has since revolutionised all biological science. As 

 it was, he came only to the lame and impotent conclu- 

 sion that they must disperse themselves over the re- 

 mainder of the world : as though Selborne church-tower 

 were the central Ararat of an unpeopled and vacant 

 continent, whence endless colonies might go forth to 

 increase and multiply and replenish the earth. In sober 

 fact, one half of them fail to pick up a living at all ; 

 the other half just keep up the standard of the race to 

 its fixed numerical average : for everybody who has 

 watched the swifts closely knows that each year just the 

 same number of pairs return punctually to just the same 

 accustomed stations in just the same ancestral towers. 



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