THE DEPARTURE OF THE SWIFTS. 169 



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 in- 

 crease ? That was the question that puzzled the natural- 

 ist 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 revolutionized all biological science. 

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

 clusion 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 con- 

 tinent, whence endless colonies might go forth to in- 

 crease 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 watch- 

 ed the swifts closely knows that each year just the same 

 number of pairs return punctually to just the same ac- 

 customed stations in just the same ancestral towers. In- 

 deed, that is the rule with the vast majority of species, 

 animal or vegetable. There are a few which, like man, 

 the Colorado beetle, and the Canadian pond- weed, are 

 rapidly increasing and overrunning the world ; there are 

 a few other which, like the great auk, the beaver, and 

 the edelweiss, are rapidly dying out before their enemies. 

 But by far the greater number seem to continue abso- 

 lutely invariable from year to year, at least within the 

 range of ordinary human observation. Out of 40,000 

 seeds of one common English weed, only a single seed on 

 an average produces a full-grown plant every season. 



