THE STRUGGLE FOR EXISTENCE. 235 



It is obvious that when once the natural limit of any 

 creature has been reached, as many must perish each year 

 as are produced in that year. Otherwise the increase 

 would go on indefinitely. A little computation will show 

 how great this yearly destruction among the bluebirds 

 must be. Bluebirds lay from four to six eggs for a brood, 

 and rear two broods a year. It is claimed that sometimes 

 they are even three-brooded. Now assume the number of 

 eggs to be five, which must be a fair average, and the 

 broods to be two. By reason of the nesting habits of the 

 bluebird, relatively few eggs fail to produce a chick. But, 

 that we may be conservative, let us assume that each pair 

 of bluebirds produces eight young in a season. In a sec- 

 tion, then, where there were 1000 bluebirds in the spring, 

 there would be 4000 young in the fall, or 5000 bluebirds in 

 all. If it were possible that these might all migrate safely 

 and return the next spring, to produce at the rate of the 

 previous year, there would be 20.000 young, which, added 

 to the 5000 parent birds, would give a total of 25,000 blue- 

 birds ready for migration the second fall. 



This rapid rate of increase explains how it is that the 

 bluebirds have so quickly come to their own again after the 

 terrible calamity of 1895, and indicates, also, how great 

 the yearly destruction must inevitably be after the normal 

 numerical limit has been reached. Some of the old, many 

 of the young, must perish — as many of old and young as 

 shall equal the number hatched each year. 



This fact — for obviously it is a fact, however disagreea- 

 ble it may appear — shows what must be the potency of 

 natural selection in this terrific struggle for existence. 

 Anything that tends in the slightest degree to the advan- 

 tage of an individual will tend to enable that individual to 

 survive, and if it survives, it may transmit its characteris- 

 tics to its young. A sweet note, a brilliant plumage, a 

 gallant bearing may help to win a mate ; strength, fleet- 



